10-02-2020, 07:29 PM
(10-02-2020, 04:07 PM)nodal Wrote: I do have a semi-answer on clamp input from an experiment, which I will post when I can work out the words as I can see what it's doing and the result but explanations are a little more difficult (and no idea on clamp output).
Nope, I am flummoxed. Attached is an image of three rectangles and a gaussian blur to spread out the tones on the histogram. Even if the 8 bit jpg is converted to 16 bit float using the mode menu, I still have issues understanding whether "levels" is actually broken.
Here's the theory as I understand it, if the output levels were really output levels then setting the black to 50, white to 205 (using 8 bit) then that should knock off all the deep blacks and whites producing a histogram that was blocky in the middle and no tones at either end. This does work as long as input levels is not touched.
If the input levels are set to the same as the output ( 50,205 ) the ends of the histogram appear which I believe is intuitively wrong. This looks like is "scaling" where the maths goes, "what's the difference between the input and output", nothing, so multiply by 1 and you end up with the tones everywhere. In the paragraph above you state "output levels, knock off the black and white ends".
Setting 30, 225 to the input and 50, 205 output does give a cut off histogram as it's scaled down but not as dramatically as only altering the output levels.
However if we go back to the initial example where input and output are set (50,205) the same and tick "clamp input", the ends of the histogram are cut off like you would expect, and if you alter the input values then it appears that the maths goes something like "clamp the input to remain inside the output histogram" whereas by default the maths appears to go scale the proportion of input to output.
I cannot find an example that gets clamp output to work so don't know the logic behind it.