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Re: [Public WebGL] The Newly Expanded Color Space Issue



Thatcher Ulrich wrote:
> Um... 0.73 in an sRGB frame buffer is (perceptually) 73% gray.  Right?
>  I can't tell from the way you phrased the question whether 73% gray
> is the wrong or right answer.
>   
Short answer: 0.73 in the final, final, ultimate output of the graphics
card is the right answer for a 50/50 mix of black/white.

A value of 0.73 in the final frame buffer causes a voltage that is 73%
of maximum to be output to (for example) an analog CRT - which will
consequently fire 73% of the maximum amount of electrons at the phosphor
which will result in only 50% of the maximum number of photons appearing
at my eye because the phosphor response follows the 'gamma' law:   
output = pow ( input, 2.2 ).  Modern LCD's, DLP's and such emulate a
gamma of 2.2 for backwards-compatibility with old-fashioned CRT's.

Hence a value of 0.73 in the final frame buffer is exactly the correct
answer for a PERCEPTUALLY 50% grey.
> Are you asking for a result where the monitor emits 50% of the photons
> of pure white, or a result that looks half as bright as pure white?
>   
This is a red herring.  It's the same thing.

When you have a white polygon that's being lit with two identical
uniform white light sources - and then you turn one of them off - you
get half as many photons arriving at your eye.    That's true whether
this is a real-world polygon with real world lights - or a virtual
polygon with virtual lights and with the photons coming out of the
CRT.   To get 50% of the number of photons - you need to put a value of
0.73 into the final frame buffer for the reasons I explained, above.

  -- Steve

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