Cycles > Volume > Alpha Channel #73152

New Issue

filip mond · 2020-01-15T23:11:34+01:00

filip mond commented

2020-01-15 23:11:34 +01:00

System Information
Operating system: Darwin-19.0.0-x86_64-i386-64bit 64 Bits
Graphics card: Intel HD Graphics 4000 OpenGL Engine Intel Inc. 4.1 INTEL-14.2.16

Blender Version
Broken: version: 2.82 (sub 6), branch: master, commit date: 2020-01-14 23:08, hash: 3edd8d5b07
Worked: (optional)

Short description of error
Transparency of scene is determinated by objects using non-volumetric material only.

Exact steps for others to reproduce the error

Keep Cube
Add Sphere
Add Sphere a Material with Emission node plunged into Volume socket
Film > Transparent

Example use Cube with Visibility > Holdout to rise problem more visible.
Sorry, probably a bit confusing.

Solid Mode

Render Viewport Mode is OK

Render (F12) results with ERROR - spheres are rendered, but object itself is ignored for alpha channel

Render viewed as Color (without alpha)

test_Edge.blend
Note: spheres (volumetric) are not calculated also for ID Mask or Cryptomatte as well

**System Information** Operating system: Darwin-19.0.0-x86_64-i386-64bit 64 Bits Graphics card: Intel HD Graphics 4000 OpenGL Engine Intel Inc. 4.1 INTEL-14.2.16 **Blender Version** Broken: version: 2.82 (sub 6), branch: master, commit date: 2020-01-14 23:08, hash: `3edd8d5b07` Worked: (optional) **Short description of error** Transparency of scene is determinated by objects using non-volumetric material only. **Exact steps for others to reproduce the error** - Keep Cube - Add Sphere - Add Sphere a Material with Emission node plunged into Volume socket - Film > Transparent Example use Cube with Visibility > **Holdout** to rise problem more visible. Sorry, probably a bit confusing. Solid Mode ![Screenshot 2020-01-15 at 22.22.21.png](https://archive.blender.org/developer/F8280726/Screenshot_2020-01-15_at_22.22.21.png) Render Viewport Mode is OK ![Screenshot 2020-01-15 at 22.20.34.png](https://archive.blender.org/developer/F8280732/Screenshot_2020-01-15_at_22.20.34.png) Render (F12) results with ERROR - spheres are rendered, but object itself is ignored for alpha channel ![Screenshot 2020-01-15 at 22.21.36.png](https://archive.blender.org/developer/F8280728/Screenshot_2020-01-15_at_22.21.36.png) Render viewed as Color (without alpha) ![Screenshot 2020-01-15 at 22.29.14.png](https://archive.blender.org/developer/F8280742/Screenshot_2020-01-15_at_22.29.14.png) [test_Edge.blend](https://archive.blender.org/developer/F8280760/test_Edge.blend) Note: spheres (volumetric) are not calculated also for ID Mask or Cryptomatte as well

filip mond commented

2020-01-15 23:11:34 +01:00

Added subscriber: @FilipMond

filip mond changed title from ~~Cycles > Volumetric > Alpha Channel~~ to Cycles > Volume > Alpha Channel

2020-01-15 23:46:06 +01:00

Philipp Oeser commented

2020-01-16 11:10:43 +01:00

Added subscribers: @brecht, @lichtwerk

Philipp Oeser commented

2020-01-16 11:10:43 +01:00

Changed status from 'Needs Triage' to: 'Needs Developer To Reproduce'

Philipp Oeser commented

2020-01-16 11:10:43 +01:00

Using a proper Volume Shader (Principled Volume / Volume Scatter) instead of emission directly will give proper alpha (but also adds to rendertime of course):
(Also shouldnt Emission be mixed with transparent if used at all?)
test_Edge_corrected.blend

However I think it was possible to use an Emission shader with Transparent and get alpha when saving to EXR (but honestly cannot get that to work atm)
Correction: this looks like it is expected behavior

In #65993#704571, @brecht wrote:
If you have only emission and zero density, as in this node setup, then alpha is supposed to be zero.

In any case, there was also the issue with seeing this correctly in blenders Image Editor, see e.g. #42755 (Volumes do not appear when transparent film is enabled) and the mother of the Image Editor Issue #52680 (Alpha difference Viewport/F12 )
However part II: this now also seems to work now (I can see the alpha from the volumes in the Image Editor now with my file)... bit confused, will get a coffee :)

Using a proper Volume Shader (Principled Volume / Volume Scatter) instead of emission directly will give proper alpha (but also adds to rendertime of course): (Also shouldnt Emission be mixed with transparent if used at all?) [test_Edge_corrected.blend](https://archive.blender.org/developer/F8281307/test_Edge_corrected.blend) However I think it was possible to use an Emission shader with Transparent and get alpha when saving to EXR (but honestly cannot get that to work atm) Correction: this looks like it is expected behavior > In #65993#704571, @brecht wrote: > If you have only emission and zero density, as in this node setup, then alpha is supposed to be zero. In any case, there was also the issue with seeing this correctly in blenders Image Editor, see e.g. #42755 (Volumes do not appear when transparent film is enabled) and the mother of the Image Editor Issue #52680 (Alpha difference Viewport/F12 ) However part II: this now also seems to work now (I can see the alpha from the volumes in the Image Editor now with my file)... bit confused, will get a coffee :)

Troy Sobotka commented

2020-01-17 15:40:02 +01:00

Added subscriber: @troy_s

Troy Sobotka commented

2020-01-17 15:40:02 +01:00

Viewer is broken. Has been for nearly two decades.

Only associated alpha can encode the values, and the viewer pipeline breaks the association.

Just to be clear, it would be useful to add a composite against a coloured background to show that the alpha is being encoded properly.

This is a known bug though, due to broken alpha handling throughout Blender.

Due to TIFF and other proper file encodings being mishandled, the only way to encode this image is in an EXR if you seek to save this to disk.

Viewer is broken. Has been for nearly two decades. Only associated alpha can encode the values, and the viewer pipeline breaks the association. Just to be clear, it would be useful to add a composite against a coloured background to show that the alpha is being encoded properly. This is a known bug though, due to broken alpha handling throughout Blender. Due to TIFF and other proper file encodings being mishandled, the only way to encode this image is in an EXR if you seek to save this to disk.

Brecht Van Lommel commented

2020-01-17 16:24:57 +01:00

Closed as duplicate of #52680

Brecht Van Lommel closed this issue

2020-01-17 16:24:57 +01:00

Brecht Van Lommel commented

2020-01-17 16:25:32 +01:00

There is indeed a bug in the viewer, but the alpha channel contents is as designed.

Christian F commented

2022-05-01 16:50:00 +02:00

Added subscriber: @Christian-Fritz

Christian F commented

2022-05-01 16:50:00 +02:00

Wait, so having density at 0 and emission at != 0 or only connecting emission shader to volume renders correctly from the looks of it but has alpha = 0 AND this is designed to be like that? ALPHA = DENSITY? I really do hope this is a joke...

Troy Sobotka commented

2022-05-01 18:15:33 +02:00

Alpha is, and always has been, degree of occlusion of the resulting spatial sample. It’s a rasterized representation of geometry. Emission without occlusion is extremely common.

Sign in to join this conversation.

No Label

Download

What's New

Blender Studio

Manual

Developers Blog

Documentation

Benchmark

Blender Conference

Development Fund

One-time Donations

Cycles > Volume > Alpha Channel #73152