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Mirrored objects render black when using the Bevel Node
Closed, ResolvedPublic

Description

System Information
Operating system: elementaryOS 0.4.1
Graphics card: GTX1080

Blender Version
Broken: 2.80, 7b05b2564d8

Short description of error
Objects that use the Bevel Node render almost black. Happens to regular objects as well as Collection Instances. But only on F12! The error is not visible in Viewport Rendering.
Here's how it looks in the viewport:

And this is how it looks when rendered (F12):

Happens also in 2.79 (master)
Exact steps for others to reproduce the error
Open this file and compare viewport render and F12:

Event Timeline

Brecht Van Lommel (brecht) triaged this task as Confirmed, Medium priority.Feb 1 2019, 5:03 PM

Hi, I am interested in trying to fix this issue. I was wondering in which files would I need to look in order to start tracking down the source of the bug?

The bevel shader code is in intern/cycles/kernel/svm/svm_bevel.h.

Ok, so I started taking a look at the shader code but I am having a hard time understanding it all, even with the reference paper in the comments. Is there a way to debug the shader code when it is running?

You can use a debugger to set a breakpoint on this function and step through the code. Which one depends on your development environment, it's not really a Blender specific thing.

This is not the easiest bug to start with though, if you're just getting start with Blender development you might want to pick an easier bug.

Yeah, I am thinking I might be a little in over my head with this bug, although it has been fun reading up on MIS and BSSRDF concepts and getting to know the code base.

Do you have a suggestion for an easier bug to start with? I had asked in the IRC channel for one and this one was suggested to me as a starter.

@Tim Stullich (tstullich) so sorry to have put you on this, I really thought this would be an easy one , especially because it works fine in preview rendering.... I better shut up next time :)

@Brecht Van Lommel (brecht) thanks so much for fixing! this makes so many things so much easier now! :)