Cycles refraction seems wrong compared to BI or Vray #39761

Closed
opened 2014-04-16 22:34:35 +02:00 by sarazin jean francois · 8 comments

System Information
Operating system and graphics card
Win 7 64 bits, gforce 770

Blender Version
Broken: (example: 2.69.7 4b206af, see splash screen)
Worked: (optional)
official 2.70 and last nightly blender-2.70-b179647-win64

Short description of error
When I render glass with cycles, result seems wrong. Refraction seems to lack of detail, compared to Vray, Yafaray or even Blender internal... So I made a test file with a pure refraction shader and I can confirm this. As if refraction bounces was kind of limited (my test was in full illumination mode)
By the way, I've got another little issue with cycles with CPU in viewport. Some part of refractive objects are black, if you choose GPU, it's right, or if you laucn a real render, but CPU in viewport does strange things. Maybe this is correlated.

cycles.jpg

vray.jpg

BI.jpg

Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps

hit render

cycles.blend

BI.blend

**System Information** Operating system and graphics card Win 7 64 bits, gforce 770 **Blender Version** Broken: (example: 2.69.7 4b206af, see splash screen) Worked: (optional) official 2.70 and last nightly blender-2.70-b179647-win64 **Short description of error** When I render glass with cycles, result seems wrong. Refraction seems to lack of detail, compared to Vray, Yafaray or even Blender internal... So I made a test file with a pure refraction shader and I can confirm this. As if refraction bounces was kind of limited (my test was in full illumination mode) By the way, I've got another little issue with cycles with CPU in viewport. Some part of refractive objects are black, if you choose GPU, it's right, or if you laucn a real render, but CPU in viewport does strange things. Maybe this is correlated. ![cycles.jpg](https://archive.blender.org/developer/F85018/cycles.jpg) ![vray.jpg](https://archive.blender.org/developer/F85019/vray.jpg) ![BI.jpg](https://archive.blender.org/developer/F85020/BI.jpg) **Exact steps for others to reproduce the error** Based on a (as simple as possible) attached .blend file with minimum amount of steps hit render [cycles.blend](https://archive.blender.org/developer/F85017/cycles.blend) [BI.blend](https://archive.blender.org/developer/F85016/BI.blend)

Changed status to: 'Open'

Changed status to: 'Open'

Added subscriber: @sarazinjean-francois

Added subscriber: @sarazinjean-francois

Forgotten to say : images in order are from : Cycles - Vray and BI. All material are pure refraction with an ior of 1.5. Just one light, no shadow.

Forgotten to say : images in order are from : Cycles - Vray and BI. All material are pure refraction with an ior of 1.5. Just one light, no shadow.

Added subscriber: @mont29

Added subscriber: @mont29
Brecht Van Lommel was assigned by Bastien Montagne 2014-04-16 23:12:58 +02:00

It looks ok to me, but see the description here, a refraction BSDF by itself is not so useful.
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Nodes/Shaders#Refraction

You probably want to use the Glass BSDF to get something comparable, a lot of the detail comes from the interaction between refraction and reflection. Also note that the number of glossy and transmission bounces in the light paths panel influences the result.

As for CPU and GPU differences, for me the render results look basically the same with this file.

It looks ok to me, but see the description here, a refraction BSDF by itself is not so useful. http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Nodes/Shaders#Refraction You probably want to use the Glass BSDF to get something comparable, a lot of the detail comes from the interaction between refraction and reflection. Also note that the number of glossy and transmission bounces in the light paths panel influences the result. As for CPU and GPU differences, for me the render results look basically the same with this file.

you're right, with a glass shader it works. This is strange, I thought Blender Internal, or Vray, compute no reflections in its refraction, when reflection are disabled or completly dark. Anyway, no I know I shouldn't use refraction alone. And for the CPU/GPU issue it only happens in viewport, not in render, and only with refraction shader.

you're right, with a glass shader it works. This is strange, I thought Blender Internal, or Vray, compute no reflections in its refraction, when reflection are disabled or completly dark. Anyway, no I know I shouldn't use refraction alone. And for the CPU/GPU issue it only happens in viewport, not in render, and only with refraction shader.
Member

Added subscriber: @gandalf3

Added subscriber: @gandalf3
Member

Here's a comparison of CPU and GPU (GTX 460) in the viewport (using the attached file):

GPU

examp442.png

CPU

examp443.png

The final render looks like the GPU.

Here's a comparison of CPU and GPU (GTX 460) in the viewport (using the attached file): **GPU** ![examp442.png](https://archive.blender.org/developer/F85702/examp442.png) **CPU** ![examp443.png](https://archive.blender.org/developer/F85704/examp443.png) The final render looks like the GPU.
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Reference: blender/blender#39761
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