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to Illustrate this point take a look at the hiestogram of a god scan. It should have a solid distribution of data over the whole range, y, all 256 channels are utilised. Now do some adjustments with levels or curves and look at the hiestogram again - we now se gaps. Depending on the amount of adjustments, it can look like a comb. This means that the data have ben compressed into less than 256 discreete shades of RGB. In severe cases we get posterisation, visible jumps in tonal graduation (not todo be confused with pixelisation).
Ballo de inglés ni fú ni fa, que tienes que toquetear los niveles y el avatar de i, k, digo, las curvas.