@Brecht Van Lommel (brecht) Where is this option please?

It's the shadow catcher pass that you enabled.

ah never mind, you just disable the pass. Thanks.

@Brecht Van Lommel (brecht) this still isn't producing the correct result, sorry I've never used the shadow catcher when rebuilding from passes. Perhaps I'm doing something wrong?

or is it just a case of the plane needing to be black?

This doesn't appear to be a user issue. I think the shadow catcher objects need to show as black in the diffuse direct and indirect passes (where there is no indirect lighting), this way the alpha will leave just the shadow and indirect lighting.

The glossy passes are fine because they automatically show as black where there's no reflection. Haven't tested transmission passes.

I can't do it using the shadow catcher pass, because this removes it from the alpha.

@Brecht Van Lommel (brecht) can you re-open this?
@Philipp Oeser (lichtwerk) could you also have a look just to make sure this is not a user issue at my side?

@michael campbell (3di): is the latest file the one from the report description?

Thanks @Philipp Oeser (lichtwerk) I'm pretty desperate on this one as someone just asked for refund because of it. So hopefully it is just something I'm doing wrong.

Here's a simpler version

will look at it tomorrow

thanks. I'm pretty sure it's a bug as I'm quite skilled in this area (been doing it for 20 years), but perhaps there is some Blender specific thing that I'm missing.

thanks @Brecht Van Lommel (brecht) much appreciated. Did you discard the diffuse direct only and keep the indirect pass for the shadow catcher? Indirect could be kept as that renders black already where there's no indirect light. Would be nice to keep the indirect light if possible as that's included in the alpha

Hi The shadow catcher's diffuse colour is still being written to the diffuse direct pass in today's daily build of 3.1.1:

@Philipp Oeser (lichtwerk) could you re-open this?

Fix is only in 3.2 (if this is what you meant -- or did you mean to reopen because of indirect?)

ah ok, sorry, I thought the bug fixes were applied to all daily's. I'll check now. It shouldn't have affected the indirect passes hopefully, as they are already black where there's no indirect lighting data.

@Philipp Oeser (lichtwerk) I've just checked in 3.2 and although the shadow catcher's diffuse colour have been removed from the DiffDir pass (allowing it to work), it's also been unncecessarily removed from the DiffInd and gloss passes, resulting in loss of indirect lighting and reflection. Ideally these should have been left alone, as now all the GI etc has been lost. These can be left alone because they render black where there's no data anyway.

The shadow catcher makes no distinction between diffuse and glossy, or direct and indirect, and they do not behave fundamentally differently when it comes to what renders black or not in general. Diffuse direct light is not special here and should not be treated as such.

ah ok. I just thought if you only removed the shadow catcher from the DiffDir, then we could still have indirect lighting and reflection. The only issue was the shadow catchers diffuse colour showing up on the diffDir pass (making the shadow catcher visible when combining the image).