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Cycles misses intersections when objects are too close
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System Information
macOS 10.13.4

Blender Version
Broken: 2.79b

Short description of error
When two objects are too close, Cycles can miss intersections.

Exact steps for others to reproduce the error
Open the attached .blend file and render.
There are two intersecting semitransparent planes in this file. Their transparencies are set so that any ray passing through both planes gets uniform attenuation. A ray that passes only one plane gets either a green or magenta tint.
Note how the line where the planes intersect has a coloured border.



Event Timeline

The source of the problem is the use of ray_offset() to prevent self-intersections for rays that originate from a transparent surface. A solution to this problem is outlined in the paper "Robust Iterative Find-Next-Hit Ray Traversal":

Philipp Oeser (lichtwerk) triaged this task as Normal priority.Jun 5 2018, 12:47 PM