Cycles strange render behavior!

Support Lighting and Rendering
1

Hello
I encountered a strange render behavior in Cycles. I conect a emission node in the volume. Thats the material. I enable transparent in Render>Film. I render this setup in the preview render. Every thing is OK. Now I render it, and there ist nothing I see. And restrict rendering of the Object is off, so you shuold see it. When I put a plane under a portion of the cube, it renders the cube only where the plane is!? Here a image of the setup and result.

grafik
grafik3840×2100 974 KB

What is the problem?
Thank you very much for your help.

2

The problem is that transparent film adds an alpha channel which sets alpha=0 wherever the background can be seen. Your volume emission cube is still rendered, just the image viewer doesn’t display, due to the alpha channel. If you change in the image viewer in the top right drop down the viewing mode from ‘Color and Alpha’ to ‘Color’ you should see it again.

1 Like
3

Problem is, Emission shader is considered as zero density, so the alpha is still zero. Like @omgold said, the color channel is ok.

If you really want to get the volume on transparent background, you need to mix it with some dense shader like a volume scatter :

Screenshot%20from%202019-10-01%2021-19-35
Screenshot%20from%202019-10-01%2021-19-351395×860 99.3 KB

(But the render result will be different, as it’s a mix between two shaders)

Another “trick” would be to get back RGB channels only, mean them (saturation zero) and use the result as a new alpha for the image, with the “set alpha” node.
Like this :

Screenshot%20from%202019-10-01%2021-17-12
Screenshot%20from%202019-10-01%2021-17-121442×890 136 KB

This way you don’t need dense shader, but the result would be crap if an object appears behind (like your plane example)

See you :slight_smile: ++
Tricotou

4

Thank you for that. But the problem is, that I would like to save the image with th transparet background, like in the preview. Is that possible?

5

Thank you tricotou. This with the volume scatter I did, but it is not quite the same like in the preview.

But I do not unserstand why in the prevview render it is possible and in the finel not!?
I do understand all the other things you said, but this not.

Thank you any way for the fast reply from all of you.