如何使用javascript在页面中获取图像的RGB值?

I am trying to make a bookmark-let. But I'm stuck at a point. I need to extract the RGB or the color value of an image of very x pixel using the "For Next" statement. But i don't know what function can help me to extract the RGB value. Can someone please help me out with the function that tells the value of RCG of a given point in a picture?I am trying to make a bookmark-let. But I'm stu