Motion blur drops on random frames in Cycles/Eevee when using alembic file #90806

New Issue

Robert Rioux · 2021-08-20T17:16:06+02:00

Robert Rioux commented

2021-08-20 17:16:06 +02:00

System Information
Operating system: macOS-10.15.7-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon Pro Vega 64 OpenGL Engine ATI Technologies Inc. 4.1 ATI-3.10.18

Blender Version
Broken: version: 2.93.2, branch: master, commit date: 2021-08-03 05:58, hash: 1eb06de260
Worked: Probably never

Short description of error
Motion blur drops on random frames in Eevee/Cycles when using alembic file

Exact steps for others to reproduce the error
Please try to render the file in the link with motion blur

#90806.zip

**System Information** Operating system: macOS-10.15.7-x86_64-i386-64bit 64 Bits Graphics card: AMD Radeon Pro Vega 64 OpenGL Engine ATI Technologies Inc. 4.1 ATI-3.10.18 **Blender Version** Broken: version: 2.93.2, branch: master, commit date: 2021-08-03 05:58, hash: `1eb06de260` Worked: Probably never **Short description of error** Motion blur drops on random frames in Eevee/Cycles when using alembic file **Exact steps for others to reproduce the error** Please try to render the file in the link with motion blur [#90806.zip](https://archive.blender.org/developer/F10319126/T90806.zip)

Robert Rioux commented

2021-08-20 17:16:06 +02:00

Added subscriber: @Funnybob

Philipp Oeser commented

2021-08-24 16:17:59 +02:00

Added subscriber: @lichtwerk

Philipp Oeser commented

2021-08-24 16:17:59 +02:00

Changed status from 'Needs Triage' to: 'Needs Developer To Reproduce'

Philipp Oeser commented

2021-08-24 16:17:59 +02:00

Can confirm, same is true for Cylces btw.

This abc has changing vertex count across the frames though and afaict, the alembic does not provide a special .velocities attribute.
Since I am unsure how motionvectors are derrived in this case, would kindly as rendering devs to have a look.

Can confirm, same is true for Cylces btw. This abc has changing vertex count across the frames though and afaict, the alembic does not provide a special `.velocities` attribute. Since I am unsure how motionvectors are derrived in this case, would kindly as rendering devs to have a look.

Philipp Oeser changed title from ~~Motion blur drops on random frames in Eevee when using alembic file~~ to Motion blur drops on random frames in Cycles/Eevee when using alembic file

2021-08-24 16:23:13 +02:00

Robert Rioux commented

2021-08-24 16:29:31 +02:00

You have been able to render this in Cycles with motion blur? I could only get it to work partially in Eevee.

Philipp Oeser commented

2021-08-24 16:41:04 +02:00

In #90806#1210854, @Funnybob wrote:
You have been able to render this in Cycles with motion blur? I could only get it to work partially in Eevee.

This is frame 23 [cropped] with Cycles in 2.93.3 from above zip, yes (but it droppes motionblur on certain frames -- same as Eevee)

> In #90806#1210854, @Funnybob wrote: > You have been able to render this in Cycles with motion blur? I could only get it to work partially in Eevee. This is frame 23 [cropped] with Cycles in 2.93.3 from above zip, yes (but it droppes motionblur on certain frames -- same as Eevee) ![image.png](https://archive.blender.org/developer/F10319257/image.png)

Robert Rioux commented

2021-08-24 17:45:46 +02:00

This must be a fix in 2.93.3 because it doesn't render at all with MB in 2.93.0. I found another strange thing. I imported the same alembic 5 times in my scene and for some reason the 4th one didn't render with MB. That's in Cycles. In Eevee they all have MB. I also tried with the "validate meshes" on and off but I don't think it makes a difference. I couldn't find any information on that option in the documentation. What is it for?

