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

On Tue, Sep 7, 2010 at 8:56 PM, Steve Baker <steve@sjbaker.org> wrote:
> 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.

Quick rebuttal: it's not the same thing.  Human visual perception of
lightness (how it looks) is a non-linear function of light intensity
(how much light energy hits your eye).  This is covered in the Gamma
FAQ, http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html

Some of your other explanation I agree with; I'll elaborate more later.


> 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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