The next project in my GPGPU series is a particle physics
engine that computes the entire physics simulation on the GPU.
Particles are influenced by gravity and will bounce off scene
geometry. This WebGL demo uses a shader feature not strictly required
by the OpenGL ES 2.0 specification, so it may not work on some
platforms, especially mobile devices. It will be discussed later in
the article.
It's interactive. The mouse cursor is a circular obstacle that the
particles bounce off of, and clicking will place a permanent obstacle
in the simulation. You can paint and draw structures through which the
the particles will flow.
Here's an HTML5 video of the demo in action, which, out of necessity,
is recorded at 60 frames-per-second and a high bitrate, so it's pretty
big. Video codecs don't gracefully handle all these full-screen
particles very well and lower framerates really don't capture the
effect properly. I also added some appropriate sound that you won't
hear in the actual demo.
On a modern GPU, it can simulate and draw over 4 million particles
at 60 frames per second. Keep in mind that this is a JavaScript
application, I haven't really spent time optimizing the shaders, and
it's living within the constraints of WebGL rather than something more
suitable for general computation, like OpenCL or at least desktop
OpenGL.
Encoding Particle State as Color
Just as with the Game of Life and path finding
projects, simulation state is stored in pairs of textures and the
majority of the work is done by a fragment shader mapped between them
pixel-to-pixel. I won't repeat myself with the details of setting this
up, so refer to the Game of Life article if you need to see how it
works.
For this simulation, there are four of these textures instead of two:
a pair of position textures and a pair of velocity textures. Why pairs
of textures? There are 4 channels, so every one of these components
(x, y, dx, dy) could be packed into its own color channel. This seems
like the simplest solution.
The problem with this scheme is the lack of precision. With the
R8G8B8A8 internal texture format, each channel is one byte. That's 256
total possible values. The display area is 800 by 600 pixels, so not
even every position on the display would be possible. Fortunately, two
bytes, for a total of 65,536 values, is plenty for our purposes.
The next problem is how to encode values across these two channels. It
needs to cover negative values (negative velocity) and it should try
to take full advantage of dynamic range, i.e. try to spread usage
across all of those 65,536 values.
To encode a value, multiply the value by a scalar to stretch it over
the encoding's dynamic range. The scalar is selected so that the
required highest values (the dimensions of the display) are the
highest values of the encoding.
Next, add half the dynamic range to the scaled value. This converts
all negative values into positive values with 0 representing the
lowest value. This representation is called Excess-K. The
downside to this is that clearing the texture (glClearColor) with
transparent black no longer sets the decoded values to 0.
Finally, treat each channel as a digit of a base-256 number. The
OpenGL ES 2.0 shader language has no bitwise operators, so this is
done with plain old division and modulus. I made an encoder and
decoder in both JavaScript and GLSL. JavaScript needs it to write the
initial values and, for debugging purposes, so that it can read back
particle positions.
The fragment shader that updates each particle samples the position
and velocity textures at that particle's "index", decodes their
values, operates on them, then encodes them back into a color for
writing to the output texture. Since I'm using WebGL, which lacks
multiple rendering targets (despite having support for gl_FragData),
the fragment shader can only output one color. Position is updated in
one pass and velocity in another as two separate draws. The buffers
are not swapped until after both passes are done, so the velocity
shader (intentionally) doesn't uses the updated position values.
There's a limit to the maximum texture size, typically 8,192 or 4,096,
so rather than lay the particles out in a one-dimensional texture, the
texture is kept square. Particles are indexed by two-dimensional
coordinates.
It's pretty interesting to see the position or velocity textures drawn
directly to the screen rather than the normal display. It's another
domain through which to view the simulation, and it even helped me
identify some issues that were otherwise hard to see. The output is a
shimmering array of color, but with definite patterns, revealing a lot
about the entropy (or lack thereof) of the system. I'd share a video
of it, but it would be even more impractical to encode than the normal
display. Here are screenshots instead: position, then velocity. The
alpha component is not captured here.
Entropy Conservation
One of the biggest challenges with running a simulation like this on a
GPU is the lack of random values. There's no rand() function in the
shader language, so the whole thing is deterministic by default. All
entropy comes from the initial texture state filled by the CPU. When
particles clump up and match state, perhaps from flowing together over
an obstacle, it can be difficult to work them back apart since the
simulation handles them identically.
To mitigate this problem, the first rule is to conserve entropy
whenever possible. When a particle falls out of the bottom of the
display, it's "reset" by moving it back to the top. If this is done by
setting the particle's Y value to 0, then information is destroyed.
This must be avoided! Particles below the bottom edge of the display
tend to have slightly different Y values, despite exiting during the
same iteration. Instead of resetting to 0, a constant value is added:
the height of the display. The Y values remain different, so these
particles are more likely to follow different routes when bumping into
obstacles.
The next technique I used is to supply a single fresh random value via
a uniform for each iteration This value is added to the position and
velocity of reset particles. The same value is used for all particles
for that particular iteration, so this doesn't help with overlapping
particles, but it does help to break apart "streams". These are
clearly-visible lines of particles all following the same path. Each
exits the bottom of the display on a different iteration, so the
random value separates them slightly. Ultimately this stirs in a few
bits of fresh entropy into the simulation on each iteration.
Alternatively, a texture containing random values could be supplied to
the shader. The CPU would have to frequently fill and upload the
texture, plus there's the issue of choosing where to sample the
texture, itself requiring a random value.
Finally, to deal with particles that have exactly overlapped, the
particle's unique two-dimensional index is scaled and added to the
position and velocity when resetting, teasing them apart. The random
value's sign is multiplied by the index to avoid bias in any
particular direction.
To see all this in action in the demo, make a big bowl to capture all
the particles, getting them to flow into a single point. This removes
all entropy from the system. Now clear the obstacles. They'll all fall
down in a single, tight clump. It will still be somewhat clumped when
resetting at the top, but you'll see them spraying apart a little bit
(particle indexes being added). These will exit the bottom at slightly
different times, so the random value plays its part to work them apart
even more. After a few rounds, the particles should be pretty evenly
spread again.
The last source of entropy is your mouse. When you move it through the
scene you disturb particles and introduce some noise to the
simulation.
Textures as Vertex Attribute Buffers
This project idea occurred to me while reading the OpenGL ES shader
language specification (PDF). I'd been wanting to do a particle
system, but I was stuck on the problem how to draw the particles. The
texture data representing positions needs to somehow be fed back into
the pipeline as vertices. Normally a buffer texture -- a texture
backed by an array buffer -- or a pixel buffer object --
asynchronous texture data copying -- might be used for this, but WebGL
has none these features. Pulling texture data off the GPU and putting
it all back on as an array buffer on each frame is out of the
question.
However, I came up with a cool technique that's better than both those
anyway. The shader function texture2D is used to sample a pixel in a
texture. Normally this is used by the fragment shader as part of the
process of computing a color for a pixel. But the shader language
specification mentions that texture2D is available in vertex
shaders, too. That's when it hit me. The vertex shader itself can
perform the conversion from texture to vertices.
It works by passing the previously-mentioned two-dimensional particle
indexes as the vertex attributes, using them to look up particle
positions from within the vertex shader. The shader would run in
GL_POINTS mode, emitting point sprites. Here's the abridged version,
The real version also samples the velocity since it modulates the
color (slow moving particles are lighter than fast moving particles).
However, there's a catch: implementations are allowed to limit the
number of vertex shader texture bindings to 0
(GL_MAX_VERTEX_TEXTURE_IMAGE_UNITS). So technically vertex shaders
must always support texture2D, but they're not required to support
actually having textures. It's sort of like food service on an
airplane that doesn't carry passengers. These platforms don't support
this technique. So far I've only had this problem on some mobile
devices.
Outside of the lack of support by some platforms, this allows every
part of the simulation to stay on the GPU and paves the way for a pure
GPU particle system.