This must be a fix in 2.93.3 because it doesn't render at all with MB in 2.93.0. I found another strange thing. I imported the same alembic 5 times in my scene and for some reason the 4th one didn't render with MB. That's in Cycles. In Eevee they all have MB. I also tried with the "validate meshes" on and off but I don't think it makes a difference. I couldn't find any information on that option in the documentation. What is it for? ![5guys.jpg](https://archive.blender.org/developer/F10319423/5guys.jpg)

Robert Rioux commented

2021-08-25 15:43:55 +02:00

Hummm.... something very strange here. If you duplicate the guy, you lose the MB on both.

Evan Wilson commented

2021-08-25 16:51:47 +02:00

Added subscribers: @kevindietrich, @EAW

Evan Wilson commented

2021-08-25 16:51:47 +02:00

The dropping of the motion blur seems to occur only on frames before or after topology changes.
This is easier to see if you create a shader outputting UVs.

19

20

21

I think you will have to wait for D12305: Modifiers: export motion blur velocity through attribute to land for this to work.

Advantages:

Geometry and particle nodes can output velocity through the same mechanism, or read the attribute coming from earlier modifiers.

The velocity can be preserved through modifiers like subdivision surface or auto smooth.

USD and Alembic previously only output velocity from fluid simulation, now they work with velocity from other sources too.

Simplifies the code for renderers like Cycles and exporters like Alembic and USD.

@kevindietrich said

Tested rendering an Alembic file with motion blur and varying topology, this seems to work fine.

The dropping of the motion blur seems to occur only on frames before or after topology changes. This is easier to see if you create a shader outputting UVs. 19 ![0019.png](https://archive.blender.org/developer/F10325090/0019.png) 20 ![0020.png](https://archive.blender.org/developer/F10325092/0020.png) 21 ![0021.png](https://archive.blender.org/developer/F10325094/0021.png) I think you will have to wait for [D12305: Modifiers: export motion blur velocity through attribute](https://archive.blender.org/developer/D12305) to land for this to work. >Advantages: > * Geometry and particle nodes can output velocity through the same mechanism, or read the attribute coming from earlier modifiers. > * The velocity can be preserved through modifiers like subdivision surface or auto smooth. > * USD and Alembic previously only output velocity from fluid simulation, now they work with velocity from other sources too. > * Simplifies the code for renderers like Cycles and exporters like Alembic and USD. @kevindietrich [said](https://developer.blender.org/D12305#319322) >Tested rendering an Alembic file with motion blur and varying topology, this seems to work fine.

Brecht Van Lommel commented

2021-08-25 17:18:45 +02:00

Added subscriber: @brecht

Brecht Van Lommel commented

2021-08-25 17:18:45 +02:00

Changed status from 'Needs Developer To Reproduce' to: 'Needs User Info'

Brecht Van Lommel commented

2021-08-25 17:18:45 +02:00

You must have either have a fixed topology, or velocities in the Alembic file. Renderers can not figure out motion blur with varying topology. D12305 will not change this requirement.

Cycles and Eevee can render differently depending on the settings, Cycles can look at a wider range of frames and subframes and they all must have the same topology.

If multiple identical meshes render with different motion blur, that is a bug. But I have not been able to reproduce that, we'd need an example .blend file to investigate that.

You must have either have a fixed topology, or velocities in the Alembic file. Renderers can not figure out motion blur with varying topology. [D12305](https://archive.blender.org/developer/D12305) will not change this requirement. Cycles and Eevee can render differently depending on the settings, Cycles can look at a wider range of frames and subframes and they all must have the same topology. If multiple identical meshes render with different motion blur, that is a bug. But I have not been able to reproduce that, we'd need an example .blend file to investigate that.

Robert Rioux commented

2021-08-25 19:39:35 +02:00

The files are too big to upload. I basically took the abc from the link above and imported it on my file. Then duplicated it. I don't understand why the file size gets 10 megs bigger for every guy I link in the Blender file. They are links, alembic files. The Blender file should be tiny.

