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Cycles refraction seems wrong compared to BI or Vray
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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.

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

Event Timeline

sarazin jean francois (dddjef) raised the priority of this task from to 90.
sarazin jean francois (dddjef) updated the task description. (Show Details)
sarazin jean francois (dddjef) edited a custom field.

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.

Bastien Montagne (mont29) lowered the priority of this task from 90 to Normal.

It looks ok to me, but see the description here, a refraction BSDF by itself is not so useful.

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.

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



The final render looks like the GPU.