Accessing image/texture data (texels) on WebGL

I have the following code snippet on WebGL:

var texture = gl.createTexture();
texture.image = new Image();
texture.image.onload = function() { 
     .... // I want to read the pixels once the image has been loaded
};
texture.image.src = urlImage;

I want to obtain the contents (RGBA) of the texture ma开发者_运维百科p once it has been loaded. Similar, capability of readPixels() to obtain the contents of the drawing buffer.

Is it possible? If so, what's the best to do it?

I am using Chrome Canary build for my development.

Thanks in advance for your help.

Note: Cross post on http://www.khronos.org/message_boards/viewtopic.php?f=43&t=3439

You can create a framebuffer backed by the texture and then use readPixels() to get the raw RGBA data.

var texture = gl.createTexture();
texture.image = new Image();
texture.image.onload = function() { 

  // Push Texture data to GPU memory
  gl.bindTexture(gl.TEXTURE_2D, texture);
  gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, true);
  gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, this);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);

  // Create a framebuffer backed by the texture
  var framebuffer = gl.createFramebuffer();
  gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);
  gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, texture, 0);

  // Read the contents of the framebuffer (data stores the pixel data)
  var data = new Uint8Array(this.width * this.height * 4);
  gl.readPixels(0, 0, this.width, this.height, gl.RGBA, gl.UNSIGNED_BYTE, data);

  gl.deleteFramebuffer(framebuffer);
};
texture.image.src = urlImage;

I use HTML5 to read the texels with the following code snippets:

var myCanvas = document.createElement("canvas");
myCanvas.width = texture.image.width; 
myCanvas.height = texture.image.height;
var myCanvasContext = myCanvas.getContext("2d"); // Get canvas 2d context
myCanvasContext.drawImage(texture.image, 0, 0); // Draw the texture
var texels = myCanvasContext.getImageData(0,0, width, height); // Read the texels/pixels back

It does what I want to do. Please let me know if there is a better way