08-04-2019, 09:07 AM
If you mean the things that look like contour lines, this is due to color quantization (only 256 values in an 8-bit channel). So you can have uniform area between which the value jumps by more that a couple of units and your eyes are very sensitive to this (in other words it a bit like an optical illusion). This can be checked wit the Histogram dialog: in the image below, the histogram corresponds to the little selected strip at the top (that is drew perpendicular to the "contour lines"). Visually, your eyes mostly see the jumps between the spikes.
If you need to remain in 8-bit you can mitigate the problem using "Spread noise" (`Filters>Noise>Spread). Despite the name th is more like a blur, it swaps pixels with neighboring pixels, so the value jumps are spread over a wider area and are less visible.
If you need to remain in 8-bit you can mitigate the problem using "Spread noise" (`Filters>Noise>Spread). Despite the name th is more like a blur, it swaps pixels with neighboring pixels, so the value jumps are spread over a wider area and are less visible.