=?ISO-8859-1?Q?J=F6rg_=27Yadgar=27_Bleimann?= <yaz### [at] gmxde> wrote:
> Now my question: as the albedo value gives the percentage of incoming
> light reflected from a body, is it correct to assume that HSL brightness
> values directly correspond to albedo

AFAIU, that would be the case if the celestial body in question would exhibit
only perfectly diffuse reflection.

> How can I adjust the overall (=average) brightness of a bitmap texture
> to a certain value? Can it be done within PoV-Ray or should I better
> relay on GIMP to do this?

You might use an "average" pseudo-pattern, as in

pigment {
  average {
    [ 0.1 myBitmapPigment ]
    [ 0.9 color rgb 0 ]
  }
}

to e.g. tune the pigment to 10% the original bitmap's brightness.

Of course in order to tune to a particular overall brightness, you'd have to
compute the original bitmap's brightness first; you could do this by sampling a
few points using the eval_pigment() builtin macro.

Also note that you may have gamma issues with the input imagery.

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