Problem Merge Over

hi, I have a problem when I go to add a background, to a render exported in exr multilayer from blender. Practically putting a merge (over) I get what you see in the image.
What am I doing wrong?

What does your alpha channel look like for your A input (merge7)?

compositing.ntp (166.5 KB)

Intent thanks for the reply.
This is the ntp file, the file links are missing, but at least you can see how the nodes are set up.
I hope it is useful

Can’t really see much without plugging the files in but I suspect your alpha just hasn’t been brought through your breakout. Hit A in the viewer when looking at merge 7, if there is an alpha try adding a premult node to apply it to the RGB channels, if there isn’t you may need to bring it right from your EXR.

In Blender
Make sure you output an alpha from blender:

  • render properties > film > check the “transparent” box
  • render with a format that support alpha
  • output properties > output > color RGBA

In Natron
reinject the alpha with a shuffle node just before the final merge over the background

Note that the output of blender is premultiplied. So you should unpremult the passes before reconstructing the lighting with all the merge plus and multiply of your node tree

The “layercontactsheet” node is very usefull for this kind of task.

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thanks for the advice, I tried without success, I am attaching the project with the files, at a lower resolution …

Thanks again (8.9 MB)

@bonalex01 Was 100% on the money with this one.* I’ve added in the shuffle node for you. In a render engine only the direct diffuse pass should have an alpha channel that corresponds to the geometry of the scene, why Cycles (or you? Maybe this one isn’t on the program and you’ve messed up somewhere in Blender, haven’t personally needed to get individual passes from that program yet) has flooded alpha for every other lighting pass is beyond me.

demo.ntp (122.4 KB)

* Except for the unpremult part, don’t know where you got that from, you only need to do that with colour transforms, don’t think it’s required for compositing operations.

Thanks so much

@Shrinks99 is absolutely right about unpremult not being necessary if you just stack the different passes. I have the habbit to unpremult each pass because I usually add some gradings here and there and don’t want to forget that step.

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