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Cycles: MIS is broken in branched pathtracing integrator
Confirmed, NormalPublicBUG

Description

System Information
Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 431.70

Blender Version
Broken: version: 2.81 (sub 13), branch: master, commit date: 2019-10-04 22:33, hash: rBab519b91b2c4
Worked: 2.79b release

Short description of error
Enabling MIS in branched PT introduces a lot of noise in every release after 2.79b.

Example picture of the Agent demo:

2.79b: https://imgur.com/a/rATmFQy

2.81 Alpha: https://imgur.com/a/uaJ34FV

Exact steps for others to reproduce the error
Download for example "barbershop_interior_cpu.blend" from https://www.blender.org/download/demo-files/, render with a current build in BPT with MIS on and compare it to the much cleaner 2.79b result.

Event Timeline

anu manu (anaho) added a comment.EditedDec 6 2019, 3:37 PM

Demo file Junk shop @ (BPT: 12²AA/1/1/1/1/1/1/1):

BPT MIS ON 2.79b: https://imgur.com/a/trPE6P1

BPT MIS ON 2.81: https://imgur.com/a/YUVtIsU

BPT MIS OFF 2.81: https://imgur.com/a/uo9xehy

While MIS ON still reduces glossy noise in BPT mode, it introduces general noise in every release after 2.79b. Turning MIS in BPT off significantly reduces general noise but of course suffers glossy noise due to lack from MIS.

I cant stress enough how bad of a regression this is in term of render times from 2.79b.
I have time on the weekend left. Is there a way to download older buildbot builds and check when exactly that bug was introduced?

There dont seem to be archives of daily builds from buildbot.
Rundown so far:
-The problem didn´t occur in 2.79b release, is present in the last 2.79 nightly build and was present in 2.79 nightly at least by December 2018

  • The problem is independent of OS used and happens on both CPU and GPU devices
  • The problem happens regardless of Lamp type (tested Point and Area)
  • Applying the scale of lamps didnt address the issue
  • Changing BVH type or CPU feature set also didnt do anything about the problem
Richard Antalik (ISS) changed the task status from Needs Triage to Needs Information from User.Jan 14 2020, 1:06 PM

@anu manu (anaho) Is this still an issue with latest build? https://builder.blender.org/download/

I tested with latest 2.83 Alpha and situation is unchanged from my first report.
Having MIS on with BPT increases noise levels.
Turning MIS off reduces overall noise but of course suffers from heavy glossy noise.
Only "workaround" is using 2.79b where BPT worked as expected.

BPT/all lamps on 1 sample/ MIS ON/ 12²AA everything else at 1 sample:

2.79b: https://imgur.com/a/ErsJXOO

2.83 Alpha: https://imgur.com/a/LGxukJk

Richard Antalik (ISS) changed the task status from Needs Information from User to Confirmed.Jan 15 2020, 11:08 AM

@anu manu (anaho) thanks for update.

Bastien Montagne (mont29) changed the subtype of this task from "Report" to "Bug".Jan 15 2020, 4:24 PM
anu manu (anaho) added a comment.EditedJul 24 2020, 8:54 AM

Still broken with current builds.
The bug was introduced with one of the perfomance patches after the 2.79b release, as 2.79b renders fine but the last build glitches.

MIS ON:https://imgur.com/a/wl2F3gI

MIS OFF:https://imgur.com/a/m6HkZev

Shift select all lights and hold down alt click the MIS option for quick toggling MIS settings on all lights.

I just tested the scene in a 2.80 Alpha dated 14.03.2018 and the bug was already present at that time.
So it was introduced between 11.09.2017-14.03.2018.