Obstacles
An important observation is that particles do not interact with each
other. This is not an n-body simulation. They do, however, interact
with the rest of the world: they bounce intuitively off those static
circles. This environment is represented by another texture, one
that's not updated during normal iteration. I call this the obstacle
texture.
The colors on the obstacle texture are surface normals. That is, each
pixel has a direction to it, a flow directing particles in some
direction. Empty space has a special normal value of (0, 0). This is
not normalized (doesn't have a length of 1), so it's an out-of-band
value that has no effect on particles.
(I didn't realize until I was done how much this looks like the
Greendale Community College flag.)
A particle checks for a collision simply by sampling the obstacle
texture. If it finds a normal at its location, it changes its velocity
using the shader function reflect. This function is normally used
for reflecting light in a 3D scene, but it works equally well for
slow-moving particles. The effect is that particles bounce off the the
circle in a natural way.
Sometimes particles end up on/in an obstacle with a low or zero
velocity. To dislodge these they're given a little nudge in the
direction of the normal, pushing them away from the obstacle. You'll
see this on slopes where slow particles jiggle their way down to
freedom like jumping beans.
To make the obstacle texture user-friendly, the actual geometry is
maintained on the CPU side of things in JavaScript. It keeps a list of
these circles and, on updates, redraws the obstacle texture from this
list. This happens, for example, every time you move your mouse on the
screen, providing a moving obstacle. The texture provides
shader-friendly access to the geometry. Two representations for two
purposes.
When I started writing this part of the program, I envisioned that
shapes other than circles could place placed, too. For example, solid
rectangles: the normals would look something like this.
So far these are unimplemented.
Future Ideas
I didn't try it yet, but I wonder if particles could interact with
each other by also drawing themselves onto the obstacles texture. Two
nearby particles would bounce off each other. Perhaps the entire
liquid demo could run on the GPU like this. If I'm imagining
it correctly, particles would gain volume and obstacles forming bowl
shapes would fill up rather than concentrate particles into a single
point.
I think there's still some more to explore with this project.
Last time I demonstrated how to run Conway's Game of Life
entirely on a graphics card. This concept can be generalized to any
cellular automaton, including automata with more than two states. In
this article I'm going to exploit this to solve the shortest path
problem for two-dimensional grids entirely on a GPU. It will be
just as fast as traditional searches on a CPU.
The JavaScript side of things is essentially the same as before -- two
textures with fragment shader in between that steps the automaton
forward -- so I won't be repeating myself. The only parts that have
changed are the cell state encoding (to express all automaton states)
and the fragment shader (to code the new rules).
Included is a pure JavaScript implementation of the cellular
automaton (State.js) that I used for debugging and experimentation,
but it doesn't actually get used in the demo. A fragment shader
(12state.frag) encodes the full automaton rules for the GPU.
Maze-solving Cellular Automaton
There's a dead simple 2-state cellular automaton that can solve any
perfect maze of arbitrary dimension. Each cell is either OPEN or a
WALL, only 4-connected neighbors are considered, and there's only one
rule: if an OPEN cell has only one OPEN neighbor, it becomes a WALL.
On each step the dead ends collapse towards the solution. In the above
GIF, in order to keep the start and finish from collapsing, I've added
a third state (red) that holds them open. On a GPU, you'd have to do
as many draws as the length of the longest dead end.
A perfect maze is a maze where there is exactly one solution. This
technique doesn't work for mazes with multiple solutions, loops, or
open spaces. The extra solutions won't collapse into one, let alone
the shortest one.
To fix this we need a more advanced cellular automaton.
Path-solving Cellular Automaton
I came up with a 12-state cellular automaton that can not only solve
mazes, but will specifically find the shortest path. Like above, it
only considers 4-connected neighbors.
OPEN (white): passable space in the maze
WALL (black): impassable space in the maze
BEGIN (red): starting position
END (red): goal position
FLOW (green): flood fill that comes in four flavors: north, east, south, west
ROUTE (blue): shortest path solution, also comes in four flavors
If we wanted to consider 8-connected neighbors, everything would be
the same, but it would require 20 states (n, ne, e, se, s, sw, w, nw)
instead of 12. The rules are still pretty simple.
WALL and ROUTE cells never change state.
OPEN becomes FLOW if it has any adjacent FLOW cells. It points
towards the neighboring FLOW cell (n, e, s, w).
END becomes ROUTE if adjacent to a FLOW cell. It points towards the
FLOW cell (n, e, s, w). This rule is important for preventing
multiple solutions from appearing.
FLOW becomes ROUTE if adjacent to a ROUTE cell that points towards
it. Combined with the above rule, it means when a FLOW cell touches
a ROUTE cell, there's a cascade.
BEGIN becomes ROUTE when adjacent to a ROUTE cell. The direction is
unimportant. This rule isn't strictly necessary but will come in
handy later.
This can be generalized for cellular grids of any arbitrary dimension,
and it could even run on a GPU for higher dimensions, limited
primarily by the number of texture uniform bindings (2D needs 1
texture binding, 3D needs 2 texture bindings, 4D needs 8 texture
bindings ... I think). But if you need to find the shortest path along
a five-dimensional grid, I'd like to know why!
So what does it look like?
FLOW cells flood the entire maze. Branches of the maze are search in
parallel as they're discovered. As soon as an END cell is touched, a
ROUTE is traced backwards along the flow to the BEGIN cell. It
requires double the number of steps as the length of the shortest
path.
Note that the FLOW cell keep flooding the maze even after the END was
found. It's a cellular automaton, so there's no way to communicate to
these other cells that the solution was discovered. However, when
running on a GPU this wouldn't matter anyway. There's no bailing out
early before all the fragment shaders have run.
What's great about this is that we're not limited to mazes whatsoever.
Here's a path through a few connected rooms with open space.
Maze Types
The worst-case solution is the longest possible shortest path. There's
only one frontier and running the entire automaton to push it forward
by one cell is inefficient, even for a GPU.
The way a maze is generated plays a large role in how quickly the
cellular automaton can solve it. A common maze generation algorithm
is a random depth-first search (DFS). The entire maze starts out
entirely walled in and the algorithm wanders around at random plowing
down walls, but never breaking into open space. When it comes to a
dead end, it unwinds looking for new walls to knock down. This methods
tends towards long, winding paths with a low branching factor.
The mazes you see in the demo are Kruskal's algorithm mazes. Walls are
knocked out at random anywhere in the maze, without breaking the
perfect maze rule. It has a much higher branching factor and makes for
a much more interesting demo.
Skipping the Route Step
On my computers, with a 1023x1023 Kruskal maze it's about an order
of magnitude slower (see update below) than A* (rot.js's
version) for the same maze. Not very impressive! I believe
this gap will close with time, as GPUs become parallel faster than
CPUs get faster. However, there's something important to consider:
it's not only solving the shortest path between source and goal,
it's finding the shortest path between the source and any other
point. At its core it's a breadth-first grid search.
Update: One day after writing this article I realized that
glReadPixels was causing a gigantic bottlebeck. By only checking for
the end conditions once every 500 iterations, this method is now
equally fast as A* on modern graphics cards, despite taking up to an
extra 499 iterations. In just a few more years, this technique
should be faster than A*.
Really, there's little use in ROUTE step. It's a poor fit for the GPU.
It has no use in any real application. I'm using it here mainly for
demonstration purposes. If dropped, the cellular automaton would
become 6 states: OPEN, WALL, and four flavors of FLOW. Seed the source
point with a FLOW cell (arbitrary direction) and run the automaton
until all of the OPEN cells are gone.
Detecting End State
The ROUTE cells do have a useful purpose, though. How do we know when
we're done? We can poll the BEGIN cell to check for when it becomes a
ROUTE cell. Then we know we've found the solution. This doesn't
necessarily mean all of the FLOW cells have finished propagating,
though, especially in the case of a DFS-maze.
