Finding number of quantizing layers in MATLAB

I'm working on image processing, and I have an image that has been DCT'd and quantized开发者_StackOverflow中文版 for 8 x 8 blocks of the 512 x 512 matrix, now I have to find how many quantizing levels that the image has. Do I need to take the top left pixel and place it in to an array and then place this on a graph calling hist?

length(unique(x(:))), where x is your image array. This is appropriate for grayscale images.