knusperli

If you want to remove banding from jpeg images, check out https://github.com/google/knusperli. When jpeg quantizes coefficients, there is an entire interval that quantizes to the same number. A traditional decoder uses the quantized value, the center of the interval. When there's is a subtle gradient, the low-order coefficients quantize to zero, and you get banding because the gradient disappears. Knusperli does not use the center of the interval, it picks the value in the interval that makes the block boundaries the least discontinous. This means that it can recover subtle gradients.