Blender Foundation: Blender 2.x BF release: Detail: 23658 Alpha-Values > 1 when rendering with premultiplied alpha (scene-settings), fixed in render-branch.

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[#23658] Alpha-Values > 1 when rendering with premultiplied alpha (scene-settings), fixed in render-branch.

Date:
2010-09-03 10:38 Priority:
3

State:
Closed

Submitted by:
Sebastian König (sebastian_k) Assigned to:
Ton Roosendaal (ton)

Category:
None Status:
Fixed / Closed

Relates to: Duplicates:

Patches:

Summary:
Alpha-Values > 1 when rendering with premultiplied alpha (scene-settings), fixed in render-branch.

Detailed description

When you have an object with raytraced transparency and set the scene-alpha to "Premultiplied" the alpha-pass has values that are greater than 1.
That leads to some errors when you try to work with renderpasses and compositing.
There are workarounds, like cut off the values greater than one with a map-value-node, or multiplying the manually combined passes in the very end with the alpha-pass, but it shouldn't happen nonetheless.
It happens with current SVN and down to previous versions like Blender 2.51.
I'll attach a file with a demo-setup.
Cheers and best regards!

Sebastian

System:
10.6.4, MacPro, GT120, Blender 2.53 (r31719)

Followup

Message

Date: 2010-10-02 09:22
Sender: Campbell Barton
looked into this but it seems to be fixed in the render branch.
assigning to brecht.

Date: 2010-12-15 11:12
Sender: Ton Roosendaal
Stealing bugs from brecht, this one I could do :)

Date: 2010-12-16 12:31
Sender: Ton Roosendaal
This is caused by alpha-hacking in raytracer, we tried to get a bit of alpha stored in raytraced transparency for compositing, even though that's not extremely accurate or useful. I've added an alpha-clamp in the code there.

Attached Files:

Name	Date	Download

premultiplied_alpha_problems.blend	2010-09-03 10:38	Download

Changes:

Field	Old Value	Date	By

status_id	Open	2010-12-16 12:31	ton
close_date	None	2010-12-16 12:31	ton
Status	Investigate	2010-12-16 12:31	ton
assigned_to	blendix	2010-12-15 11:12	ton
Status	New	2010-12-15 11:12	ton
Status	None	2010-12-10 11:56	jesterking
summary	Alpha-Values > 1 when rendering with premultiplied alpha (scene-settings)	2010-10-02 09:22	campbellbarton
assigned_to	none	2010-10-02 09:22	campbellbarton
File Added	12453: premultiplied_alpha_problems.blend	2010-09-03 10:38	sebastian_k