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Color adjustment: white point.
CG works are almost painted on true white canvas, hence white point must equal value 255. but the scans are not equal.
one reason is paper color. see forum #3137.
another reason is scanning/printing artifacts. due to that, even white borders aren't have same value. in this case, there are 3 ways to define white point.
1) take lowest value on white area. this will erase all noises on the white without filtering, but the result image is partly destroyed, and its contrast must get too high.
2) take midst value. since filtering mixes colors, it might be mostly used way. sometimes the result is bit high contrasted, so it might need curve adjustment.
3) take highest value. this will preserve original tone, but it requires lighten only filtering on the white area.

on the other hand, adjusting black point is more difficult. the darkest points of many original CG works are not value 0, and it has tough screening, and more, adjusting RGB in the mid tone also modifies RGB in the dark tone.