@jeffcliff Now, if you simply apply the sobel filter, you will see the edges - but you will end up with spikes in the histogram. When this happens, the filtered image looks smooth when it is not, as the small differences in pixel values are meaningful. You need to do something to amplify those small differences so that you can actually see the noise. Equalizing the histograms or normalizing the image are both good ways to do this. This is why I :keku: at you for clobbering the noise earlier, when you should have amplified it to see what you were looking for.

Moreover, you should also take the sobel filters in both the X and Y directions, and view them separately.

Note that there is a good argument here for taking a Laplacian filter instead. I have not investigated this myself, but I suspect it to be a good idea - although what you will see will look different, and will require a new way of explaining it and a new heuristic.

Moreover, we only do this for the Luminance, because it is where the bulk of the interpretable image data is.