In a CPU-based solution, I'd keep a counter and increment it every
time an OPEN cell changes state. The the counter doesn't change after
an iteration, I'm done. OpenGL 4.2 introduces an atomic
counter that could serve this role, but this isn't available in
OpenGL ES / WebGL. The only thing left to do is use glReadPixels to
pull down the entire thing and check for end state on the CPU.
The original 2-state automaton above also suffers from this problem.
Encoding Cell State
Cells are stored per pixel in a GPU texture. I spent quite some time
trying to brainstorm a clever way to encode the twelve cell states
into a vec4 color. Perhaps there's some way to exploit
blending to update cell states, or make use of some other kind
of built-in pixel math. I couldn't think of anything better than a
straight-forward encoding of 0 to 11 into a single color channel (red
in my case).
This leaves three untouched channels for other useful information. I
experimented (uncommitted) with writing distance in the green channel.
When an OPEN cell becomes a FLOW cell, it adds 1 to its adjacent FLOW
cell distance. I imagine this could be really useful in a real
application: put your map on the GPU, run the cellular automaton a
sufficient number of times, pull the map back off (glReadPixels),
and for every point you know both the path and total distance to the
source point.
Performance
As mentioned above, I ran the GPU maze-solver against A* to test its
performance. I didn't yet try running it against Dijkstra’s algorithm
on a CPU over the entire grid (one source, many destinations). If I
had to guess, I'd bet the GPU would come out on top for grids with a
high branching factor (open spaces, etc.) so that its parallelism is
most effectively exploited, but Dijkstra's algorithm would win in all
other cases.
Overall this is more of a proof of concept than a practical
application. It's proof that we can trick OpenGL into solving mazes
for us!
The biggest flaw with so many OpenGL tutorials is trying to teach two
complicated topics at once: the OpenGL API and 3D graphics. These are
only loosely related and do not need to be learned simultaneously.
It's far more valuable to focus on the fundamentals, which can
only happen when handled separately. With the programmable pipeline,
OpenGL is useful for a lot more than 3D graphics. There are many
non-3D directions that tutorials can take.
I think that's why I've been enjoying my journey through WebGL so
much. Except for my sphere demo, which was only barely 3D,
none of my projects have been what would typically be
considered 3D graphics. Instead, each new project has introduced me to
some new aspect of OpenGL, accidentally playing out like a great
tutorial. I started out drawing points and lines, then took a dive
into non-trivial fragment shaders, then textures and
framebuffers, then the depth buffer, then general
computation with fragment shaders.
The next project introduced me to alpha blending. I ported my old
feedback applet to WebGL!
Since finishing the port I've already spent a couple of hours just
playing with it. It's mesmerizing. Here's a video demonstration in
case WebGL doesn't work for you yet. I'm manually driving it to show
off the different things it can do.
Drawing a Frame
On my laptop, the original Java version plods along at about 6 frames
per second. That's because it does all of the compositing on the CPU.
Each frame it has to blend over 1.2 million color components. This is
exactly the sort of thing the GPU is built to do. The WebGL version
does the full 60 frames per second (i.e. requestAnimationFrame)
without breaking a sweat. The CPU only computes a couple of 3x3 affine
transformation matrices per frame: virtually nothing.
Similar to my WebGL Game of Life, there's texture stored on the
GPU that holds almost all the system state. It's the same size as the
display. To draw the next frame, this texture is drawn to the display
directly, then transformed (rotated and scaled down slightly), and
drawn again to the display. This is the "feedback" part and it's where
blending kicks in. It's the core component of the whole project.
Next, some fresh shapes are drawn to the display (i.e. the circle for
the mouse cursor) and the entire thing is captured back onto the state
texture with glCopyTexImage2D, to be used for the next frame. It's
important that glCopyTexImage2D is called before returning to the
JavaScript top-level (back to the event loop), because the screen data
will no longer be available at that point, even if it's still visible
on the screen.
Alpha Blending
They say a picture is worth a thousand words, and that's literally
true with the Visual glBlendFunc + glBlendEquation Tool. A
few minutes playing with that tool tells you pretty much everything
you need to know.
While you could potentially perform blending yourself in a fragment
shader with multiple draw calls, it's much better (and faster) to
configure OpenGL to do it. There are two functions to set it up:
glBlendFunc and glBlendEquation. There are also "separate"
versions of all this for specifying color channels separately, but I
don't need that for this project.
The enumeration passed to glBlendFunc decides how the colors are
combined. In WebGL our options are GL_FUNC_ADD (a + b),
GL_FUNC_SUBTRACT (a - b), GL_FUNC_REVERSE_SUBTRACT (b - a). In
regular OpenGL there's also GL_MIN (min(a, b)) and GL_MAX (max(a,
b)).
The function glBlendEquation takes two enumerations, choosing how
the alpha channels are applied to the colors before the blend function
(above) is applied. The alpha channel could be ignored and the color
used directly (GL_ONE) or discarded (GL_ZERO). The alpha channel
could be multiplied directly (GL_SRC_ALPHA, GL_DST_ALPHA), or
inverted first (GL_ONE_MINUS_SRC_ALPHA). In WebGL there are 72
possible combinations.
In this project I'm using GL_FUNC_ADD and GL_SRC_ALPHA for both
source and destination. The alpha value put out by the fragment shader
is the experimentally-determined, magical value of 0.62. A little
higher and the feedback tends to blend towards bright white really
fast. A little lower and it blends away to nothing really fast. It's a
numerical instability that has the interesting side effect of making
the demo behave slightly differently depending on the floating
point precision of the GPU running it!
Saving a Screenshot
The HTML5 canvas object that provides the WebGL context has a
toDataURL() method for grabbing the canvas contents as a friendly
base64-encoded PNG image. Unfortunately this doesn't work with WebGL
unless the preserveDrawingBuffer options is set, which can introduce
performance issues. Without this option, the browser is free to throw
away the drawing buffer before the next JavaScript turn, making the
pixel information inaccessible.
By coincidence there's a really convenient workaround for this
project. Remember that state texture? That's exactly what we want to
save. I can attach it to a framebuffer and use glReadPixels just
like did in WebGL Game of Life to grab the simulation state. The pixel
data is then drawn to a background canvas (without using WebGL) and
toDataURL() is used on that canvas to get a PNG image. I slap this
on a link with the new download attribute and call it done.
Anti-aliasing
At the time of this writing, support for automatic anti-aliasing in
WebGL is sparse at best. I've never seen it working anywhere yet, in
any browser on any platform. GL_SMOOTH isn't available and the
anti-aliasing context creation option doesn't do anything on any of my
computers. Fortunately I was able to work around this using a cool
smoothstep trick.
The article I linked explains it better than I could, but here's the
gist of it. This shader draws a circle in a quad, but leads to jagged,
sharp edges.
The improved version uses smoothstep to fade from inside the circle
to outside the circle. Not only does it look nicer on the screen, I
think it looks nicer as code, too. Unfortunately WebGL has no fwidth
function as explained in the article, so the delta is hardcoded.
Up until this point I had avoided matrix uniforms. I was doing
transformations individually within the shader. However, as transforms
get more complicated, it's much better to express the transform as a
matrix and let the shader language handle matrix multiplication
implicitly. Rather than pass half a dozen uniforms describing the
transform, you pass a single matrix that has the full range of motion.
My Igloo WebGL library originally had a vector library that
provided GLSL-style vectors, including full swizzling. My long term
goal was to extend this to support GLSL-style matrices. However,
writing a matrix library from scratch was turning out to be far more
work than I expected. Plus it's reinventing the wheel.
So, instead, I dropped my vector library -- I completely deleted it --
and decided to use glMatrix, a really solid
WebGL-friendly matrix library. Highly recommended! It doesn't
introduce any new types, it just provides functions for operating on
JavaScript typed arrays, the same arrays that get passed directly to
WebGL functions. This composes perfectly with Igloo without making it
a formal dependency.
