> Maybe I'm missing something obvious, but whenever I try to render an
> image with a transparent background, any emissive media that does not
> have a solid object between it and the background just becomes
> invisible, instead of partially transparent which is the intent.
>
> Essentially, I'm trying to render an engine glow via emission, and it
> works fine when I have a backdrop, but it disappears when I remove the
> background.
>
> minimal scene:
> sphere{0,1
> texture{pigment{color rgbt 1}}
> hollow
> interior{
> media{
> emission <1,0,0>
> method 3
> intervals 1
> density{color rgb 0.5}
> }
> }
> }
> camera{location <0,2,-2> look_at <0,0,0>}
>
> Setting -UA in the command line shows a red sphere, +UA is completely
> transparent. Upon opening in photoshop, it shows that the red channel is
> in fact correct, but the alpha channel is at 100% for the entire image.
>
> Is there a way to get around this?

Fact is that any emissive media IS totaly transparent by design. It adds 
to whatever is behind, wighout filtering anything out.

What you can do:
Add some absorbing media with absorbtion<0,1,1>. Removes the blue and 
green, leaving the red.
Add some scattering media and a light_source.
Change the pigment to rgbt<1,1,1,0.9> or rgbf<1,1,1,0.9>


Alain

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