TextureState

I hope this pseudo code sample will help make it clearer how activeTexture, bindTexture, and the various texture functions in WebGL effect the state of textures.

Imagine textures are defined like this

    Texture2D = function() {
      this.levels = [];
      this.state = {};
    };

And the WebGLRenderingContext is defined like this

    WebGLRenderingContext = function() {
      // activeTexture is just an index into textureUnits
      this.activeTexture = 0;
  
      // these are the textureUnits for this context.
      this.textureUnits = [
        { TEXTURE_2D: default2DTexture, TEXTURE_CUBE_MAP: defaultCubeMapTexture, },
        { TEXTURE_2D: default2DTexture, TEXTURE_CUBE_MAP: defaultCubeMapTexture, },
        { TEXTURE_2D: default2DTexture, TEXTURE_CUBE_MAP: defaultCubeMapTexture, },
        ...
      ];
    };

Then activeTexture would be implemented like this

    WebGLRenderingContext.prototype.activeTexture = function(texture) {
      this.activeTexture = texture - this.TEXTURE0;
    };

bindTexture would be implemented like this

    WebGLRenderingContext.prototype.bindTexture = function(target, texture) {
      this.textureUnits[this.activeTexture][target] = texture;
    };

And texture functions like texParameter or texImage2D would be implemented like this

    WebGLRenderingContext.prototype.texParameteri = function(target, pname, value) {
      var texture = this.textureUnits[this.activeTexture][target];
      texture.state[pname] = value;
    };