Here's my function for creating the mat3 uniform that transforms both
the main texture as well as the individual shape sprites. This use of
glMatrix looks a lot like java.awt.geom.AffineTransform, does it
not? That's one of my favorite parts of Java 2D, and I've been
missing it.
/* Translate, scale, and rotate. */Feedback.affine=function(tx,ty,sx,sy,a){varm=mat3.create();mat3.translate(m,m,[tx,ty]);mat3.rotate(m,m,a);mat3.scale(m,m,[sx,sy]);returnm;};
The return value is just a plain Float32Array that I can pass to
glUniformMatrix3fv. It becomes the placement uniform in the
shader.
To move to 3D graphics from here, I would just need to step up to a
mat4 and operate on 3D coordinates instead of 2D. glMatrix would still
do the heavy lifting on the CPU side. If this was part of an OpenGL
tutorial series, perhaps that's how it would transition to the next
stage.
Conclusion
I'm really happy with how this one turned out. The only way it's
indistinguishable from the original applet is that it runs faster. In
preparation for this project, I made a big pile of improvements to
Igloo, bringing it up to speed with my current WebGL knowledge. This
will greatly increase the speed at which I can code up and experiment
with future projects. WebGL + Skewer + Igloo has really
become a powerful platform for rapid prototyping with OpenGL.
GNU Emacs is seven years older than Unicode. Support for Unicode had
to be added relatively late in Emacs' existence. This means Emacs has
existed longer without Unicode support (16 years) than with it (14
years). Despite this, Emacs has excellent Unicode support. It feels as
if it was there the whole time.
However, as a natural result of Unicode covering all sorts of edge
cases for every known human language, there are pitfalls and
complications. As a user of Emacs, you're not particularly affected
by these, but extension developers might run into trouble while
handling Emacs character-oriented data structures: strings and
buffers.
In this article I'll go over Elisp's Unicode surprises. I've been
caught by some of these myself. In fact, as a result of writing this
article, I've discovered subtle encoding bugs in some of my own
extensions. None of these pitfalls are Emacs' fault. They're just the
result of complexities of natural language.
Unicode and Code Points
First, there are excellent materials online for learning Unicode. I
recommend starting with UTF-8 and Unicode FAQ for Unix/Linux.
There's no reason for me to repeat all this information here, but I'll
attempt to quickly summarize it.
Unicode maps code points (integers) to specific characters, along
with a standard name. As of this writing, Unicode defines over 110,000
characters. For backwards compatibility, the first 128 code points are
mapped to ASCII. This trend continues for other character standards,
like Latin-1.
In Emacs, Unicode characters are entered into a buffer with C-x 8
RET (insert-char). You can enter either the official name of the
character (e.g. "GREEK SMALL LETTER PI" for π) or the hexadecimal code
point. Outside of Emacs it depends on the application, but C-S-u
followed by the hexadecimal code works for most of the applications I
care about.
Encodings
The Unicode standard also describes several methods for encoding
sequences of code points into sequences of bytes. Obviously a selection
of 110,000 characters cannot be encoded with one byte per letter, so
these are multibyte encodings. The two most popular encodings are
probably UTF-8 and UTF-16.
UTF-8 was designed to be backwards compatible with ASCII, Unix, and
existing C APIs (null-terminated C strings). The first 128 code points
are encoded directly as a single byte. Every other character is
encoded with two to six bytes, with the highest bit of each byte set
to 1. This ensures that no part of a multibyte character will be
interpreted as ASCII, nor will it contain a null (0). The latter means
that C programs and C APIs can handle UTF-8 strings with few or no
changes. Most importantly, every ASCII encoded file is automatically a
UTF-8 encoded file.
UTF-16 encodes all the characters from the Basic Multilingual Plane
(BMP) with two bytes. Even the original ASCII characters get two bytes
(16 bits). The BMP covers virtually all modern languages and is
generally all you'll ever practically need. However, this doesn't
include the important TROPICAL DRINK or PILE OF POO
characters from the supplemental ("astral") plane. If you need to use
these characters in UTF-16, you're going to run into problems:
characters outside the BMP don't fit in two bytes. To accommodate
these characters, UTF-16 uses surrogate pairs: these characters are
encoded with two 16-bit units.
Because of this last point, UTF-16 offers no practical advantages
over UTF-8. Its existence was probably a big mistake. You
can't do constant-time character lookup because you have to scan for
surrogate pairs. It's not backwards compatible and cannot be stored in
null-terminated strings. In both Java and JavaScript, it leads to the
awkward situation where the "length" of a string is not the number of
characters, code points, or even bytes. Worst of all, it has serious
security implications. New applications should avoid it
whenever possible.
Emacs and UTF-8
Emacs internally stores all text as UTF-8. This was an excellent
choice! When text leaves Emacs, such as writing to a file or to a
process, Emacs automatically converts it to the coding system
configured for that particular file or process. When it accepts text
from a file or process, it either converts it to UTF-8 or preserves it
as raw bytes.
There are two modes for this in Emacs: unibyte and multibyte. Unibyte
strings/buffers are just raw bytes. They have constant access O(1)
time but can only hold single-byte values. The byte-code compiler
outputs unibyte strings.
Multibyte strings/buffers hold UTF-8 encoded code points. Character
access is O(n) because the string/buffer has to be scanned to count
characters.
The actual encoding is rarely relevant because there's little way (and
need) to access it directly. Emacs automatically converts text as
needed when it leaves Emacs and arrives in Emacs, so there's no need
to know the internal encoding. If you really want to see it anyway,
you can use string-as-unibyte to get a copy of a string with the
exact same bytes, but as a byte-string.
(string-as-unibyte"π");; => "\317\200"
This can be reversed with string-as-multibyte), to change a unibyte
string holding UTF-8 encoded text back into a multibyte string. Note
that these functions are different than string-to-unibyte and
string-to-multibyte, which will attempt a conversion rather than
preserving the raw bytes.
The length and buffer-size functions always count characters in
multibyte and bytes in unibyte. Being UTF-8, there are no surrogate
pairs to worry about here. The string-bytes and position-bytes
functions return byte information for both multibyte and unibyte.
To specify a Unicode character in a string literal without using the
character directly, use \uXXXX. The XXXX is the hexadecimal code
point for the character and is always 4 digits long. For characters
outside the BMP, which won't fit in four digits, use a capital U with
eight digits: \UXXXXXXXX.
"\u03C0";; => "π""\U0001F4A9";; => "💩" (PILE OF POO)
Finally, Emacs extends Unicode with 256 additional "characters"
representing raw bytes. This allows raw bytes to be embedded
distinctly within UTF-8 sequences. For example, it's used to
distinguish the code point U+0041 from the raw byte #x41. As far as I
can tell, this isn't used very often.
Combining Characters
Some Unicode characters are defined as combining characters. These
characters modify the non-combining character that appears before it,
typically with accents or diacritical marks.
For example, the word "naïve" can be written as six characters as
"nai\u0308ve". The fourth character, U+0308 (COMBINING DIAERESIS),
is a combining character that changes the "i" (U+0069 LATIN SMALL
LETTER I) into an umlaut character.
The most commonly accented characters have a code of their own. These
are called precomposed characters. This includes ï (U+00EF LATIN
SMALL LETTER I WITH DIAERESIS). This means "naïve" can also be written
as five characters as "na\u00EFve".
Normalization
So what happens when comparing two different representations of the
same text? They're not equal.
(string="nai\u0308ve""na\u00EFve");; => nil
To deal with situations like this, the Unicode standard defines four
different kinds of normalization. The two most important ones are NFC
(composition) and NFD (decomposition). The former uses precomposed
characters whenever possible and the latter breaks them apart. The
functions ucs-normalize-NFC-string and ucs-normalize-NFD-string
perform this operation.
