Hullo and happy New Year! I'm rendering a glass bottle--100% transmissive--lit only with some area lights in Karma (for contour reflections/highlights), but I still can't get an alpha channel so I can place it over a background in post. The alpha is always 100% opaque. How can I get the glass to be glassy?

bentway23
Hullo and happy New Year! I'm rendering a glass bottle--100% transmissive--lit only with some area lights in Karma (for contour reflections/highlights), but I still can't get an alpha channel so I can place it over a background in post. The alpha is always 100% opaque. How can I get the glass to be glassy?

This is done through an LPE using the background event (B). This only works if nothing is refracting in your object ( nothing is behind it)

Thanks. I found an old thread that showed that, but when there is no background that AOV still shows a solid opaque alpha, so it doesn't help here.

After many brains being picked, it looks like the problem is that renderers are now too smart to do dumbeasy stuff any more. (The background in question is a solid color, refractions are not needed.)

I never thought I would miss mentalray!

The while purpose of B event for LPEs is to be able to isolate specific paths ending with empty background

Since you didn't link the mentioned thread, nor the LPE itself, it's difficult to say what exactly you tried and why it didn't work for you

bentway23
Thanks. I found an old thread that showed that, but when there is no background that AOV still shows a solid opaque alpha, so it doesn't help here.

After many brains being picked, it looks like the problem is that renderers are now too smart to do dumbeasy stuff any more. (The background in question is a solid color, refractions are not needed.)

I never thought I would miss mentalray!

Not sure if you are aware, but you will be rendering an aov with that so you need to shuffle that in comp.