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:

Functioning Object Outline on White Background

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:

Nonfunctional Object Outline on Transparent Background

My thanks in advance for any assistance or pointers in the right direction.


1 Answer 1


Not the most elegant solution, and probably there is a better one, but if I understand correctly maybe this will help:

enter image description here

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.

  • 1
    $\begingroup$ This worked beautifully for me. (And I'd say that adding only one node is a pretty elegant solution.) Thanks very much! @Timaroberts $\endgroup$ Commented Jan 23, 2018 at 19:37
  • $\begingroup$ @ColinConwell, you're welcome. Glad to help. $\endgroup$
    – Timaroberts
    Commented Jan 23, 2018 at 21:42
  • $\begingroup$ One question: Would it possible to somehow animate this? Such that the color drained out over some number of frames? $\endgroup$ Commented Jan 23, 2018 at 22:14

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