>High!
>
>During the process of getting together the astronomical data for my 
>Solar System project ("PoVSolar"), I usually also find albedo values 
>for the various planetary objects.
>
>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 (I know that there are 
>different ways to compute astronomical albedo, but for the time being, 
>I would stick to the basic definition), so that a dark grey asteroid 
>with an albedo of 0.04 would be best modeled as rgb 0.04? (Yes, I've 
>also already heard of a PoV macro to convert HSL vectors into RGB 
>vectors, within colors.inc, if I remember correctly...)
>
>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?

Using rgb 0.04 is fine if the object has zero saturation.

For non-grey colours, the albedo is probably best represented by .gray 
(watch out for that American spelling, the UK English spelling doesn't 
work) which returns a value where the three components are weighted in a 
way that corresponds to the way the human eye responds. As you say, 
there are various definitions of albedo, but using the response of the 
human eye is a reasonable one to go with.

   #declare C = rgb <0.5,0.3,0.2>;

then C.gray, the albedo of that colour, returns 0.348

If you have an object that has the same hue and saturation as C but has 
albedo A, then you can write

   pigment {C*A/C.gray}

-- 
Mike Williams
Gentleman of Leisure

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