Multires Rebuild Subdivision shrinks shape #76302

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce RTX 2070 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 442.92

Blender Version
Broken: version: 2.90 (sub 0), branch: master, commit date: 2020-05-01 14:06, hash: 60741cfe18
Worked: (newest version of Blender that worked as expected)

*Short description of error
Multires Rebuild Subdivision shrinks shape

Exact steps for others to reproduce the error
Create cube
Create multires with 2 levels and apply
Create multires and Rebuild subdivision
Start and rebuild shapes are different

Same for simple subdivision modifier

Added subscriber: @borschberry

Added subscriber: @Regnas

Indeed.

Added subscribers: @ankitm, @PabloDobarro

Changed status from 'Needs Triage' to: 'Confirmed'

Added subscriber: @nudelZ

press "apply base" after that operation the new base mesh should roughly be the same as the previous one (its not 100% perfect..)

Much better, but the differences are still significant. Moreover, this is a cube example where everything should work well

Thanks a lot for the tip though.

Added subscriber: @Andruxa696

I think it is not correct to look as an example always on cube. Here is I did some comparison on more real examples.
For me in 90% rebuild subdivision works better then apply base , it try to be more close to high poly model then apply base. For example look on lips - in apply base lips getting too much close to each other and overlapping too much.

on hard surface rebuild subdivision also works a little bit better, but in more specific some cases apply base works better.
@borschberry rebuild subdivision task is to make level 0 and other levels as close to highpoly as possible, not to make original cube. I am artist, not developer, but in my opinion it is works as it should, the same way as it works in Zbrush.

It's not about examples at all. The question is whether the algorithm works correctly or not. The cube is the simplest example, so I use it as a volume preservation marker (it's not about saving the shape, it's about saving the volume first).

@Andruxa696 As an artist, you may be satisfied with the result, but I am not, it does not matter. If the algorithm is not working correctly, a small error can accumulate and lead to unpredictable results.

Added subscriber: @ebarranco

it looks like the problem may be in the new "limit surface" feature in the subdivision algorithm.

Added subscriber: @Debuk

This is no bug and there is no danger that this is introducing problems. If the algorithm finds patches that can be evenly unsubivided it simply takes the position of the "remaining points" of the finegrained mesh itself. This is just the simplest solution for positioning the basemeshes points. Technically this is as valid as any other position and is rather a safe choice. The only thing that decides here about right or wrong is if the baseshape and the displacement combine to be the original smooth input surface.

It might be worth to think about if this should not have the bigger dimensions of a plain subdiv basemesh/controlcage though.
But it's no bug.

Added subscriber: @Frozen_Death_Knight

@borschberry This is definitely not a bug and @Andruxa696 is completely right about Rebuild Subdivisions producing the most correct results for sculpting.

Pablo Dobarro just uploaded a patch where he changed the Apply Base algorithm to the Rebuild Subdivisions/Unsubdivide algorithm and made the current algorithm into a separate button called Refit Base. After having tested 2.90 for the last week and having previous ZBrush experience, this is the way forward as far as I'm concerned. Current Apply Base is bad for what sculptors need in the vast majority of cases and I hope the new Pablo patch gets added asap.

Changed status from 'Confirmed' to: 'Archived'

I also don't think this is a bug. I made unsubdivide work this way to make the Delete Lower operator. This way the resulting base mesh will always match the positions of the displacement. To improve this it is necessary to do a refit step after calculating the new base mesh and it will work like the current Apply Base. To make this more clear, I made a patch to split Apply Base into two separate buttons, one that refits (which inflates the mesh) and one that does not refit, matching the vertex positions of the displacement (like rebuild subdivisions) D7644

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Multires Rebuild Subdivision shrinks shape #76302