Android :: How To Compare Two Simple Images?

I'll get straight to the point. Please note that I haven't code anything in this project just yet. This is only an idea. In human language, my goal is to: 1. Take a picture of a blank white A4 paper. 2. Verify (by code) that the picture is "White". (this's why I include 'simple' in the topic) I mean, the program should be able to tell if the user just took the picture of "white" paper and not any other colors. For the code... I can guess that the actual process should be: 1. Use Camera API to take a picture. // this step should be easy 2. Convert it to Bitmap. // this step should also be easy 3. Verify that it is "White". //*** now I need some help here. How can I do step 3? I look up the docs and found this is the Bitmap class: getPixels (int[] pixels, int offset, int stride, int x, int y, int width, int height) I can use this and compare the data pixel-by-pixel but it seems expensive. Could anyone recommend me a more proper way(if there is) to do this please?