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Hair density texture map, low render performance in cycles
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Description

System Information
Operating system: window 10
Graphics card: 2080 Ti

blender 2.93.00

Worked: dont know when it worked

Low render performance in cycles when use density map for hair. both viewport and offline render.

  1. Open the attached file and first turn of and turn on and of interpolated children
  2. Hit viewport render and rotate the view.
  3. Go into render texture and disable density influence. set it to length{F10208719}
  4. see the difference in render performance

Test File:

Event Timeline

Richard Antalik (ISS) changed the task status from Needs Triage to Needs Information from User.Jul 8 2021, 8:43 AM

Can't reproduce this issue on CPU.

@Daniel Westlund (dawe) is performance different on GPU only?

I can reproduce it on CPU. But its most noticable in the viewport. I get a halt every time when trying to rotate the viewport that can freez the viewport for up to a fiew seconds, campared to when I turn of the density or set it to length, as I then get instant feedback in the rendered viewport
Its alot more significant on my gpu though.
Tried it on two systems.
Rtx 2080 ti and mobile rtx 3070
Dont forget to turn of and on the children, as its a nother bug that decreases the children on file reopening.

Philipp Oeser (lichtwerk) changed the task status from Needs Information from User to Confirmed.Jul 8 2021, 10:09 AM

Can confirm a slowdown, but this is due the bug T89598: Hair density maps increases hair children amount on reopening scene file (or rendering which will get slow).
You just end up with many more strands because of that, resulting in the slowdown.
Will merge these reports.