I'm using a background plate LOP, and I can successfully go through the whole rigamarole of setting up the AOVs and compositing it all together. But I'm running into a single problem that when I use karma physical sky under the Atmosphere sky mode, no matter what I do the background holdout shadow catcher geometry shows up as blue in the holdout_shadows pass. It shows up in other holdout passes also, but it's particularly prominent in the holdout_shadows as you can see from the screenshot.



So long story short whenever I try to composite it all together, the blue area that's supposed to be black because it's a shadow catcher gets inverted in the composite to a yellowish tint and then my whole render gets a yellowish tint to it. So I'm trying to figure out how to get it to only render the shadow and nothing else.

Thing is I don't have any such problems when I just use a normal light, not Karma Physical Sky set to atmosphere. As you can see here everything looks good and the shadow catcher doesn't show up in the render at all:




Is there anything I can do to get Karma Physical Sky in atmosphere mode to work with the background plate node? Being able to do this is pretty important to what I'm trying to accomplish, and I've spent a lot of hours tinkering with LPE tags and whatnot trying to get the blue backdrop to go away, but haven't figured anything out yet, so help!




(Here's a screenshot of my simplified node flow that I threw together just to isolate the problem. Every single setting is copied and pasted identical except for the light source)

Hi.
Under atmospheric mode, the atmosphere exists as volume object, so the blueness you're seeing is the actual shadow of the volume cast onto the plane. While this is technically the correct shadow pass, it's also assuming that the atmosphere exists only as CG/foreground element and the plate doesn't have any atmosphere in it (ie the photo was taken outside of earth's atmosphere).
Seeing as how your plate photograph likely already includes atmosphere, you can make the volume object part of the background geometry (in your background plate LOP, include the light's 'atmosphere' object under Primitives field). This will produce the shadows you seek, but this will also omit the atmosphere from the foreground elements - which is again technically correct since an object (including atmosphere) is either part of the background or foreground (it can't be both), but it may not be the result you seek either since it's hard to separate atmosphere from the photograph in order to composite atmosphere on top of the CG element.
So it's tricky... my suggestion may be to render shadows as a separate pass.

Ok, that worked in terms of getting the shadows to show up properly. Thanks.

But then I do want the atmospherics to affect the CG element, and I guess that's not happening anymore? I'm actually rending objects into top down aerial footage from some distance, so the whole point is to use the atmospheric diffraction to modify how the CG element is rendered in a way that is more or less consistent with the physics of the situation.

So when you say to render shadows as a separate pass, what do you mean by that? Are you sayin to render the shadows using a background plate node by adding the atmosphere to the primitives field, and then combining the shadows AOVs that I get from that pass with a totally separate pass that renders the foreground elements properly with all the atmospheric effects? Am I understanding that right?

dlee
Hi.
Under atmospheric mode, the atmosphere exists as volume object, so the blueness you're seeing is the actual shadow of the volume cast onto the plane. While this is technically the correct shadow pass, it's also assuming that the atmosphere exists only as CG/foreground element and the plate doesn't have any atmosphere in it (ie the photo was taken outside of earth's atmosphere).
Seeing as how your plate photograph likely already includes atmosphere, you can make the volume object part of the background geometry (in your background plate LOP, include the light's 'atmosphere' object under Primitives field). This will produce the shadows you seek, but this will also omit the atmosphere from the foreground elements - which is again technically correct since an object (including atmosphere) is either part of the background or foreground (it can't be both), but it may not be the result you seek either since it's hard to separate atmosphere from the photograph in order to composite atmosphere on top of the CG element.
So it's tricky... my suggestion may be to render shadows as a separate pass.

ubietyworld
So when you say to render shadows as a separate pass, what do you mean by that? Are you sayin to render the shadows using a background plate node by adding the atmosphere to the primitives field, and then combining the shadows AOVs that I get from that pass with a totally separate pass that renders the foreground elements properly with all the atmospheric effects? Am I understanding that right?

Hi. Yes, that's correct. For the shadows to be attenuated convincingly you may have to tweak it in comp (either manually or use the alpha of the atmospheric).