webgl - What are Vertex Arrays in OpenGL & WebGL2?

Question

I've been working with WebGL1 for some time but now that I'm learning more about WebGL2, I'm confused what Vertex Arrays actually do. For example, in the following example, I can remove all references to them (e.g. creation, binding, deletion) and the example continues to work.

Answer 1

This has been explained elsewhere but you can consider both WebGL1 and WebGL2 to have a vertex array. It's just WebGL1 by default only has one where as WebGL2 you can create multiple vertex arrays (although 99.9% of all WebGL1 implementations support them as an extension)

A Vertex Array is the collection of all attribute state plus the ELEMENT_ARRAY_BUFFER binding.

You can think of WebGL state like this

class WebGLRenderingContext {
  constructor() {
   // internal WebGL state
   this.lastError: gl.NONE,
   this.arrayBuffer = null;
   this.vertexArray = {
     elementArrayBuffer: null,
     attributes: [
       { enabled: false, type: gl.FLOAT, size: 3, normalized: false, 
         stride: 0, offset: 0, value: [0, 0, 0, 1], buffer: null },
       { enabled: false, type: gl.FLOAT, size: 3, normalized: false, 
         stride: 0, offset: 0, value: [0, 0, 0, 1], buffer: null },
       { enabled: false, type: gl.FLOAT, size: 3, normalized: false, 
         stride: 0, offset: 0, value: [0, 0, 0, 1], buffer: null },
       { enabled: false, type: gl.FLOAT, size: 3, normalized: false, 
         stride: 0, offset: 0, value: [0, 0, 0, 1], buffer: null },
       { enabled: false, type: gl.FLOAT, size: 3, normalized: false, 
         stride: 0, offset: 0, value: [0, 0, 0, 1], buffer: null },
       ...
     ],
   }
   ...

And you can think of gl.bindBuffer as implemented like this

   // Implementation of gl.bindBuffer. 
   // note this function is doing nothing but setting 2 internal variables.
   this.bindBuffer = function(bindPoint, buffer) {
     switch(bindPoint) {
       case gl.ARRAY_BUFFER;
         this.arrayBuffer = buffer;
         break;
       case gl.ELEMENT_ARRAY_BUFFER;
         this.vertexArray.elementArrayBuffer = buffer;
         break;
       default:
         this.lastError = gl.INVALID_ENUM;
         break;
     }
   };

So you can see above, calling gl.bindBuffer with gl.ELEMENT_ARRAY_BUFFER sets the elementArray part of the current vertexArray

You can also see vertexArray has a number of attributes. They define how to pull data out of the buffers to supply to your vertex shader. Calling gl.getAttribLocation(someProgram, "nameOfAttribute") tells you which attribute the vertex shader will look at to get data out of a buffer.

There's 4 functions that you use to configure how an attribute will get data from a buffer. gl.enableVertexAttribArray, gl.disableVertexAttribArray, gl.vertexAttribPointer, and gl.vertexAttrib??.

They're effectively implemented something like this

this.enableVertexAttribArray = function(location) {
  const attribute = this.vertexArray.attributes[location];
  attribute.enabled = true;  // true means get data from attribute.buffer 
};

this.disableVertexAttribArray = function(location) {
  const attribute = this.vertexArray.attributes[location];
  attribute.enabled = false; // false means get data from attribute.value
};

this.vertexAttribPointer = function(location, size, type, normalized, stride, offset) {
  const attribute = this.vertexArray.attributes[location];
  attribute.size       = size;       // num values to pull from buffer per vertex shader iteration
  attribute.type       = type;       // type of values to pull from buffer
  attribute.normalized = normalized; // whether or not to normalize
  attribute.stride     = stride;     // number of bytes to advance for each iteration of the vertex shader. 0 = compute from type, size
  attribute.offset     = offset;     // where to start in buffer.

  // IMPORTANT!!! Associates whatever buffer is currently *bound* to 
  // "arrayBuffer" to this attribute
  attribute.buffer     = this.arrayBuffer;
};

this.vertexAttrib4f = function(location, x, y, z, w) {
  const attribute = this.vertexArray.attributes[location];
  attribute.value[0] = x;
  attribute.value[1] = y;
  attribute.value[2] = z;
  attribute.value[3] = w;
};

Now, when you call gl.drawArrays or gl.drawElements the system knows how you want to pull data out of the buffers you made to supply your vertex shader. See here for how that works.

There's then 3 functions that will manage all state connected to this.vertexArray. They are gl.createVertexArray, gl.bindVertexArray and gl.deleteVertexArray. In WebGL1 they are available on the OES_vertex_array_object extension slightly renamed. On WebGL2 they are just available by default which is also a feature of WebGL 2.0.

Calling gl.createVertexArray makes new vertex array. Calling gl.bindVertexArray sets this.vertexArray to point to the one you pass in. You can imagine it implemented like this

 this.bindVertexArray = function(vao) {
   this.vertexArray = vao ? vao : defaultVertexArray;
 }    

The benefit should be obvious. Before each thing you want to draw you need to set all the attributes. Setting each attribute requires a minimum of one call per used attribute. More commonly 3 calls per attribute. One call to gl.bindBuffer to bind a buffer to ARRAY_BUFFER and one call to gl.vertexAttribPointer to then bind that buffer to a specific attribute and set how to pull data out and one call to gl.enableVertexAttribArray to turn on getting data from a buffer for the attribute.

For a typical model with positions, normals, and texture coordinates that's 9 calls, +1 more if you're using indices and need to bind a buffer to ELEMENT_ARRAY_BUFFER.

With vertex arrays all those calls happen at init time. You make a vertex array for each thing you want to draw then setup the attributes for that thing. At draw time it then only takes one call to gl.bindVertexArray to setup all the attributes and the ELEMENT_ARRAY_BUFFER.

If you'd like to just always use vertex arrays you can use this polyfill in WebGL1. It uses the built in one if the extension exists or else emulates it. Of course emulation is slower but any GPU that would need the emulation is probably already too slow.

note if you're looking for samples maybe compare the corresponding examples on https://webglfundamentals.org to https://webgl2fundamentals.org. The WebGL2 site uses vertex arrays everywhere. You'll notice in the WebGL1 examples just before drawing, for each piece of vertex data the buffer for that data is bound and then the attribute for that data is setup. In the WebGL2 examples that happens at init time instead of draw time. At draw time all that happens is calling gl.bindVertexArray

One extra thing about vertex arrays to be aware of is that they generally require more organization. If you're going to draw the same object more than once with different shader programs then it's possible one shader program will use different attributes for the same data. In other words, with no extra organization shaderprogram1 might use attribute 3 for position where as shaderprogram2 might attribute 2 for position. In that case the same vertex array would not work with both programs for the same data.

The solution is to manually assign the locations. You can do this in the shaders themselves in WebGL2. You can also do it by calling gl.bindAttribLocation before linking the shaders for each shaderprogram in both WebGL1 and WebGL2. I tend to think using gl.bindAttribLocation is better than doing it in GLSL because it's more D.R.Y.