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Cycles > Volume > Alpha Channel
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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: rB3edd8d5b075a
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)

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

Event Timeline

filip mond (vklidu) updated the task description. (Show Details)
filip mond (vklidu) renamed this task from Cycles > Volumetric > Alpha Channel to Cycles > Volume > Alpha Channel.Jan 15 2020, 11:46 PM
filip mond (vklidu) updated the task description. (Show Details)
Philipp Oeser (lichtwerk) changed the task status from Needs Triage to Needs Developer to Reproduce.Jan 16 2020, 11:10 AM

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?)

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

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. T42755: Volumes do not appear when transparent film is enabled and the mother of the Image Editor Issue T52680: 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 :)

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.

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