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First of all, I'm new to the forum so please forgive me any mistakes (also in my english).

Setup: I'm currently rendering a Smoke simulation in Cycles, for which I've also setup a ground and wall plane as a collision object, and more importantly, a shadow catcher.

Setup wide view Setup camera view

The smoke domain material is the default setup created by the "Quick Smoke" effect.

Node setup domain material

The issue: When rendering the image, theres a grayish Box at the intersecting point between the ground plane and the smoke domain (which is set to adaptive, the box lines up with the edge of the adaptive domain).

low sample render of the issue (low sample demo render, also background color is set to white for clearer visibility of the gray box)

What I've tried so far:

  • moving the ground plane

  • changing the light in the scene

  • checking AO

  • Disabling "Shadow" Ray Visibility in the Cycles Settings of both the domain and the ground plane

It looks like I'm not the first one to experience this problem, but I've not discovered an answer that worked for me yet.

Any help is highly appreciated.

Cheers

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  • $\begingroup$ I can't reproduce the error. Please use the edit link at the bottom of your question (i.stack.imgur.com/lXFuK.png) and add more information on how you set up your project. Consider sharing your .blend file so that others can inspect it. You can upload it at blend-exchange.giantcowfilms.com and then paste the resulting link as part of your question. $\endgroup$ – cegaton Mar 30 '18 at 2:21
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This appears to be a form of z-fighting - where two faces exactly coincide. Presumably the bottom face of the Smoke Domain rests exactly on the plane. This causes Blender to have trouble in determining exactly which surface has been hit by a ray and, despite the edge of the Smoke Domain being 100% transparent, this can result in the shadow.

In order to demonstrate this effect, here's the default cube, moved 1 blender-unit up so that it's bottom face rests on the Z=0 plane with a plane added also at Z=0. The Cube has been assigned a 100% transparent shader.

scene setup

And here is the render - note the square shadow of the face of the cube that should not be present :

rendered

The solution is to slightly move the Cube or the Plane so as to separate those faces - only a very slight movement is required - moving the cube only 0.00001BU up was sufficient to avoid the issue.

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