I have a gray scale uint8 image. When I smooth it with a Gaussian filter and then compute the gradient image with a Sobel filter in OpenCV, ripples would appear.
The first image is what happens when I use a 25x25 Gaussian filter with sigma value 4 and compute the second order derivative in the x direction with a size 3 Sobel filter.
The more I smooth the image, the more significant the ripples become.
The second image is what I got when I use a 49x49 Gaussian filter with sigma value 7. The Sobel filter is the same as the first image.
Why is the Gaussian filter creating more noise when it's suppose to get rid of them? How should I deal with this?