Rendering a Transparent Object (with Outline added by Nodes) on a Transparent Background Hey there Blender Community,

I am attempting to render just the outline of a fully transparent object on a transparent background (in Cycles). Using a material index pass and an inverse Sobel filter, I am able to produce the outline effect successfully on a white background. See below:



However, when I make the film (the background) transparent, the outline disappears. Is there a way to keep the black outline of the fully transparent object on an a transparent background?

The same node setup, but with film set to transparent:



My thanks in advance for any assistance or pointers in the right direction.
Not the most elegant solution, and probably there is a better one, but if I understand correctly maybe this will help:



click to enlarge

Using a Color Key node to replace the white with transparent pixels, and adding the key to the image  seems to work well in this case.