How to composite render over backdrop with AgX config

Continuing my journey into Blender compositing, I’ve tried adjusting a shadow catcher pass. Boy, is it interesting. The pass is rendered on a (1,1,1) white background. I assume this is for compositing purposes, but it messes with my goal of being able to composite on top of an sRGB brand color. I can’t simply transform this pass to sRGB as then the (1,1,1) values would get compressed and I won’t be able to use them anymore.

The solution I found was this, but I’m not entirely sure it makes sense.

  1. extract the white values as alpha mask from the shadow catcher pass
  2. apply this mask to the shadow catcher pass and transform it to sRGB with AgX.
  3. Tint the shadows blue-ish for demonstrating purposes. (In a real case I’d choose a similar tint as the background color). I have to be honest I have no clue what the color node is doing exactly here.
  4. Composite this modified shadow catcher pass over an sRGB green background color
  5. Composite the original shadowless render over this.


Image pass has alpha (sRGB through AgX)


Shadow catcher pass does not have alpha (Linear BT.709 I-D65)


Composite (sRGB) Yes it looks horrible, but it looks like I intended it to.

I’ll tag @troy_s as well to slap the nonsense out of me