[Public WebGL] Linear-to-sRGB conversion to the drawing buffer using sRGB extension

[Oskar Skrinjar <oskar@scientificiv.com>]

Hello,

I have a question to those who know how to use sRGB extension with WebGL 1.

When I use an image as a texture, if I specify the texture to be sRGB,  
it seems that it works properly, since the pixel color values get  
smaller in the fragment shader and if I directly display that texture  
using a full screen quad, I get a darker version of the image, which  
should be the case. Thus, the fragment shader gets the colors in the  
linear space and then I can do processing in the linear space.

I have problems going back from linear values to sRGB values at the  
end of the processing pipeline, i.e. when I want to render the final  
result to the drawing buffer. If I directly render the linear color  
values into the drawing buffer the displayed image looks darker, as  
expected. Since it is not possible to specify that the drawing buffer  
does linear-to-sRGB conversion, I tried to use an FBO for that purpose.

If I create an FBO with an sRGB texture as COLOR ATTACHMENT 0, then  
rendering linear colors into that FBO seems to work properly. E.g.  
when I write inside the fragment shader a gray value of 54 (R=54,  
G=54,B=54) into the FBO and then use pixelRead to check the stored  
value, the stored value in the FBO is (127,127,127), which should be  
the case. Thus, that FBO stores colors in the sRGB space, as expected.

What I do not know how to do is displaying the content of that FBO to  
the screen. If I do a “transfer pass”, i.e. use the FBO’s color  
attachment texture as an input, and read it from a fragment shader and  
then write the color values into the drawing buffer I end up with  
linear values. This is because that color attachment texture is sRGB  
and reading it from a fragment shader converts the values from the  
sRGB to linear space and the resulting image is darker. Of course, I  
could do linear-to-sRGB conversion inside the fragment shader, but  
that is what the sRGB extension should somehow do.

Could someone refer me to examples or explain how to convert linear  
color values into sRGB color values that end up in the drawing buffer  
using the sRGB extension. Any help will be appreciated.

Thanks,

Oskar




Oskar Skrinjar, PhD
Scientific Imaging and Visualization, LLC
Website: http://www.scientificiv.com
Email: oskar@scientificiv.com
Phone: 404 863 2371


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