HowTo:Migrate old scenes to work with the new gamma system

Before "assumed_gamma 1.0" or the most recent 3.7 betas, the colour math was simply wrong. So in order to get exactly the same result, you'd have to mimic that wrong colour math - which 3.7 refuses to, except for legacy scenes using "#version 3.6" (or anything smaller than 3.7) and "assumed_gamma 2.2".

If you do want to port the scene to the right math, here's what I expect to be the most important steps:

1. In the ini-file, set the "Display_Gamma" to whatever fits your system; set "File_Gamma" to the same. (Ideally, your system should have a Display_Gamma of 2.2.)
2. In the scene file, use "#version 3.7", and remove any "assumed_gamma" statement.
3. Gamma-adjust all colour literals, by raising the R,G,B components to the power of 2.2 (or wait for the next beta, and add "gamma 2.2" to the color literal). Check your scene for colour values that are computed from some other values - you may (or may not) want to gamma-adjust these as well.
4. Check your scene for input image files, and whether they are handled properly regarding gamma. For images that are used as anything non-color (e.g. height field data, bump map, texture map or what-have-you), explicitly specify "file_gamma 1.0" after the filename (next beta will also support "gamma 1.0" here). For any images that do wind up being used as a colour, try the default first (i.e. no "file_gamma" statement); if the colours appear too washed-out or too strong, try explicitly specifying "file_gamma srgb".
5. Toy around with overall brightness and/or brightness of individual light sources, until you feel comfortable with the illumination. You may also want to tweak some of the colours in your scene (both pigment and light sources). If you're using self-made input image files, you may also want to tweak their file_gamma setting, but try to avoid this with 3rd party input images (you should usually find a way to make them work by tweaking the scene rather than the image).

Here's a macro that can be used to achieve the color conversions pending the next beta or if you are still using version 3.6:

#macro gamma_color_adjust(in_color)
	#local out_gamma = 2.2;
	rgbft <
		pow(in_color.red, out_gamma),
		pow(in_color.green, out_gamma),
		pow(in_color.blue, out_gamma),
		in_color.filter,
		in_color.transmit
	>
#end

And a usage example:

pigment {color gamma_color_adjust(rgb <110,160,008>/255)}
pigment {color gamma_color_adjust(rgbft <1,1,1,0,1>)}
pigment {color gamma_color_adjust(rgbt <1,1,1,1>)}
pigment {color gamma_color_adjust(red 1 green 1 blue 1 transmit 1)}

As of version 3.7.0.beta.38, POV-Ray will natively support gamma-adjustment of color literals:

pigment {color rgb <110,160,008>/255) gamma 2.2}
pigment {color rgbft <1,1,1,0,1> gamma 2.2}
pigment {color rgbt <1,1,1,1> gamma 2.2}
pigment {color red 1 green 1 blue 1 transmit 1 gamma 2.2}

Certain scenes with partial transparency (filter and/or transmit?) suffer additional degradation that is not resolved by the above fix. For instance, Chris Colefax's lens flare macro. It is not clear yet how to resolve this issue. See here.

See also

• Converting pre-gamma scenes to assumed_gamma = 1? - discussion thread