Adding Glass shaders with different IORs gives different results for Path Tracing and Branched Path Tracing #45583

New Issue

Lukas Stockner · 2015-07-27T23:59:20+02:00

Lukas Stockner commented

2015-07-27 23:59:20 +02:00

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.blend

**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.blend](https://archive.blender.org/developer/F213567/PT-BPT-bug.blend)

Lukas Stockner commented

2015-07-27 23:59:20 +02:00

Changed status to: 'Open'

Lukas Stockner self-assigned this 2015-07-27 23:59:20 +02:00

Lukas Stockner commented

2015-07-27 23:59:20 +02:00

Added subscriber: @LukasStockner

Sergey Sharybin commented

2015-07-28 10:54:29 +02:00

Added subscriber: @Sergey

Sergey Sharybin commented

2015-07-28 10:54:29 +02:00

Changed status from 'Open' to: 'Archived'

Sergey Sharybin closed this issue

2015-07-28 10:54:29 +02:00

Sergey Sharybin commented

2015-07-28 10:54:29 +02:00

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).

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).

blender-admin commented

2015-07-29 22:04:36 +02:00

This issue was referenced by 4737b12462

This issue was referenced by 4737b1246241fd575921f93ddf229da656c7fb5a

Thomas Dinges commented

2015-07-29 22:06:08 +02:00

Changed status from 'Archived' to: 'Resolved'

blender-admin commented

2015-07-30 10:37:26 +02:00

This issue was referenced by blender/cycles@46bdbb2bd4

This issue was referenced by blender/cycles@46bdbb2bd4bf82513699869859510ce58958a17c

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Adding Glass shaders with different IORs gives different results for Path Tracing and Branched Path Tracing #45583