I've been adjusting my textures using a different trick: I filter the 
texture over itself and over an opaque white surface, effectively raising 
the colour to a power of 2, which isn't perfect gamma correction but it 
suits my needs (the texture had to be filtered anyway for some effects I'm 
doing).

I was going to do it with functions like you have, but the pigment 
definitions came out of poseray and I was too lazy to retype them all :)

-- 
Tek
http://evilsuperbrain.com

"Tim Nikias" <JUSTTHELOWERCASE:timISNOTnikias(at)gmx.netWARE> wrote in 
message news:439b2aa2@news.povray.org...
> (see thread "using assumed_gamma of 1.0... a discussion", located here:
>
http://news.povray.org/povray.general/thread/%3Cweb.439a1b692d1f46002a1c213f0%40news.povray.org%3E/)
>
> Anyways. My experience is this: I have a nice image_map, I follow the docs
> and supply assumed_gamma of 1.0 and have Display_Gamma properly set to 
> 2.2,
> and what happens? The image_maps turn out brighter than their source. Why?
> Due to the internal gamma correction of POV-Ray.
>
> Personally, I'd like an image-map to look like it's source when its lit 
> with
> 100% white light (e.g. by applying finish{ambient 1 diffuse 0}), but the
> gamma-correction prevents that. The image shows this. The upper-left 
> corner
> is the pure image-map in POV-Ray. The lower-left shows the original 
> source.
>
> What's on the right side you ask? Well, the upper right is the image_map
> "uncorrected", but passed through my gamma-correction macro using a gamma 
> of
> 1.0 (to show that it doesn't "harm" the image to pass through the macro).
> The lower right is the uncorrected PNG version of the image. PNG comes 
> with
> internal gamma values, and POV-Ray makes proper use of them. The problem
> lies within the usual floating point (in-)accuracy. If you take a closer
> look at the ocean's horizon, you'll notice color-banding and
> color-clipping-like effects. That's why I'd rather not use PNGs, at least
> not if they're as dark as this source.
>
> Here's the macro:
> #macro Gamma_Corrected(Image_Pigment,Gamma)
>  #local Image_Function =
>    function{
>      pigment{Image_Pigment}
>    }
>  #declare Gamma_Corrected_Return=
>  pigment{
>    average pigment_map{
>      [1 function{Image_Function(x,y,z).x} poly_wave Gamma color_map{[0 rgb
> 0][1 rgb 3*x]}]
>      [1 function{Image_Function(x,y,z).y} poly_wave Gamma color_map{[0 rgb
> 0][1 rgb 3*y]}]
>      [1 function{Image_Function(x,y,z).z} poly_wave Gamma color_map{[0 rgb
> 0][1 rgb 3*z]}]
>    }
>  }
>  //Return value
>  Gamma_Corrected_Return
> #end
>
> All you do is #declare your image_map as a pigment and instead of applying
> it directly onto your object, you pass it through the macro, like so:
>
> #declare Image_Map = pigment{image_map{sys "test.bmp"}}
> box{-.5,.5 pigment{Gamma_Corrected(Image_Map,1.0)}
>  finish{ambient 1 diffuse 0}
> }
>
> I'll probably add this to my website some time soon, but wanted to get 
> this
> on the web for the ones joining the above-mentioned thread and wondering 
> how
> to do what they want to do. :-)
>
> Regards,
> Tim
>
> -- 
> aka "Tim Nikias v2.0"
> Homepage: <http://www.nolights.de>
>
>
>

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