Render Unlit fog transparent for a logo

Question

I have a problem with rendering a logo which consists of two main parts :

• Front half of the logo is a rather normal solid set of objects creating my font.

• Back half of the logo is a slightly smaller version of the same logo emitting light.

I would now like to create a glow effect by lighting up fog with the emitting objects...

As such, I am able to create the desired effect in Cycles. There is only the problem that I need a thick set of fog in order to create the desired fog... I would now like to be able to render it in a way where I only get the colourfull slighty transparent fog rendered and everything else fom the fog transparent. As of now I am only able to create my desired image with black background because of the fog.

I would be very gratefull for help on this problem !

Answer 1

Scene setup

Let's say you have this scene, with emission objects. First you want to enable Film transparency.

Compositing

Then in the compositor, enable Use Nodes and add a Glare node as follows :

By setting the Mix to 1, you'll only get the glare without the original image.

Glare/bloom effects are meaned to be used with an additive Blend mode. This means that you'll only add values to your pixels. Thus, black won't do anything and brighter parts will add brightness. My setup here could be better, by enabling the Emit render pass, and use it as input of the Glare node. In that case, there will no transparency at all.

So my point is, to my knowledge, a Glare/bloom layer should not be transparent. Or at least it doesn't have to be.

Compositing example

The final setup would look like that :

You can then decrease the glare with the Factor value of the Add node. Or you could increase the effect of the glare.