Blender Foundation: Blender 2.x BF release: Detail: 31935 Two equivalent shader which use "add shader" yield different results

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[#31935] Two equivalent shader which use "add shader" yield different results

Date:
2012-06-26 12:22 Priority:
3

State:
Closed

Submitted by:
Yasuhiro Fujii (y-fujii) Assigned to:
Brecht Van Lommel (blendix)

Category:
Rendering: Cycles Status:
Todo / Closed

Relates to: Duplicates:

Patches:

Summary:
Two equivalent shader which use "add shader" yield different results

Detailed description

The two equivalent shaders described below yield different results.
The bug is reproduced on both 2.63a and r48298 SVN, with CPU rendering.

The shader at the left of the attached image:
AddShader {
GlassBSDF{ RGB = 0.8, 0.0, 0.8 }
GlassBSDF{ RGB = 0.0, 0.8, 0.0 }
}

Right:
GlassBSDF{ RGB = 0.8, 0.8, 0.8 }

Followup

Message

Date: 2012-06-29 11:11
Sender: Sergey Sharybin
Brecht, could you please look on this? Thanks!

Date: 2012-07-02 12:12
Sender: Yasuhiro Fujii
Additional information:

0. This problem seems to be independent from following condition.
- IOR value
- bounce limit
- connection order of the add shader input.

1. Mix shader also has this problem.

2. Glossy BSDF also has this problem.
- Glass BSDF, sharp distribution: incorrect, magenta-tinted (attached image)
- Glossy BSDF, sharp distribution: incorrect, magenta-tinted but incorrect region is small.
- (Glass|Glossy) BSDF, (Beckmann|GGX) distribution, roughness = 0.0: incorrect, green-tinted
- (Glass|Glossy) BSDF, (Beckmann|GGX) distribution, roughness = 0.01: correct, good convergence

Is this bug due to numerical instability?

Date: 2012-09-28 17:20
Sender: Brecht Van Lommel
Looked into this, and it's actually noise that you are seeing here. If you see the minimum bounces higher, it will converge faster and you can see it becoming less purple, but still very slow. The high IOR and number of bounces mean that the path tracer has difficulty finding all paths, this would work better with e.g. a bidirectional path tracer.

There might be a way to make the sampling more clever here, and in this simple case we could do an optimization to merge the two closure, but would not consider this a bug, rather a todo item.

Attached Files:

Name	Date	Download

equiv-shaders.png	2012-06-26 12:22	Download
equiv-shaders.blend	2012-06-26 12:22	Download

Changes:

Field	Old Value	Date	By

status_id	Open	2012-09-28 17:20	blendix
close_date	None	2012-09-28 17:20	blendix
Status	New	2012-09-28 17:20	blendix
assigned_to	none	2012-06-29 11:11	nazgul
File Added	21268: equiv-shaders.blend	2012-06-26 12:22	y-fujii
File Added	21267: equiv-shaders.png	2012-06-26 12:22	y-fujii