art with code

2012-01-27

Animating a million letters with WebGL





Here's an WebGL animated book demo! It's got just 150000 letters, but it does scale up to two million.


Writing efficient WebGL is a bit funny. The basic idea is to collect as many operations as possible into a single draw call, as changing the WebGL state machine state and doing WebGL calls is relatively expensive. If you want to draw more than a couple thousand objects at once, you need to adopt a quite different strategy for drawing.

The usual way of drawing with WebGL is to set up your uniforms, buffers and shaders for each object, followed by a call to draw the object. Unless your object is very complex, the time taken in this way of drawing is dominated by the state setup. To draw in a fast way, you can either do some buffer editing in JavaScript, followed by re-uploading the buffer and the draw call. If you need to go even faster, you can push more computation to the shaders.

My goal in this article is to draw a million animated letters on the screen at a smooth framerate. This task should be quite possible with modern GPUs. Each letter consists of two textured triangles, so we're only talking about two million triangles per frame.

Ok, let's start. First I'm going to create a texture with the letter bitmaps on it. I'm using the 2D canvas for this. The resulting texture has all the letters I want to draw. Then I'm going to create a buffer with texture coordinates to the letter sprite sheet. While this is an easy and straightforward method of setting up the letters, it’s a bit wasteful as it uses two floats per vertex for the texcoords. A shorter way would be to pack the letter index and corner index into one number and convert that back to texture coordinates in the vertex shader.

I also upload a two-million triangle array to the GPU. These vertices are used by the vertex shader to put the letters on the screen. The vertices are set to the letter positions in the text so that if you render the triangle array as-is, you get a basic layout rendering of the text.

With a simple vertex shader, I get a flat view of the text. Nothing fancy. Runs well, but if I want to animate it, I need to do the animation in Javascript. And JavaScript is kinda slow for animating the six million vertices involved, especially if you want to do it on every frame. Maybe there is there a faster way.

Why yes, we can do procedural animation. What that means is that we do all our position and rotation math in the vertex shader. Now I don't need to run any JavaScript to update the positions of the vertices. The vertex shader runs very fast and I get a smooth framerate even with a million triangles being individually animated every frame. To address the individual triangles, I round down the vertex coordinates so that all four points of a letter quad map to a single unique coordinate. Now I can use this coordinate to set the animation parameters for the letter in question.

The only problem now is that JavaScript doesn’t know about the particle positions. If you really need to know where your particles are, you could duplicate the vertex shader logic in JavaScript and update them in, say, a web worker every time you need the positions. That way your rendering thread doesn’t have to wait for the math and you can continue animating at a smooth frame rate.

For more controllable animation, we could use render-to-texture functionality to tween between the JavaScript-provided positions and the current positions. First we render the current positions to the texture, then tween from the JS array towards these positions, updating the texture on each frame. The nice thing about this is that we can update a small fraction of the JS positions per frame and still continue animating all the letters every frame. The vertex shader is tweening the positions.
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Built art installations, web sites, graphics libraries, web browsers, mobile apps, desktop apps, media player themes, many nutty prototypes, much bad code, much bad art.

Have freelanced for Verizon, Google, Mozilla, Warner Bros, Sony Pictures, Yahoo!, Microsoft, Valve Software, TDK Electronics.

Ex-Chrome Developer Relations.