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Black artefact when mixing transparency and glass at a certain ratio
Confirmed, NormalPublicBUG


System Information
Operating system: WIn 10
Graphics card: GTX 1080

Blender Version

Short description of error
Black artefact appear on flatter surfaces when mixing 99.9% transparent and 0.1% glass shaders. Glass should be rough.

Exact steps for others to reproduce the error
Make a cube, switch to Cycles, add material, add Glass and Transparent Shaders, Mix them together, put factor on 0.999, increase Glass roughness, depending on roughness a entirely black circle will appear on the cube when viewed straight on.

Event Timeline

Germano Cavalcante (mano-wii) changed the task status from Needs Triage to Confirmed.Thu, Mar 25, 4:23 PM
Germano Cavalcante (mano-wii) changed the subtype of this task from "Report" to "Bug".

It is a very specific problem, but I can confirm it.
In fact it looks like a bug.

It becomes more annoying and actually less specific, the more I fiddle with it.

I basically need to make a shader that is blurry refracting/glass but clear when viewed head on, so I use Layer Weight and color ramp to tune it in:

The problem becomes worse when I change the color ramp slightly:

Also, this happens when you make glass IOR 1 with some roughness and mix it with transparent and render with GPU:

it is almost black but should be 80% transparent

BUT, when I switch to CPU, then:

It's fine, thought I just mention it, maybe related somehow, turning it on CPU doesn't resolve the main problem though.

Also, it would seem that it is somewhat dependent on the glass shaders roughness.

With roughness 0.2 it is somewhat usable at certain other settings:

But with 0.4

And 0.6