Pitfall #1: Proper string comparison requires normalization. It
doesn't matter which normalization you use (though NFD should be
slightly faster), you just need to use it consistently. Unfortunately
this can get tricky when using equal to compare complex data
structures with multiple strings.
(string=(ucs-normalize-NFD-string"nai\u0308ve")(ucs-normalize-NFD-string"na\u00EFve"));; => t
Emacs itself fails to do this. It doesn't normalize strings before
interning them, which is probably a mistake. This means you can have
differently defined variables and functions with the same canonical
name.
(eq(intern"nai\u0308ve")(intern"na\u00EFve"));; => nil(defunprint-résumé()"NFC-normalized form."(print"I'm going to sabotage your team."))(defunprint-résumé()"NFD-normalized form."(print"I'd be a great asset to your team."))(print-résumé);; => "I'm going to sabotage your team."
String Width
There are three ways to quantify multibyte text. These are often the
same value, but in some circumstances they can each be different.
length: number of characters, including combining characters
bytes: number of bytes in its UTF-8 encoding
width: number of columns it would occupy in the current buffer
Most of the time, one character is one column (a width of one). Some
characters, like combining characters, consume no columns. Many Asian
characters consume two columns (U+4000, 䀀). Tabs consume tab-width
columns, usually 8.
Generally, a string should have the same width regardless of which
whether it's NFD or NFC. However, due to bugs and incomplete Unicode
support, this isn't strictly true. For example, some combining
characters, such as U+20DD ⃝, won't combine correctly in Emacs nor in
other applications.
Pitfall #2: Always measure text by width, not length, when laying
out a buffer. Width is measured with the string-width function.
This comes up when laying out tables in a buffer. The number of
characters that fit in a column depends on what those characters are.
Fortunately I accidentally got this right in Elfeed because
I used the format function for layout. The %s directive operates
on width, as would be expected. However, this has the side effect that
the output of may format change depending on the current buffer!
Pitfall #3: Be mindful of the current buffer when using the format
function.
Regular expressions operate on code points. This means combining
characters are counted separately and the match may change depending
on how characters are composed. To avoid this, you might want to
consider NFC normalization before performing some kinds of regular
expressions.
;; Like string= from before:(string-match-p"na\u00EFve""nai\u0308ve");; => nil;; The . only matches part of the composition(string-match-p"na.ve""nai\u0308ve");; => nil
Pitfall #5: Be mindful of combining characters when using regular
expressions. Prefer NFC normalization when dealing with regular
expressions.
Another potential problem is ranges, though this is quite uncommon.
Ranges of characters can be expressed in inside brackets, e.g.
[a-zA-Z]. If the range begins or ends with a decomposed combining
character you won't get the proper range because its parts are
considered separately by the regular expression engine.
It's especially important to keep all of this in mind when
sanitizing untrusted input, such as when using Emacs as a web server.
An attacker might use a denormalized or strange grapheme cluster to
bypass a filter.
Interacting with the World
Here's a mistake I've made twice now. Emacs uses UTF-8 internally,
regardless of whatever encoding the original text came in. Pitfall #6:
When working with bytes of text, the counts may be different than
the original source of the text.
For example, HTTP/1.1 introduced persistent connections. Before this,
a client connects to a server and asks for content. The server sends
the content and then closes the connection to signal the end of the
data. In HTTP/1.1, when Connection: close isn't specified, the
server will instead send a Content-Length header indicating the
length of the content in bytes. The connection can then be re-used for
more requests, or, more importantly, pipelining requests.
The main problem is that HTTP headers usually have a different
encoding than the content body. Emacs is not prepared to handle
multiple encodings from a single source, so the only correct way to
talk HTTP with a network process is raw. My mistake was allowing Emacs
to do the UTF-8 conversion, then measuring the length of the content
in its UTF-8 encoding. This just happens to work fine about 99.9% of
the time since clients tend to speak UTF-8, or something like it,
anyway, but it's not correct.
Further Reading
A lot of this investigation was inspired by JavaScript's and other
languages' Unicode shortcomings.
The universe is a two-dimensional grid of 8-connected square cells.
A cell is either dead or alive.
A dead cell with exactly three living neighbors comes to life.
A live cell with less than two neighbors dies from underpopulation.
A live cell with more than three neighbors dies from overpopulation.
These simple cellular automata rules lead to surprisingly complex,
organic patterns. Cells are updated in parallel, so it's generally
implemented using two separate buffers. This makes it a perfect
candidate for an OpenGL fragment shader.
Preparing the Textures
The entire simulation state will be stored in a single, 2D texture in
GPU memory. Each pixel of the texture represents one Life cell. The
texture will have the internal format GL_RGBA. That is, each pixel
will have a red, green, blue, and alpha channel. This texture is not
drawn directly to the screen, so how exactly these channels are used
is mostly unimportant. It's merely a simulation data structure. This
is because I'm using the OpenGL programmable pipeline for general
computation. I'm calling this the "front" texture.
Four multi-bit (actual width is up to the GPU) channels seems
excessive considering that all I really need is a single bit of
state for each cell. However, due to framebuffer completion
rules, in order to draw onto this texture it must be GL_RGBA.
I could pack more than one cell into one texture pixel, but this would
reduce parallelism: the shader will run once per pixel, not once per
cell.
Because cells are updated in parallel, this texture can't be modified
in-place. It would overwrite important state. In order to do any real
work I need a second texture to store the update. This is the "back"
texture. After the update, this back texture will hold the current
simulation state, so the names of the front and back texture are
swapped. The front texture always holds the current state, with the
back texture acting as a workspace.
Here's how a texture is created and prepared. It's wrapped in a
function/method because I'll need two identical textures, making two
separate calls to this function. All of these settings are required
for framebuffer completion (explained later).
A texture wrap of GL_REPEAT means the simulation will be
automatically torus-shaped. The interpolation is
GL_NEAREST, because I don't want to interpolate between cell states
at all. The final OpenGL call initializes the texture size
(this.statesize). This size is different than the size of the
display because, again, this is actually a simulation data structure
for my purposes.
The null at the end would normally be texture data. I don't need to
supply any data at this point, so this is left blank. Normally this
would leave the texture content in an undefined state, but for
security purposes, WebGL will automatically ensure that it's zeroed.
Otherwise there's a chance that sensitive data might leak from another
WebGL instance on another page or, worse, from another process using
OpenGL. I'll make a similar call again later with glTexSubImage2D()
to fill the texture with initial random state.
In OpenGL ES, and therefore WebGL, wrapped (GL_REPEAT) texture
dimensions must be powers of two, i.e. 512x512, 256x1024, etc. Since I
want to exploit the built-in texture wrapping, I've decided to
constrain my simulation state size to powers of two. If I manually did
the wrapping in the fragment shader, I could make the simulation state
any size I wanted.
Framebuffers
A framebuffer is the target of the current glClear(),
glDrawArrays(), or glDrawElements(). The user's display is the
default framebuffer. New framebuffers can be created and used as
drawing targets in place of the default framebuffer. This is how
things are drawn off-screen without effecting the display.
A framebuffer by itself is nothing but an empty frame. It needs a
canvas. Other resources are attached in order to make use of it. For
the simulation I want to draw onto the back buffer, so I attach this
to a framebuffer. If this framebuffer is bound at the time of the draw
call, the output goes onto the texture. This is really powerful
because this texture can be used as an input for another draw
command, which is exactly what I'll be doing later.
Here's what making a single step of the simulation looks like.
First, bind the custom framebuffer as the current framebuffer with
glBindFramebuffer(). This framebuffer was previously created with
glCreateFramebuffer() and required no initial configuration. The
configuration is entirely done here, where the back texture is
attached to the current framebuffer. This replaces any texture that
might currently be attached to this spot -- like the front texture
from the previous iteration. Finally, the size of the drawing area is
locked to the size of the simulation state with glViewport().
