I have some new findings for POVers who wish to use the named colors in 
colors.inc with assumed_gamma 1 in old versions of POV-Ray.

You may recall that I noted a few years ago that the named colors in 
colors.inc seemed to have been determined without taking non-linear 
representation into consideration.  In other words, they don't look 
right with assumed_gamma 1.  So, I recommended applying srgb to these 
colors, as one would do with color values from third-party software, and 
ignoring the "suspicious expression" warning.  Shortly thereafter, I 
discovered the srgbft keyword, which eliminates the warning on colors 
that are already defined.

But while fooling around with my render rig wish list the other day, I 
discovered that 3.5 and 3.6 renders of the dark grays were lighter than 
3.7 and 3.8 renders!

So I wrote a scene to render some of the grays with different gamma 
configurations under different POV-Ray versions, then sampled the output 
images and created montages with the sampled values.  The ground plane, 
sky colors, and ambient were adjusted per frame to be comparable 
regardless of assumed_gamma, which is why the assumed_gamma 1 renders do 
not appear much different from the others.  In each frame, the light 
source is rgb 1 and diffuse was set such that the sum of diffuse and 
ambient is exactly one; this ensures that a Lambertian surface facing 
the light source reflects the pigment value.

The number in the center of each frame is a sample of the upper left 
facet, which directly faces the light source.

My goal was to use the unregulated color (upper left frame) as a 
control, and match that color using assumed_gamma 1.  The closest 
results are:

   Color    Control   POV-Ray 3.6          POV-Ray 3.7
   -----    -------   -----------          -----------
   Gray05   0.0037    Gamma 2.2 (0.0037)   sRGB (0.0040)
   Gray25   0.0497    Gamma 2.2 (0.0497)   sRGB (0.0513)
   Gray50   0.2122    Gamma 2.2 (0.2122)   Either (0.2159)
   Gray75   0.5210    Gamma 2.2 (0.5210)   sRGB (0.5210)

So, it appears that my initial recommendation holds for POV-Ray 3.7:

   #version 3.7;
   global_settings { assumed_gamma 1 }
   #include "colors.inc"
   pigment { srgbft ColorFromColorsDotInc }

But for earlier versions of POV-Ray, the power function should be used:

   #version 3.6; // or #version 3.5;
   global_settings { assumed_gamma 1 }
   #include "colors.inc"
   #include "math.inc"
   pigment { rgb VPow (ColorFromColorsDotInc, 2.2) }

The 3.5 renders are not shown because they sampled identical to the 
corresponding 3.6 frames.  I haven't sampled 3.8 frames, but unless 
someone knows differently, I have no reason to believe that changes were 
made in its gamma handling.  I have not tested any versions prior to 
3.5, nor have I tested any versions on an old Macintosh, which I 
understand used an unconventional gamma.

An implication that I have realized is that this rendering difference is 
not limited to stock colors.  *Any* color you define with rgb will look 
slightly different from 3.6 to 3.7.

In doing the automated sampling, I also discovered that a 3.7 image map 
loaded from a 3.5 or 3.6 rendered PNG source will look different from 
the source image!  To avoid this, put gamma 2.2 into the image_map 
block.  Whether this is a property of the rendering engine or just how 
the PNG chunks were set up, I don't know.  At the moment, I have no 
means of a proper comparison of the other supported image formats.

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