Forums Technical Discussion Very basic question about shadow mat

Very basic question about shadow mat

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Island
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I haven't tried using a shadow material before in Houdini with Mantra, but I can't get the equivalent of a shadow catcher in Modo, Blender, or C4D. What setting am I missing? In actual render, the png lacks the environmental light behind the floor/grid material but the grid comes in as a partial transparency.

The only workaround I can use is to do two renders, with objects hidden in the second render. Then in the first render, do not render environment. Finally, put them together in Davinci resolve (or photoshop). Is there an easier way?
Edited by Island - June 20, 2020 15:01:13

Attachments:
ShadowTest.hiplc (516.4 KB)
ShadowProblem.jpg (202.3 KB)
ActualRender.jpg (1.2 MB)

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ziconic
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I'm not sure I understand the question. If you take your png output (the second attached image) and composite it over a background image, the alpha in the image (which are in the areas where the pixel value is black) results in a shadow over the visible part of the background image. The grid is partially transparent because the environment light causes a very diffused shadow. Is this not what you were expecting?
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Island
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In modo, blender, and cinema 4d - when the environment is rendered with a shadow matte on a ground plane, it renders as if objects had a shadow on the environment. There is no extra step of post processing composite needed.
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ziconic
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Oh I see. Well you can go inside the shadowmatte shader and rewire it as follows:
Edited by ziconic - June 22, 2020 19:25:41

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shadowmatte.png (64.8 KB)

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Island
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Unfortunately that is worse. It does not render the shadow and the ground plane is completely transparent. Thanks for looking though.
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ziconic
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Did you set Cf to 0 and Af to 1 and plug alpha into Of (not Af)? This is what I got.

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ShadowTest.png (958.1 KB)

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Island
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Brilliant! My mistake. I did not set one of the constants correctly. It works perfectly now.
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Ivan L
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Hi, decided to just ask here.

I need the same thing, but in Karma and the setup from above doesn't really work there.
If anybody knows how to get this to work in Karma let me know, please

Thank you
Edited by Ivan L - March 31, 2021 14:37:22
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