Using Igloo again to keep the call concise, a fullscreen quad
is rendered so that the fragment shader runs exactly once for each
cell. That state uniform is the front texture, bound as
GL_TEXTURE0.
With the drawing complete, the buffers are swapped. Since every pixel
was drawn, there's no need to ever use glClear().
The Game of Life Fragment Shader
The simulation rules are coded entirely in the fragment shader. After
initialization, JavaScript's only job is to make the appropriate
glDrawArrays() call over and over. To run different cellular automata,
all I would need to do is modify the fragment shader and generate an
appropriate initial state for it.
The get(int, int) function returns the value of the cell at (x, y),
0 or 1. For the sake of simplicity, the output of the fragment shader
is solid white and black, but just sampling one channel (red) is good
enough to know the state of the cell. I've learned that loops and
arrays are are troublesome in GLSL, so I've manually unrolled the
neighbor check. Cellular automata that have more complex state could
make use of the other channels and perhaps even exploit alpha channel
blending in some special way.
Otherwise, this is just a straightforward encoding of the rules.
Displaying the State
What good is the simulation if the user doesn't see anything? So far
all of the draw calls have been done on a custom framebuffer. Next
I'll render the simulation state to the default framebuffer.
First, bind the default framebuffer as the current buffer. There's no
actual handle for the default framebuffer, so using null sets it to
the default. Next, set the viewport to the size of the display. Then
use the "copy" program to copy the state to the default framebuffer
where the user will see it. One pixel per cell is far too small, so
it will be scaled as a consequence of this.viewsize being four times
larger.
Here's what the "copy" fragment shader looks like. It's so simple
because I'm storing the simulation state in black and white. If the
state was in a different format than the display format, this shader
would need to perform the translation.
Since I'm scaling up by four -- i.e. 16 pixels per cell -- this
fragment shader is run 16 times per simulation cell. Since I used
GL_NEAREST on the texture there's no funny business going on here.
If I had used GL_LINEAR, it would look blurry.
You might notice I'm passing in a scale uniform and using
gl_FragCoord. The gl_FragCoord variable is in window-relative
coordinates, but when I sample a texture I need unit coordinates:
values between 0 and 1. To get this, I divide gl_FragCoord by the
size of the viewport. Alternatively I could pass the coordinates as a
varying from the vertex shader, automatically interpolated between the
quad vertices.
An important thing to notice is that the simulation state never
leaves the GPU. It's updated there and it's drawn there. The CPU is
operating the simulation like the strings on a marionette -- from a
thousand feet up in the air.
User Interaction
What good is a Game of Life simulation if you can't poke at it? If all
of the state is on the GPU, how can I modify it? This is where
glTexSubImage2D() comes in. As its name implies, it's used to set
the values of some portion of a texture. I want to write a poke()
method that uses this OpenGL function to set a single cell.
Bind the front texture, set the region at (x, y) of size 1x1 (a single
pixel) to a very specific RGBA value. There's nothing else to it. If
you click on the simulation in my demo, it will call this poke method.
This method could also be used to initialize the entire simulation
with random values, though it wouldn't be very efficient doing it one
pixel at a time.
Getting the State
What if you wanted to read the simulation state into CPU memory,
perhaps to store for reloading later? So far I can set the state and
step the simulation, but there's been no way to get at the data.
Unfortunately I can't directly access texture data. There's nothing
like the inverse of glTexSubImage2D(). Here are a few options:
Call toDataURL() on the canvas. This would grab the rendering of
the simulation, which would need to be translated back into
simulation state. Sounds messy.
Take a screenshot. Basically the same idea, but even messier.
Use glReadPixels() on a framebuffer. The texture can be attached to
a framebuffer, then read through the framebuffer. This is the right
solution.
I'm reusing the "step" framebuffer for this since it's already
intended for these textures to be its attachments.
Voilà! This rgba array can be passed directly back to
glTexSubImage2D() as a perfect snapshot of the simulation state.
Conclusion
This project turned out to be far simpler than I anticipated, so much
so that I was able to get the simulation running within an evening's
effort. I learned a whole lot more about WebGL in the process, enough
for me to revisit my WebGL liquid simulation. It uses a
similar texture-drawing technique, which I really fumbled through that
first time. I dramatically cleaned it up, making it fast enough to run
smoothly on my mobile devices.
Also, this Game of Life implementation is blazing fast. If rendering
is skipped, it can run a 2048x2048 Game of Life at over 18,000
iterations per second! However, this isn't terribly useful because
it hits its steady state well before that first second has passed.
I just added a formal tags page along with individual feeds
for each tag. I've had tags for a couple of years now, but they were
really only useful for traveling sideways to similar articles. So now,
if you're only interested in a subset of my content, you can subscribe
to one or more tags rather than the main Atom feed.
What prompted this? In my Emacs Chat, Sacha asked me if this
blog was part of Planet Emacsen (currently, it's not). If
my tags are accurate, only about 25% of my articles are about Emacs,
so most of my blog isn't relevant there. Tag feeds will go a long way
to help support these "planet" aggregators, should they want to
include my articles. For example, Planet Emacsen would use my Emacs
feed.
Static Site Generation
I couldn't practically support these extra feeds until recently.
Remember, this blog is statically generated. More feeds means
more content to generate, because articles are duplicated in whole for
each feed. In past years, Jekyll would probably take on the order of
an hour to do all this for a single build. Fortunately, Jekyll has
improved dramatically, especially in the past year or so, and these
feeds have little impact on the total build time. It's currently
around 10 seconds or so. Not bad at all!
A consequence of being statically generated is that you can't ask for
a combination of tags as a single feed. It would be a combinatorial
nightmare (billions of feeds). Plus, the request would have to
normalize the tag order (e.g. alphabetical) or else the combinatorial
explosion to be far worse (i.e. exceeding the number of atoms in the
universe). So I hope you can forgive me when subscribing to each tag
individually.
Duplicate Articles
What if an article matches multiple tags? It will appear in each feed
where it's tagged, possibly showing up multiple times in your web feed
reader. Fortunately, this is where Atom saves the day! I'm leveraging
Atom's prudent design to make this work cleanly. Articles' UUIDs are
consistent across all of these feeds, so if your web feed reader is
smart enough, it will recognize these as being the same article. For
example, this article is f47e5404-cc4a-3cc0-01ce-a844c04721b8 regardless of which feed
you see it in.
Unfortunately, Elfeed isn't smart enough for this. Sorry! In
order to better support all the broken RSS feeds out there, I had to
compromise on entry keying. I couldn't trust RSS feeds to provide
me a reasonably unique key, so, transitively, Elfeed doesn't fully
trust Atom's UUIDs either. These RSS feeds are broken largely because
RSS itself is a broken mess. When making new feeds in the
future, please use Atom!
Atom requires that every feed and article have a proper UUID. It
doesn't matter where you get the feed from. You could subscribe to the
same exact feed at three different URLs (mirrors perhaps) and your
reader could reliably use the UUIDs to avoid duplication. Or, if
you're subscribed to an aggregator like Planet Emacsen, and it
includes content from a feed to which you're also directly subscribed,
your reader client should be able to merge these articles. In
comparison, RSS not only doesn't require UUIDs, it actively
discourages them with its broken guid tag, so merging content from
multiple sources is impossible with RSS.
Anyway, if most of my content doesn't suit you, you can now subscribe
to the subset that does. Aren't Atom feeds cool?
The April 5th, 2014 draft of the ECMA-262 6th Edition
specification -- a.k.a the next major version of
JavaScript/ECMAScript -- contained a subtle, though very significant,
change to the semantics of the for loop (13.6.3.3). Loop variables
are now fresh bindings for each iteration of the loop: a
per-iteration binding. Previously loop variables were established
once for the entire loop, a per-loop binding. The purpose is an
attempt to fix an old gotcha that effects many languages.
