On Thu, 03 Sep 2009 15:36:49 +0200, Warp <war### [at] tagpovrayorg> wrote:
>
>   I have demonstrated this phenomenon here:
>
> http://warp.povusers.org/povray_gamma_issue/index.html
>
>   The PNG basically ignores File_Gamma (as a net effect), while a TGA
> doesn't, and thus the latter matches the preview window.

When you specify a Display_Gamma that does not match the actual gamma  
response of your monitor, you are effectively lying to POV-Ray.  
Consequently, POV-Ray shows a preview with incorrect brightness.

When you specify a File_Gamma, you are telling POV-Ray to apply that gamma  
curve to the image data it produces on file. When you output to a  
gamma-aware format like PNG the gamma value is recorded in the file so  
other programs will know what gamma curve to apply in order to make the  
image look right on a given display. Many modern image viewers perform  
this gamma correction. When you output to a gamma-unaware format like TGA  
there is no way to record the gamma value in the file. It is therefore up  
to the user to remember the gamma value and apply gamma correction as  
necessary. If you do not want to have to perform this gamma correction  
yourself, you should use a File_Gamma that matches the gamma of the  
intended output display.

What you seem to want is to use colour values that look a certain way in a  
gamma-2.2 environment and have them look the same in the output *and* have  
the same numerical value in the output as they did on input. To the extent  
that I understand these things, the correct way is to alter the colour  
values on input, because POV-Ray expects linear colours on input. This is  
handled automatically for input images in PNG-format if they have a  
correctly set gAMA chunk, but other inputs must be corrected "by hand".



-- 
FE

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