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Maniphest T45583

Adding Glass shaders with different IORs gives different results for Path Tracing and Branched Path Tracing
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Description

System Information
Linux Mint 17.1, 64bit

Blender Version
Broken: 2.75a release (hash c27589e)

Short description of error
Combining three Glass BSDF nodes with different IORs where one is RGB(1,0,0), one is RGB(0,1,0) and one is RGB(0,0,1) with two Add nodes, probably to create a dispersion-like effect, results in different results when rendered with either Branched Path Tracing or regular Path Tracing. Also, modifying the order in which the nodes are added together changes the result.

Exact steps for others to reproduce the error

  • Open the attached .blend file
  • Render
  • Switch to Branched Path Tracing
  • Render on a different slot
  • Compare

A similar .blend file was posted by bubblebobble on #blendercoders, so credit for finding this one goes to him.
Also, I've just assigned this one to me as I'll try to trace down the issue, but thought that posting it here might be a good idea.

PT-BPT-bug.blend580 KBDownload

Revisions and Commits

rC Cycles
D1436rC46bdbb2bd4bf Fix issue in closure merge function
rB Blender
D1436rB4737b1246241 Cycles: Fix issue in closure merge function.

Related Objects

Event Timeline

Lukas Stockner (lukasstockner97) created this task.Jul 27 2015, 11:59 PM
Lukas Stockner (lukasstockner97) claimed this task.
Lukas Stockner (lukasstockner97) raised the priority of this task from to 90.
Lukas Stockner (lukasstockner97) updated the task description. (Show Details)
Lukas Stockner (lukasstockner97) added projects: BF Blender, Cycles.
Lukas Stockner (lukasstockner97) edited a custom field.
Lukas Stockner (lukasstockner97) added a subscriber: Lukas Stockner (lukasstockner97).
Sergey Sharybin (sergey) changed the task status from Unknown Status to Unknown Status.Jul 28 2015, 10:54 AM
Sergey Sharybin (sergey) added a subscriber: Sergey Sharybin (sergey).

First of all, it doesn't seem it is an energy conserving shader? So it's possible that branched path tracing will give different result, because it evaluates all the closures instead of picking random one.

Modifying order of shaders changes which exact closure controls whether ray is reflected or transmitted, changing overall look of the render result.

Don't really think you can solve this issue, non-conserving energy is usually asking for such kind of artifacts. However if you're still looking into the issue and want some ground-truth image, you can split the scene into three and render R,G,B channels separately in them and them merge in compositor (you'll need to use single Glass shader in each of the scenes).

Lukas Stockner (lukasstockner97) mentioned this in D1436: Cycles: Only merge closures when data2 is equal.Jul 28 2015, 9:03 PM
Thomas Dinges (dingto) changed the task status from Unknown Status to Resolved by committing rB4737b1246241: Cycles: Fix issue in closure merge function..Jul 29 2015, 10:06 PM
Thomas Dinges (dingto) added a commit: rB4737b1246241: Cycles: Fix issue in closure merge function..
Sergey Sharybin (sergey) added a commit: rC46bdbb2bd4bf: Fix issue in closure merge function.Jul 30 2015, 11:35 AM