Maniphest T101818

Viewer node doesn't display correctly added Alpha channel in Combine RGBA node
Closed, Archived
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Assigned To
None
Authored By
Corrado Piscitelli (GigiSpligi)
Oct 14 2022, 11:20 AM
Tags
  • BF Blender
Subscribers
Corrado Piscitelli (GigiSpligi)
Ivan Cappiello (icappiello)
Jeroen Bakker (jbakker)
Omar Emara (OmarSquircleArt)

Description

System Information
Operating system: macOS-12.6-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon Pro Vega 64 OpenGL Engine ATI Technologies Inc. 4.1 ATI-4.8.101

Blender Version
Broken: version: 3.3.1, branch: master, commit date: 2022-10-04 18:35, hash: rBb292cfe5a936
Worked: (N/A)

Short description of error
When combining a separate RGBA and a combine RGBA nodes to manually input a different alpha channel on an image, the resulting expected output from the viewer node connected to combine RGBA node should be RGB from input A and alpha from input B, but it seems instead the inputed alpha is not processed correctly or ignored.
Alpha seems to be applied correctly when more compositing is applied afterwards or image is saved to a file (png).

Expected result

Actual result

Exact steps for others to reproduce the error
Open the attached blender file, click render and look at the viewer node in the compositor.

combine_rgba_b331.blend876 KBDownload

Checked in 2.93 it seems to also work in the same way. Included a 2.93 version for you to see.
combine_rgba_b293.blend874 KBDownload

Related Objects

Event Timeline

Corrado Piscitelli (GigiSpligi) created this task.Oct 14 2022, 11:20 AM
Corrado Piscitelli (GigiSpligi) added subscribers: Ivan Cappiello (icappiello), Jeroen Bakker (jbakker).
Omar Emara (OmarSquircleArt) closed this task as Archived.Oct 14 2022, 12:53 PM
Omar Emara (OmarSquircleArt) added a subscriber: Omar Emara (OmarSquircleArt).

This is just how Blender views zero alpha when the pixel has color data, see for instance T81199. But the actual data is correct.
To get the result you expect, just premultiply the alpha, which is what happens when you save the image.