Nigel Stewart wrote in message <3714BFE3.BF82787E@eisa.net.au>...

>
>PovAlphaChannel (is a ????)
>
>  Notes:
>
>  Where does alpha channel fit?  Is it simply
>  the degree of "background" that shows for
>  each pixel, or is it something more?  Perhaps
>  alpha channel should be an optional extention
>  of PovCanvas.

Background visibility is a good definition.
Having it as a member of PovCanvas seems right.
During the raytracing process, rays that
partially hit objects or never hit them
set a transparency value into the alpha channel.

Having other compositing channels/canvasses
would be good too, like one for shadows only.

Ray

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