Eraser/rubber for Android Canvas

Can any one tell me how to erase the color done on a canvas..i.e., I hv a colorful image on canvas that has been made B/W by drawing white color all over the canvas now 开发者_Go百科it appeared to be a sketched image only which can be painted similarly as other B/W image. Now the question is that My application has one option to fill the image with its original color with stroke of mouse(finger) only, According to me if i can erase the white color(or any of its color done above the original image) then i 'll achieve the required option.. Any help... Please suggest me any other way to achieve this also, if there is.....

Thanks,,,

It would be possible by creating a photo-shop functionality named Masking(or alpha masking or image masking) in android using porterduff and xfermode.