It's rather obvious. The formula is laid out for black=0, white=255. But in YV12 with TV levels, it's black=16, white=235.
Replacing all "256" with "236 16 -", and all "x" or "y" with "x 16 -" and "y 16 -" respectively, should result in a correct screen formula for 16-235 TV scale.
Code:
236 16 - 236 16 - x 16 - - 236 16 - y 16 - - * 236 16 - / -
In your 2nd script line ... do you really expect mt_lut
xy to reckognize the letters "U" and "L"?
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