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Old 09 February 2016, 15:45   #93
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Join Date: Aug 2004
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Originally Posted by Toni Wilen View Post
Originally Posted by mark_k View Post
By the way, it looks like you do ST palette correction after gamma correction. Would it be better to do that before gamma correction?
I don't know and don't care
Thinking about it some more, you should definitely do ST-correction before gamma correction. [Maybe the rest of this could/should be moved to a different thread...]

I'll refer to brightness as ranging from 0 (black) to 1 (white) as that's simpler when talking about gamma correction (just raise to a power). Whatever the gamma correction factor y, 0^y = 0 and 1^y = 1. Black and white are always unchanged.

Suppose the user gamma-corrects with a value of 2. Unadjusted "ST white" is value 7/15. That becomes (7/15)^2 = 0.2178 approx after gamma correction. If we then double it (correcting ST to Amiga 4-bit) you get 0.4356 approx. So even ST white becomes less than 50% grey after ST-correction.

Alternatively, suppose the gamma correction factor is 0.5. Then ~half-brightness ST is 4/7, i.e. 4/15 Amiga = $44/255 8-bit = 0.2667 approx. Raise that to power 0.5 ==> 0.5164 approx. If we do ST-correction after that, end up with 1.0323 which clips to 1 (i.e. full white). So anything above ST value 3 gets clipped to white in that case.
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