[Radiance-general] Alpha channel/object matte

[Peter Apian-Bennewitz]

Kirk Thibault wrote:

> Hi Folks,
>
> Is there a straightforward way to generate alpha channel or object 
> matte 8 bit grayscale images in radiance for compositing purposes?
>
Hi Kirk,

At least not automatically by rpict itself. rayshade (and likely others 
raytracers too) generate an optional alpha channel, and I nagged Greg to 
add an optional fourth channel to the image format in ( - hang on, a 
quick seach in my mail folders..) June 1993. That never made it, since a 
change of format would have produced various nasty side effects. The 
main reason for my wish was to paste the image into existing (tpically 
outside) scenery easily. With fisheye HDRs being on-vogue now, 
surrounding the model with an HDR source-primitive is a better way to 
solve that merging problem.

So, you'll have to generate a z-file (-z option), use the z-value to 
differentiate between an object hit (alpha opaque) and infinity (alpha 
transparent) and convert that to whatever image format you need. 
Suitable pcomb/pfilt/pvalue scripts are left as an exercise to the 
reader ...

-Peter

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