Subdivided Cube with multi-res is slow, subdivided Suzanne is not. #77081

Closed
opened 2020-05-26 10:40:14 +02:00 by Steve White · 12 comments

System Information
Operating system: Windows 10 Pro
Graphics card: 1080ti

Blender Version
Broken: 2.9, 2.8x

Short description of error
Starting with a (default) cube, adding a multirez modifier slows down with as little as 2 or 3 million polygons while other objects subdivide to much greater numbers without the slowdown.

Exact steps for others to reproduce the error
With the default cube selected, add a multires modifier and subdivide the cube from the multires panel until you have 6,291, 456 faces
Switch to sculpt mode.
Try sculpting. It is incredibly slow.
Now delete the subdivided cube and add a Suzanne Monkey.
Again add the multires modifier to Suzanne and subdivide until you have 8,060,928 faces.
Switch to sculpt mode. Even with 2 million more polys, sculpting performance is much higher with much less lag.

Based on the default startup or an attached .blend file (as simple as possible).
SlowCube.blend

**System Information** Operating system: Windows 10 Pro Graphics card: 1080ti **Blender Version** Broken: 2.9, 2.8x **Short description of error** Starting with a (default) cube, adding a multirez modifier slows down with as little as 2 or 3 million polygons while other objects subdivide to much greater numbers without the slowdown. **Exact steps for others to reproduce the error** With the default cube selected, add a multires modifier and subdivide the cube from the multires panel until you have 6,291, 456 faces Switch to sculpt mode. Try sculpting. It is incredibly slow. Now delete the subdivided cube and add a Suzanne Monkey. Again add the multires modifier to Suzanne and subdivide until you have 8,060,928 faces. Switch to sculpt mode. Even with 2 million more polys, sculpting performance is much higher with much less lag. Based on the default startup or an attached .blend file (as simple as possible). [SlowCube.blend](https://archive.blender.org/developer/F8555231/SlowCube.blend)
Author

Added subscriber: @Massivetree

Added subscriber: @Massivetree

Added subscriber: @VertexSupa

Added subscriber: @VertexSupa

It always had this problem. It's not just about the polycount,. The more subdivision levels you have, the more slower it becomes, it doesn't matter if your model is still low poly.
Multires needs some serious optimizations, and a function to delete the lower subdivisions, and maybe then the multires will be workable.

It always had this problem. It's not just about the polycount,. The more subdivision levels you have, the more slower it becomes, it doesn't matter if your model is still low poly. Multires needs some serious optimizations, and a function to delete the lower subdivisions, and maybe then the multires will be workable.

Added subscriber: @iss

Added subscriber: @iss

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

Changed status from 'Needs Triage' to: 'Archived'
Richard Antalik self-assigned this 2020-05-27 16:08:47 +02:00

This seems to be covered by #73317 so I will close this report. If you want to monitor progress, please subscribe to that task

This seems to be covered by #73317 so I will close this report. If you want to monitor progress, please subscribe to that task

Added subscriber: @mano-wii

Added subscriber: @mano-wii

A guess for the slowness is how the BVH is configured to the shape of the objects.
To configure vertices within BVH nodes the Cube gets a single BVH node, while suzanne have many BVH nodes.
Since the final vertex count is similar in both cases, the suzanne case will have less vertices per node, emaning, brush stroke is more efficient since it can discard lot of vertices much early on.

A guess for the slowness is how the BVH is configured to the shape of the objects. To configure vertices within BVH nodes the Cube gets a single BVH node, while suzanne have many BVH nodes. Since the final vertex count is similar in both cases, the suzanne case will have less vertices per node, emaning, brush stroke is more efficient since it can discard lot of vertices much early on.

In #77081#939843, @mano-wii wrote:
A guess for the slowness is how the BVH is configured to the shape of the objects.
To configure vertices within BVH nodes the Cube gets a single BVH node, while suzanne have many BVH nodes.
Since the final vertex count is similar in both cases, the suzanne case will have less vertices per node, emaning, brush stroke is more efficient since it can discard lot of vertices much early on.

So this means that there is nothing that can be done about it?

> In #77081#939843, @mano-wii wrote: > A guess for the slowness is how the BVH is configured to the shape of the objects. > To configure vertices within BVH nodes the Cube gets a single BVH node, while suzanne have many BVH nodes. > Since the final vertex count is similar in both cases, the suzanne case will have less vertices per node, emaning, brush stroke is more efficient since it can discard lot of vertices much early on. So this means that there is nothing that can be done about it?

Added subscriber: @Frozen_Death_Knight

Added subscriber: @Frozen_Death_Knight

One reason as to why I think this is happening is because the density of the polygons are much higher on the cube than on the Suzanne model. It is just a general observation I picked up on several models whether they are using MultiRes or not. For instance, a human sized model can have the same amount or way more polygons than a head bust, but the head bust has a much higher density than the human sized model, thus leading to severe slowdowns. So a brush in general will move around much more polygons per brush stroke depending on the density of the model rather than the overall poly count.

One reason as to why I think this is happening is because the density of the polygons are much higher on the cube than on the Suzanne model. It is just a general observation I picked up on several models whether they are using MultiRes or not. For instance, a human sized model can have the same amount or way more polygons than a head bust, but the head bust has a much higher density than the human sized model, thus leading to severe slowdowns. So a brush in general will move around much more polygons per brush stroke depending on the density of the model rather than the overall poly count.

Removed subscriber: @Frozen_Death_Knight

Removed subscriber: @Frozen_Death_Knight
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Reference: blender/blender#77081
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