If you couldn't already tell, this is going to be another language
lawyer post!
Backup to C
To try to explain what this all means this in plain English, let's
step back a moment and discuss what a for loop really is. I can't
find a source for this, but I'm pretty confident the three-part for
loop originated in K&R C.
for (INITIALIZATION; CONDITION; ITERATION) {
BODY;
}
Evaluate INITIALIZATION.
Evaluate CONDITION. If zero (false), exit the for.
Evaluate BODY.
Evaluate ITERATION and go to 2.
In the original C, and all the way up to C89, no variable declarations
were allowed in the initialization expression. I can understand why:
there's a subtle complication, though it's harmless in C. We'll get to
that soon. Here's a typical C89 for loop.
The variable i is established independent of the loop, in the scope
outside the for loop, alongside count. This isn't even a per-loop
binding. As far as the language is concerned, it's just a variable
that the loop happens to access and mutate. It's very
assembly-language-like. Because C has block scoping, the body of the
for loop is another nested scope. The variable foo is in this scope,
reestablished on each iteration of the loop (per-iteration).
As an implementation detail, foo will reside at the same location
on the stack each time around the loop. If it's accessed
before being initialized, it will probably hold the value from the
previous iteration, but, as far as the language is concerned, this is
just a happy, though undefined, coincidence.
C99 Loops
Fast forward to the end of the 20th century. At this point, other
languages have allowed variable declarations in the initialization
part for years, so it's time for C to catch up with C99.
Now consider this: in what scope is the variablei? The outer
scope as before? The iteration scope with foo? The answer is
neither. In order to make this work, a whole new loop scope is
established in between: a per-loop binding. This scope holds for the
entire duration of the loop.
The variable i is constrained to the for loop without being
limited to the iteration scope. This is important because i is what
keeps track of the loop's progress. The semantic equivalent in C89
makes the additional scope explicit with a block.
This, ladies and gentlemen, is the the C-style 3-part for loop.
Every language that has this statement, and has block scope, follows
these semantics. This included JavaScript up until two months ago,
where the draft now gives it its own unique behavior.
JavaScript's Let
As it exists today in its practical form, little of the above is
relevant to JavaScript. JavaScript has no block scope, just function
scope. A three-part for-loop doesn't establish all these scopes,
because scopes like these are absent from the language.
An important change coming with 6th edition is the introduction of
let declarations. Variables declared with let will have block
scope.
If these variables had been declared with var, the last two lines
wouldn't be errors (or worse, global references). count, i, and
foo would all be in the same function-level scope. This is really
great! I look forward to using let exclusively someday.
The Closure Trap
I mentioned a subtle complication. Most of the time programmers don't
need to consider or even be aware of this middle scope. However, when
combined with closures it suddenly becomes an issue. Here's an example
with Perl,
In all three cases, one closure is created per iteration. Each closure
captures the loop variable i. It's easy to make the mistake of
thinking each closure will return a unique value. However, as pointed
out above, this is a per-loop variable, existing in a middle scope.
The closures all capture the same variable, merely bound to different
values at the time of capture. The solution is to establish a new
variable in the iteration scope and capture that instead. Below, I've
established a $value variable for this.
This is something that newbies easily get tripped up on. Because
they're still trying to wrap their heads around the closure concept,
this looks like some crazy bug in the interpreter/compiler. I can
understand why the ECMA-262 draft was changed to accommodate this
situation.
The JavaScript Workaround
The language in the new draft has two items called
perIterationBindings and CreatePerIterationEnvironment (in case
you're searching for the relevant part of the spec). Like the $value
example above, for loops in JavaScript with "lexical" (i.e. let)
loop bindings will implicitly mask the loop variable with a variable
of the same name in the iteration scope.
letclosures=[];for(leti=0;i<2;i++){closures.push(function(){returni;});}/* Before the change: */closures[0]();// => 2closures[1]();// => 2/* After the change: */closures[0]();// => 0closures[1]();// => 1
Note: If you try to run this yourself, note that at the time of this
writing, the only JavaScript implementation I could find that updated
to the latest draft was Traceur. You'll probably see the
"before" behavior for now.
You can't see it (I said it's implicit!), but under an updated
JavaScript implementation there are actually two i variables here.
The closures capture the most inner i, the per-iteration version of
i. Let's go back to the original example, JavaScript-style.
Here's what the scope looks like for the latest draft. Notice the
second i in the iteration scope. The inner i is initially assigned
to the value of the outer i.
We could emulate this in an older edition. Imagine writing a macro to
do this.
letcount=10;// ...for(leti=0;i<count;i++){let__i=i;// (possible name collision){leti=__i;letfoo;// ...}}
I have to use __i to smuggle the value across scopes without having
i reference itself. Unlike Lisp's let, the assignment value for
var and let is evaluated in the nested scope, not the outer scope.
Each iteration gets its own i. But what happens when the loop
modifies i? Simple, it's copied back out at the end of the body.
Now all the expected for semantics work -- the body can also update
the loop variable -- but we still get the closure-friendly
per-iteration variables.
Conclusion
I'm still not sure if I really like this change. It's clean fix, but
the gotcha hasn't been eliminated. Instead it's been inverted.
Sometime someone will have the unusual circumstance of wanting to
capture the loop variable, and he will run into some surprising
behavior. Because the semantics are a lot more complicated, it's hard
to reason about what's not working unless you already know JavaScript
has magical for loops.
Perl and C# each also gained per-iteration bindings in their history,
but rather than complicate or change their standard for loops, they
instead introduced it as a new syntactic construction: foreach.
In this case, per-iteration bindings definitely make sense. The
variable $i is established and bound to each value in turn. As far
as control flow goes, it's very functional. The binding is never
actually mutated.
I think it could be argued that Python and Ruby's for ... in forms
should behave like this foreach. These were probably misdesigned
early on, but it's not possible to change their semantics at this
point. Because JavaScript's var was improperly designed from the
beginning, let offers the opportunity to fix more than just var.
We're seeing this right now with these new for semantics.
My previously mentioned Emacs Chat with Sacha
Chua went well and the recording is available. At my request,
Sacha agreed to put these recordings in the public domain, so they're
completely free for any purpose with no strings attached.
I recently got an itch to play around with Voronoi diagrams.
It's a diagram that divides a space into regions composed of points
closest to one of a set of seed points. There are a couple of
algorithms for computing a Voronoi diagram: Bowyer-Watson and Fortune.
These are complicated and difficult to implement.
However, if we're interested only in rendering a Voronoi diagram as
a bitmap, there's a trivial brute for algorithm. For every pixel of
output, determine the closest seed vertex and color that pixel
appropriately. It's slow, especially as the number of seed vertices
goes up, but it works perfectly and it's dead simple!
Does this strategy seem familiar? It sure sounds a lot like an OpenGL
fragment shader! With a shader, I can push the workload off to the
GPU, which is intended for this sort of work. Here's basically what it
looks like.
If you have a WebGL-enabled browser, you can see the results for
yourself here. Now, as I'll explain below, what you see here isn't
really this shader, but the result looks identical. There are two
different WebGL implementations included, but only the smarter one is
active. (There's also a really slow HTML5 canvas fallback.)
You can click and drag points around the diagram with your mouse. You
can add and remove points with left and right clicks. And if you press
the "a" key, the seed points will go for a random walk, animating the
whole diagram. Here's a (HTML5) video showing it off.
Unfortunately, there are some serious problems with this approach. It
has to do with passing seed information as uniforms.
The number of seed vertices is hardcoded. The shader language
requires uniform arrays to have known lengths at compile-time. If I
want to increase the number of seed vertices, I need to generate,
compile, and link a new shader to replace it. My implementation
actually does this. The number is replaced with a %%MAX%%
template that I fill in using a regular expression before sending
the program off to the GPU.
