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Strand combing corrupts after F12 render (cycles)
Closed, ResolvedPublic

Description

System Information
Linux 64

Blender Version
Broken: 2.77a (official build) and 7ac126e (from builder.blender.org)

Short description of error

Particle strands combing gets corrupted after an F12 render

Exact steps for others to reproduce the error


Press Shift Z and let render preview. That is the correct result
Stop preview and press F12 for a tiled render. Shows incorrect result
Esc and press Shift Z again for incorrect result. Combing has now been corrupted


EDIT: This just got weirder. This is a new particle system created from scratch. It has never been combed!
Press Shift Z and let render preview. Notice the correct length of the strands.
Stop preview and press F12 for a tiled render.
Esc and press Shift Z again. Now strands are much shorter in the preview!

Event Timeline

Daniel Salazar (zanqdo) updated the task description. (Show Details)
Daniel Salazar (zanqdo) raised the priority of this task from to Needs Triage by Developer.
Daniel Salazar (zanqdo) set Type to Bug.
Daniel Salazar (zanqdo) updated the task description. (Show Details)
Daniel Salazar (zanqdo) updated the task description. (Show Details)

There are 2 slightly different issues here:

  1. When switching to F12 render out of the viewport render, the strands lose their edit during the render. It seems that this is a really ancient bug, goes back to at least 2.70a (before that it becomes difficult to test with this file because Cycles support for hair and viewport is incomplete in older versions).
  1. When switching back to viewport after the F12 render, the hairs would at least regain their edited shape until fairly recent versions (bug introduced somewhere between 2.74 and 2.76b).

I think part of the problem is that the file has ParticleSettings.render_step set to 0

If I set it to 3 it gets better, at least it doesn't corrupt the combing, but still rendering is really weird.

Edit: It seems rendering corrupts the particle system even without combing! After render the strands get shorter on preview rendering. Added new test file in initial description

I'm using the newest version of Blender and this bug is still persistent.

Update: This is due to rendering with a different level of subdivision surface than the hair was combed at. Hair must be combed with the level of subdivision surface that it will be rendered at.