Cycles: tapering hair transparency fails in '3D Curves' mode [regression] (probably caused by embree) #86500

New Issue

Nicolas Antille · 2021-03-11T15:18:01+01:00

Nicolas Antille commented

2021-03-11 15:18:01 +01:00

System Information
Operating system: Ubuntu 20 LTS and MacOS Mojave
Graphics card: Quadro p2000

Blender Version
Broken: 2.92 and latest 2.93.0 alpha 5f1f233dc9
Worked: 2.82a and 2.83.4

This broke somewhere between b21ba5e579 and f2b5f731d5
Assume 99436acde8

Short description of error
Setting a transparent BSDF shader to hair in 3D curves mode yields a visible error where parts of the hair are black.
This issue is visible in the viewport and at render time. It affects both CPU and GPU rendering.

In ribbons mode, transparency works almost as expected. I noticed that when hair strands overlap and they have transparency, they are not transparent but they take the background opaque color instead, so the hair strands below are "covered" with background color.

embree

before embree

#86500.blend
Open file and switch to rendered view

Original report:

Initial setup in viewport: a cube with 20 hair strands:

Render in version 2.83.4:

Render in version 2.92+

Exact steps for others to reproduce the error
Attached is a .blend file example (so that you don't need to reproduce the steps below). transparent_hair_curves_issue.blend
If you run it on Blender 2.82a, rendering should show only a cube. Blender 2.92+ will show the issue.

Steps:

Open a new default blender scene with a cube
Choose Cycles renderer
Under Render Properties > Hair, select 3D Curves
Add a hair particle system to it
Lower particle count to 10
Set segments to 5
Enable Hair Dynamics, play the timeline for a while until hair bends
Go to simulated frame 30 (just an example)
Scroll down in psys panel to Render, enable b-spline with steps 2
In Timing, set start to 0.361 (just for testing purpose)
In Hair Shape, set Diameter Root 0.5, Tip 1, Radius Scale 1.00
Render in viewport or to an image (same result)

**System Information** Operating system: Ubuntu 20 LTS and MacOS Mojave Graphics card: Quadro p2000 **Blender Version** Broken: 2.92 and latest 2.93.0 alpha 5f1f233dc97de34f6be0f51edd3cbf2d8247fbda Worked: 2.82a and 2.83.4 This broke somewhere between b21ba5e579 and f2b5f731d5 Assume 99436acde8 **Short description of error** Setting a transparent BSDF shader to hair in 3D curves mode yields a visible error where parts of the hair are black. This issue is visible in the viewport and at render time. It affects both CPU and GPU rendering. In ribbons mode, transparency works almost as expected. I noticed that when hair strands overlap and they have transparency, they are not transparent but they take the background opaque color instead, so the hair strands below are "covered" with background color. embree ![image.png](https://archive.blender.org/developer/F9886948/image.png) before embree ![image.png](https://archive.blender.org/developer/F9886952/image.png) [#86500.blend](https://archive.blender.org/developer/F9886955/T86500.blend) Open file and switch to rendered view --- Original report: Initial setup in viewport: a cube with 20 hair strands: ![hair_transparency_blender_283_vp.png](https://archive.blender.org/developer/F9885074/hair_transparency_blender_283_vp.png) Render in version 2.83.4: ![hair_transparency_blender_283.png](https://archive.blender.org/developer/F9885070/hair_transparency_blender_283.png) Render in version 2.92+ ![hair_transparency_issue_blender_292.png](https://archive.blender.org/developer/F9885072/hair_transparency_issue_blender_292.png) **Exact steps for others to reproduce the error** Attached is a .blend file example (so that you don't need to reproduce the steps below). [transparent_hair_curves_issue.blend](https://archive.blender.org/developer/F9885080/transparent_hair_curves_issue.blend) If you run it on Blender 2.82a, rendering should show only a cube. Blender 2.92+ will show the issue. Steps: - Open a new default blender scene with a cube - Choose Cycles renderer - Under Render Properties > Hair, select 3D Curves - Add a hair particle system to it - Lower particle count to 10 - Set segments to 5 - Enable Hair Dynamics, play the timeline for a while until hair bends - Go to simulated frame 30 (just an example) - Scroll down in psys panel to Render, enable b-spline with steps 2 - In Timing, set start to 0.361 (just for testing purpose) - In Hair Shape, set Diameter Root 0.5, Tip 1, Radius Scale 1.00 - Render in viewport or to an image (same result)

Nicolas Antille commented

2021-03-11 15:18:01 +01:00

Added subscriber: @nantille

Philipp Oeser commented

2021-03-12 12:36:02 +01:00

Added subscriber: @lichtwerk

Philipp Oeser commented

2021-03-12 12:36:02 +01:00

Changed status from 'Needs Triage' to: 'Confirmed'

Philipp Oeser commented

2021-03-12 12:36:02 +01:00

Can confirm, will show this in a simplified file.

For me, this broke somewhere between b21ba5e579 and f2b5f731d5
Assume 99436acde8 ?

Can confirm, will show this in a simplified file. For me, this broke somewhere between b21ba5e579 and f2b5f731d5 Assume 99436acde8 ?

Philipp Oeser changed title from ~~Cycles: hair transparency fails in curve mode [regression]~~ to Cycles: hair transparency fails in '3D Curves' mode [regression] (probably caused by embree)

2021-03-12 12:54:04 +01:00

Nicolas Antille commented

2021-03-18 19:08:55 +01:00

@lichtwerk You are right, this appeared with Embree. It's worth noting that the unwanted effect gets worse when the hair strand faces the camera as on the screenshots I provided.

Nicolas Antille changed title from ~~Cycles: hair transparency fails in '3D Curves' mode [regression] (probably caused by embree)~~ to Cycles: tapering hair transparency fails in '3D Curves' mode [regression] (probably caused by embree)

2021-03-24 15:56:03 +01:00

Philipp Oeser removed the

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Cycles: tapering hair transparency fails in '3D Curves' mode [regression] (probably caused by embree) #86500