The number of available uniform bindings is very constrained,
even on high-end GPUs: GL_MAX_FRAGMENT_UNIFORM_VECTORS. This
value is allowed to be as small as 16! A typical value on high-end
graphics cards is a mere 221. Each array element counts as a
binding, so our shader may be limited to as few as 8 seed vertices.
Even on nice GPUs, we're absolutely limited to 110 seed vertices.
An alternative approach might be passing seed and color information
as a texture, but I didn't try this.
There's no way to bail out of the loop early, at least with
OpenGL ES 2.0 (WebGL) shaders. We can't break or do any sort of
branching on the loop variable. Even if we only have 4 seed
vertices, we still have to compare against the full count. The GPU
has plenty of time available, so this wouldn't be a big issue,
except that we need to skip over the "unused" seeds somehow. They
need to be given unreasonable position values. Infinity would be an
unreasonable value (infinitely far away), but GLSL floats aren't
guaranteed to be able to represent infinity. We can't even know
what the maximum floating-point value might be. If we pick
something too large, we get an overflow garbage value, such as 0
(!!!) in my experiments.
Because of these limitations, this is not a very good way of going
about computing Voronoi diagrams on a GPU. Fortunately there's a
much much better approach!
A Smarter Approach
With the above implemented, I was playing around with the fragment
shader, going beyond solid colors. For example, I changed the
shade/color based on distance from the seed vertex. A results of this
was this "blood cell" image, a difference of a couple lines in the
shader.
That's when it hit me! Render each seed as cone pointed towards the
camera in an orthographic projection, coloring each cone according to
the seed's color. The Voronoi diagram would work itself out
automatically in the depth buffer. That is, rather than do all this
distance comparison in the shader, let OpenGL do its normal job of
figuring out the scene geometry.
Here's a video (GIF) I made that demonstrates what I mean.
Not only is this much faster, it's also far simpler! Rather than being
limited to a hundred or so seed vertices, this version could literally
do millions of them, limited only by the available memory for
attribute buffers.
The Resolution Catch
There's a catch, though. There's no way to perfectly represent a cone
in OpenGL. (And if there was, we'd be back at the brute force approach
as above anyway.) The cone must be built out of primitive triangles,
sort of like pizza slices, using GL_TRIANGLE_FAN mode. Here's a cone
made of 16 triangles.
Unlike the previous brute force approach, this is an approximation
of the Voronoi diagram. The more triangles, the better the
approximation, converging on the precision of the initial brute force
approach. I found that for this project, about 64 triangles was
indistinguishable from brute force.
Instancing to the Rescue
At this point things are looking pretty good. On my desktop, I can
maintain 60 frames-per-second for up to about 500 seed vertices moving
around randomly ("a"). After this, it becomes draw-bound because
each seed vertex requires a separate glDrawArrays() call to OpenGL.
The workaround for this is an OpenGL extension called instancing. The
WebGL extension for instancing is ANGLE_instanced_arrays.
The cone model was already sent to the GPU during initialization, so,
without instancing, the draw loop only has to bind the uniforms and
call draw for each seed. This code uses my Igloo WebGL
library to simplify the API.
Instancing works by adjusting how attributes are stepped. Normally the
vertex shader runs once per element, but instead we can ask that some
attributes step once per instance, or even once per multiple
instances. Uniforms are then converted to vertex attribs and the
"loop" runs implicitly on the GPU. The instanced glDrawArrays() call
takes one additional argument: the number of instances to draw.
ext=gl.getExtension("ANGLE_instanced_arrays");// only onceprograms.cone.use().attrib('cone',buffers.cone,3).attrib('position',buffers.positions,2).attrib('color',buffers.colors,3);/* Tell OpenGL these iterate once (1) per instance. */ext.vertexAttribDivisorANGLE(cone.vars['position'],1);ext.vertexAttribDivisorANGLE(cone.vars['color'],1);ext.drawArraysInstancedANGLE(gl.TRIANGLE_FAN,0,66,seeds.length);
The ugly ANGLE names are because this is an extension, not part of
WebGL itself. As such, my program will fall back to use multiple draw
calls when the extension is not available. It's only there for a speed
boost.
Here are the new shaders. Notice the uniforms are gone.
On the same machine, the instancing version can do a few thousand seed
vertices (an order of magnitude more) at 60 frames-per-second, after
which it becomes bandwidth saturated. This is because, for the
animation, every vertex position is updated on the GPU on each frame.
At this point it's overcrowded anyway, so there's no need to support
more.
Emacs Lisp strings are mutable, fixed-length character (multibyte) or
byte (unibyte) arrays. Any operation that would change its length
requires allocating a new string object. This is common in many
programming languages' strings. Python, Java, and JavaScript go even
further, with strings being completely immutable.
In these languages, performing many string operations at a time,
especially with the += operator, allocates many temporary strings.
It's also awkward. For these situations, Java provides a class,
StringBuilder, so that these operations can be done with a
temporary, efficient, mutable data structure that will emit the final
string when complete.
In JavaScript a popular string building idiom is to use an array. Push
the components onto an array and join() the result.
functiontoString(object){varoutput=[];for(varkinobject){output.push(k);output.push(' -> ');output.push(object[k]);output.push('\n');}returnoutput.join('');}toString({a:1,b:2});// => "a -> 1\nb -> 2\n"
Emacs Lisp
What character sequence data structure already exists in Elisp that's
efficient at insert, update, and delete? Buffers, of course! I know
it's easy to forget, but editing sequences of characters is the
primary purpose of Emacs, after all. To make use of a buffer as a
string builder, use one of my favorite macros: with-temp-buffer. I
like to combine this with setting standard-output so that all of the
printing functions go there.
Update: Jon O. pointed out that Emacs has a with-output-to-string
macro available to do this more concisely.
Internally Elisp buffers are gap buffers, a rather simple data
structure where the data is split into two sequences with a "gap" in
between. Insertion and deletion occurs at the gap, which is slid up
and down the overall sequence. This makes gap buffers efficient for
making lots of edits localized in a single area, just as a human would
do while editing text.
Each character in a buffer is a full Unicode code point and can have
an arbitrary set of properties associated with it (font-lock-face,
read-only, nonstickiness, etc.). Along with inline image objects, this
makes buffers rich enough to display rendered HTML (to a limited
extent).
The Catch
There's an important caveat to using buffers as mutable strings:
they're not managed by the garbage collector. Each buffer
goes into the global buffer list, implemented internally as an
intrusive linked list. If a buffer is not on this list, it's a dead
buffer.
Ultimately this makes buffer objects poor return values. It's an
impedance mismatch. The caller has to be careful to free ("kill") the
buffer. It's easy to miss if an error is signaled. For example,
url-retrieve and url-retrieve-synchronously return a buffer with
the response from a web server. It's not uncommon for Elisp programs
to leak these buffers during normal operation.
As a side note: alternatively you could use my finalize
package to associate the buffer with an object that
is subject to garbage collection. The buffer will be killed
immediately when the object is garbage collected.
Buffer Passing Style
To deal with this, my preferred idiom is what I call buffer-passing
style. Rather than have the callee instantiate the buffer, the caller
instantiates the buffer and "passes" it implicitly as the current
buffer. The callee fills it with something. The caller should use
something like with-temp-buffer so that the buffer has a clean
life-cycle, fully managed by the caller.
Imagine instead of returning a buffer, url-retrieve-synchronously
puts the result in the current buffer instead of returning a buffer.
If anything goes wrong, the buffer will be automatically killed by
with-temp-buffer.
Buffer-passing style is what I settled on for simple-httpd.
Servlets are called with the output buffer as the current buffer )and
with standard-output set to this buffer). The servlet is only
responsible for filling this buffer with content. Thanks to
process-send-region, the content is never actually copied into a
string.
