How to strip background color when using html5 canvas image?

If canvas can strip background color开发者_C百科 from a external image which is loaded(drawimage) into this canvas?

I think canvas can manipulate pixels, so it should strip the image background color. After some searching, I still can't find a answer or any idea for it.

in a demo I wrote (personal spam ;) there is something you could need:

the full source is here

• first it reads an image

  brush.load({
  imageUrl: "brush-2.png",
  onload: function (brush) { 
      setBG(brush);
  }
  

  });

• then it manipulates the pixels

  for(p = 0; p < l; p = p + 4){
    r = pixels.data[p + 0];
    g = pixels.data[p + 1];
    b = pixels.data[p + 2];
    a = pixels.data[p + 3];
  
    pixels.data[p + 0] = k;
    pixels.data[p + 1] = m;
    pixels.data[p + 2] = o;
    pixels.data[p + 3] = a;
  

  }

• then it sets the image from the imageData

  document.body.style.backgroundImage= "url(" + ctx.canvas.toDataURL() + ")";
  

more here