Robert Rioux commented

2021-08-26 05:31:52 +02:00

So let's say frames 1,2, and 3 are good, 4 is bad, 5 and 6 ok. 4 will probably have the geometry of 5, right? So would it be possible to reconstruct the MV of 4 by extrapolating 5 and 6?

Kévin Dietrich commented

2021-08-26 06:37:05 +02:00

Since the Alembic file does not have velocity information, Cycles "computes" motion blur by storing the meshes for the surrounding frames as well as the current one, and interpolates the meshes during path tracing. For this to work, the topologies for all frames have to match. Computing some motion blur data using some inter- or extrapolation scheme for meshes that do not have a stable topology should be done offline (outside the render engine) as this is heavily dependent on the mesh and how it evolves, and I don't think there is a single technique to achieve this. (I can think of some way to achieve this, by sampling the motion into a volume, and interpolating that back onto the vertices, but again that is not a job for a render engine.)

Philipp Oeser commented

2021-08-26 10:44:38 +02:00

So the missing MB on certain frames is not a bug (since we have changing vertex count across the frames and the alembic does not provide a special .velocities attribute).

I imported the same alembic 5 times in my scene and for some reason the 4th one didn't render with MB

Hummm.... something very strange here. If you duplicate the guy, you lose the MB on both.

If multiple identical meshes render with different motion blur, that is a bug

Since this report already has a bit of (unrelated) history, I think it would be good to open a new, fresh report for the case of missing MB on multiple meshes.
@Funnybob : can you report this separately?

So the missing MB on certain frames is not a bug (since we have changing vertex count across the frames and the alembic does not provide a special .velocities attribute). > I imported the same alembic 5 times in my scene and for some reason the 4th one didn't render with MB > Hummm.... something very strange here. If you duplicate the guy, you lose the MB on both. > If multiple identical meshes render with different motion blur, that is a bug Since this report already has a bit of (unrelated) history, I think it would be good to open a new, fresh report for the case of missing MB on multiple meshes. @Funnybob : can you report this separately?

Robert Rioux commented

2021-08-26 14:18:40 +02:00

Will do thanks

Robert Rioux commented

2021-08-26 16:46:10 +02:00

Bug report done here #90970

I would like to know why the blend file gets so much bigger when you import alembic files? The alembic resides outside the blend file after all. Thanks

Bug report done here #90970 I would like to know why the blend file gets so much bigger when you import alembic files? The alembic resides outside the blend file after all. Thanks

Kévin Dietrich commented

2021-08-27 02:01:18 +02:00

In #90806#1212069, @Funnybob wrote:
I would like to know why the blend file gets so much bigger when you import alembic files? The alembic resides outside the blend file after all. Thanks

This is because actual Blender objects are created upon imports, then a modifier is added which will load and modify the geometry if it is animated. This means that the geometry for the first frame of the animation (or just the geometry if it is static) will be written into the blender file as the original geometry.

> In #90806#1212069, @Funnybob wrote: > I would like to know why the blend file gets so much bigger when you import alembic files? The alembic resides outside the blend file after all. Thanks This is because actual Blender objects are created upon imports, then a modifier is added which will load and modify the geometry if it is animated. This means that the geometry for the first frame of the animation (or just the geometry if it is static) will be written into the blender file as the original geometry.

Philipp Oeser commented

2021-08-27 09:23:11 +02:00

Changed status from 'Needs User Info' to: 'Archived'

Philipp Oeser closed this issue

2021-08-27 09:23:11 +02:00

Philipp Oeser commented

2021-08-27 09:23:11 +02:00

Thx clarifying @kevindietrich .

Since we now have #90970 (Motion blur lost when rendering multiple copies of an alembic ) separately, I will archive this report.

Thx clarifying @kevindietrich . Since we now have #90970 (Motion blur lost when rendering multiple copies of an alembic ) separately, I will archive this report.

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Motion blur drops on random frames in Cycles/Eevee when using alembic file #90806