Smoothed image produces contour-like ripples, yet it disappears in saved image? I smoothed a grayscale image using a size 10 gaussian filter and display it in Octave (imshow) which resulted in ripples which appears to be contours for image intensity.



Yet if I save the image (imwrite), I get a proper smoothed image.(The ripple image above was taken as a screenshot.)
My guess is that imshow, for some reason, has a lower color depth at which it displays greyscale images than your image viewer.

The contours are actually just the quantization steps of the lower number of shades displayable.