Blender to Nuke Compositing - Color pass issues

Support Compositing and Post Processing
1

Hi! Im totally new on the forums!
I might should have posted it in the render forum but here we go.

I have an issue when applying the Alpha pass on the reconstructed Beauty in Nuke (using openEXR). They do not match. I get the classic black halo do to premultiplyed passes. I noticed that all the Color passes in the diffuse/glossy/transmission comes out of Blender premultiplied by default (same with AO), The Direct/Indirect does not(which is what i want.)
Is there a way in Blender to render out these passes unpremulted?
Or maybe someone knows how to work around this issue in Nuke?

Thanks

2

can you post a screenshot what your node graph in nuke looks like or maybe upload the files? you can always premult in nuke any channel by any other channel with the “premult” node

3

Ok so here is my node graph setup:

node_tree.jpg1920×1040 378 KB

and here is the result i get when I enable/disable the premult node:

premult.jpg1920×1040 578 KB

and I thinks it’s due to that the colorpasses comes out of blender with a soft edge that doesn’t match the alpha pass completly?
Or am I all wrong?

soft_edges.jpg1920×1041 360 KB

4

The edge of the color passes should perfectly match to the alpha.

But why do you need to premultiply the image? Blender did it already, the black outline happens when you premultiply a second time.
If you really need to premultiply for any reason, you can of course, but you have to unpremultiply your passes, or the result of the merges before using the premultiply node.

5

What is the issue exactly? If you need unpremultiplied values, add an Unpremult node and do whatever you need to do (some heavy grading maybe). Then do Premult and combine the passes.

6

Thank you for your answers guys. I guess it wasn’t an issue after all. I started to some colorcorrection and I still maintained a good edge so false alarm. I’m pretty new when it comes to compositing and think it still quite confusing when to and not use the premultiply/unpremulty node. Cause I can recall to other multipass comps i’ve done where I had to use a premult to get the alpha to work.

But stupid question: Why does the colorpass comes with a soft edge and the direct/indirect does not?

7

I suppose this is to avoid the black artifact. The color pass act like a premult node because it is multiplied by the direct/indirect. If the direct/indirect passes were already premultiplied, then multiply it again with the color pass, it would “cut” the edge a second time, and then produce the black issue.

Not easy the deal with at the beginning but once you understand it, it makes perfectly sense.