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Refraction Crash/Extremely Slow
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Description

System Information
Microsoft Windows 7 Home Premium
Intel 4700-MQ
NVIDIA 780M
16GB 1600MHz RAM

Blender Version
Broken: 2.70, 2.71RC2
Worked: None yet.
Settings:

While following the Pancake Hobo tutorial on Blender Cookie, I was following along with my GPU as render device, using fake SSS shaders. However, near the end, after adding a refraction shader as part of the material for the syrup, I ran into problems. The render crashed because of a timeout, and when I edited the registry setting that controls it, the render was taking a very long time - 5 samples per second on the white background.

1: Open the file, enter viewport render. Notice that it works well enough.
2: Attempt an f12 render, using CUDA. Blender will likely crash due to timeout, and if overriden (as I have done), notice the slow render speeds.
3: Enter the syrup object material, and enter the Refractive Absorption nodegroup. Try removing the Light Path node; no change (there was not long ago a bug that affected using Light Path in materials with the GPU). Replace the refraction shader with a Transluscent shader, inputting the color with the Light Path node on the other end of the string.
4: Now, try another f12 render. Notice that it has sped up considerably, with no crashes.

Notes:
-The SSS shaders for the other objects are faked with a diffuse and two transluscent shaders each.
-When rendering with No Causics enabled, and the refraction shader in place, the render goes back to a decent speed.
-All textures are Creative Commons.
-The attached file is large, but the only one where I can easily recreate the issue.

https://docs.google.com/uc?id=0Bx0YaKfeuWSrd3BMbUttU05fY1E&export=download

Details

Type
Bug

Event Timeline

Sofus Rose (Darkfie9825) set Type to Bug.
Sofus Rose (Darkfie9825) created this task.
Sofus Rose (Darkfie9825) raised the priority of this task from to Needs Triage by Developer.
Brecht Van Lommel (brecht) claimed this task.

Unfortunately this is an old issue with no easy fix. It's considered a To Do item still.
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/GPU_Rendering#The_NVIDIA_OpenGL_driver_lost_connection_with_the_display_driver

If you've got many bounces, GPU rendering can take too long and interfere with the display driver. The workaround for now is to use smaller tile sizes and fewer bounces. Tricks with the Light Path node or setting No Caustics will lower the effective number of bounces.