While reading specification at Khronos, I found:
bufferData(ulong target, Object data, ulong usage)
\'usage\' parameter can be: STREAM_DRA
For 'desktop' OpenGL, there is a good explanation here:
http://www.opengl.org/wiki/Buffer_Object
Basically, usage parameter is a hint to OpenGL/WebGL on how you intend to use the buffer. The OpenGL/WebGL can then optimize the buffer depending on your hint.
The OpenGL ES docs writes the following, which is not exactly the same as for OpenGL (remember that WebGL is inherited from OpenGL ES):
STREAM
STATIC
DYNAMIC
The nature of access must be:
DRAW
The most common usage is STATIC_DRAW (for static geometry), but I have recently created a small particle system where DYNAMIC_DRAW makes more sense (the particles are stored in a single buffer, where parts of the buffer is updated when particles are emitted).
http://jsfiddle.net/mortennobel/YHMQZ/
Code snippet:
function createVertexBufferObject(){
particleBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, particleBuffer);
var vertices = new Float32Array(vertexBufferSize * particleSize);
gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.DYNAMIC_DRAW);
bindAttributes();
}
function emitParticle(x,y,velocityX, velocityY){
gl.bindBuffer(gl.ARRAY_BUFFER, particleBuffer);
// ...
gl.bufferSubData(gl.ARRAY_BUFFER, particleId*particleSize*sizeOfFloat, data);
particleId = (particleId +1 )%vertexBufferSize;
}