How to overlay a volume emission object on transparent background with another image

I am using Blender 2.73 Cycles and am very new to Blender.

I need to overlay a torus mesh object using volume emission on a second image preferably using .png files.

When I render the torus with output set to “RGBA” and film to “transparent” the result is just a .png transparency with
no torus.

Is there a way to do this or do I need to use OpenEXR or another format?

I can get the effect I need using GIMP, but this is an animation and I want Blender to be able to render it
without having to edit things frame by frame later.


Vol emission is strictly an additive effect. There’s no actual blocking of light, so the alpha remains transparent. Instead, render out the emission pass, and blend that over your background with the “add” mode. (might be called “linear dodge” in some apps).

Not sure if I understand correctly. I tried rendering the emission torus with normal background and using “add” effect strip to combine with the second image. The result is close to what I want - the torus is now superimposed on the second image but the background is much darker.

I also rendered the emission torus with RGBA and transparent background. This produces a totally blank image. When I use the “add” effect strip over the second image, the background is now normal, but the torus location is solid white instead of the torus image.
“Alpha over” produces a blank image.

What am I missing here?

Update: I think it is a bug:

Some people have tried adding some transparent shading or volume absorption as a work-around, but it does not produce what I want.

Does anyone else have some ideas on this?


I was not aware of what “emission pass” meant in your post, not having used render layers before. I did some research and created a new render layer for the emission object, masked all other layers, and rendered it a few different ways as above. Can you provide more detail on what needs to be enabled in the render layers dialog for this to work? I have only been using computer graphics software for a few months so still pretty green.