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The rendering result of hair is diffirent in the diffirent version!!!
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Description

System Information
Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce GTX 1060 6GB/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 417.01

Blender Version
Broken: version: 2.80 (sub 74), branch: master, commit date: 2019-07-10 10:48, hash: rBac8c795429f9
Worked: (optional)

Short description of error


[Hi,there is a question.The rendering result of hair is diffirent in the diffirent version(Blender2.79 and Blender2.80).I really hope you can fix this bug.Because many projects of Blender2.79 need to rendered in 2.80. But now we can’t because this bug. (T_T)]

Exact steps for others to reproduce the error
[Please describe the exact steps needed to reproduce the issue]
[Based on the default startup or an attached .blend file (as simple as possible)]

Details

Type
Bug

Event Timeline

I assume this is because of the more efficient Russian roulette termination from rBcd023b6cecb7.
In above commit Min Bounces was removed.
It has been brought back though rBc9238e638fd5 as Min Light Bounces / Min Transparent Bounces.

You can find these under Properties Editor > Render > Sampling > Advanced

Does setting this/these to something like Min Bounces in 2.79 give you a closer result?
(cannot render atm., but can check later, too...)

Philipp Oeser (lichtwerk) triaged this task as Confirmed, Medium priority.Jul 10 2019, 10:52 AM

I assume this is because of the more efficient Russian roulette termination from rBcd023b6cecb7.
In above commit Min Bounces was removed.
It has been brought back though rBc9238e638fd5 as Min Light Bounces / Min Transparent Bounces.
You can find these under Properties Editor > Render > Sampling > Advanced
Does setting this/these to something like Min Bounces in 2.79 give you a closer result?
(cannot render atm., but can check later, too...)

Forget about this comment, I now have it rendered and this is not the look, it is about the particle distribution...
Will confirm on first sight, checking...

Note that the particle distribution random number generator was intentionally changed in 2.80.

Made a couple of test (without looking at code -- it might very well be the RNG @Brecht Van Lommel (brecht) was talking about): seems like only interpolated children are affected, simple children with all kinds of randomness seem the same in 2.79 and 2.8.

[which makes T63534 come to my mind -- might be unrelated though]

Note that the particle distribution random number generator was intentionally changed in 2.80.

Is there any solution? This problem has caused us a lot of trouble.

Brecht Van Lommel (brecht) claimed this task.

No, there is no way to get the exact same result in 2.79 and 2.80 with child particles unfortunately. Further I can't see a bug here.