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Rendering multiple deformation motion blur steps with "velocity" attribute is wrong and can cause memory problems #116792

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opened 2024-01-04 17:15:10 +01:00 by Steffen Dünner · 7 comments
Steffen Dünner commented 2024-01-04 17:15:10 +01:00
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System Information
Operating system: Linux-6.5.0-14-generic-x86_64-with-glibc2.35 64 Bits, X11 UI
Graphics card: NVIDIA GeForce RTX 2070 Super/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 535.129.03

Blender Version
Broken: version: 4.1.0 Alpha, branch: main, commit date: 2024-01-04 15:38, hash: 29ed7a69e6e3

Short description of error
Rendering point clouds with a velocity attribute now finally gives proper motion blur for particles / points.
But setting the motion blur steps > 1 causes problems like artefacts and crashes due to memory consumption.

Exact steps for others to reproduce the error

  • Open the attached blend file. It's a freeze frame of an Alembic with motion vectors on points. In the end it's a point cloud with a "velocities" attribute.
  • Press F12 to render the point cloud with motion blur. All is fine.
    image
  • Now up the "Deformation Steps" to something > 1
    image
    The rendering takes a lot longer and looks "wrong".
  • Now up the steps to e.g. 7 and re-render: crash!
**System Information** Operating system: Linux-6.5.0-14-generic-x86_64-with-glibc2.35 64 Bits, X11 UI Graphics card: NVIDIA GeForce RTX 2070 Super/PCIe/SSE2 NVIDIA Corporation 4.6.0 NVIDIA 535.129.03 **Blender Version** Broken: version: 4.1.0 Alpha, branch: main, commit date: 2024-01-04 15:38, hash: `29ed7a69e6e3` **Short description of error** Rendering point clouds with a velocity attribute now finally gives proper motion blur for particles / points. But setting the motion blur steps > 1 causes problems like artefacts and crashes due to memory consumption. **Exact steps for others to reproduce the error** - Open the attached blend file. It's a freeze frame of an Alembic with motion vectors on points. In the end it's a point cloud with a "velocities" attribute. - Press F12 to render the point cloud with motion blur. All is fine. ![image](/attachments/6bbda0cd-643f-4605-a6e1-f2a65cfad100) - Now up the "Deformation Steps" to something > 1 ![image](/attachments/626587e2-e250-46ca-bf59-35a125b214a9) The rendering takes a lot longer and looks "wrong". - Now up the steps to e.g. 7 and re-render: crash!
BUG_MotionBlur_v01.blend
297 KiB
image.png
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image.png
1.1 MiB
image.png
Steffen Dünner added the
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 labels 2024-01-04 17:15:11 +01:00
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Philipp Oeser commented 2024-01-05 09:24:33 +01:00
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Was introduced in 472cf44166 will have a look

Was introduced in 472cf44166aff98c0cf481cc063406be4b0aa79c will have a look
Philipp Oeser commented 2024-01-05 09:37:43 +01:00
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Was introduced in 472cf44166 will have a look

Ah, misread this, this is not about the alembic part at all

> Was introduced in 472cf44166aff98c0cf481cc063406be4b0aa79c will have a look Ah, misread this, this is not about the alembic part at all
Steffen Dünner commented 2024-01-05 09:40:26 +01:00
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Yes, it's not really related to the new Alembic import feature but I noticed it while testing it yesterday.

Yes, it's not really related to the new Alembic import feature but I noticed it while testing it yesterday.
Philipp Oeser commented 2024-01-05 09:53:06 +01:00
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It is the same for meshes

1 MB Step

image

3 MB Steps

image

5 MB Steps

crash

It is the same for meshes 1 MB Step ![image](/attachments/030ac559-d50b-46cc-87e7-3c210dfef3b2) 3 MB Steps ![image](/attachments/08841414-c158-4383-b6ce-37c3f86e0c3c) 5 MB Steps crash
image.png
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Steffen Dünner commented 2024-01-05 10:01:26 +01:00
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To me it looks like the motion vectors of each step get added instead of "subdivided" by the number of steps and then added.
On the other hand, when using a motion vector attribute to add motion blur, the result should look identical no matter how many steps, because the vectors only exist for the full frame, not the subframes.

To me it looks like the motion vectors of each step get added instead of "subdivided" by the number of steps and then added. On the other hand, when using a motion vector attribute to add motion blur, the result should look identical no matter how many steps, because the vectors only exist for the full frame, not the subframes.
Philipp Oeser changed title from Rendering point clouds with motion blur can cause memory problems to Rendering multiple deformation motion blur steps with "velocity" attribute is wrong and can cause memory problems 2024-01-05 10:06:07 +01:00
Philipp Oeser commented 2024-01-05 11:31:13 +01:00
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128eb6cbe9 states that when using the velocity attribute

/* Only export previous and next frame, we don't have any in between data. */

Other parts of code will still go over all steps and might end up accessing garbage data because of that @brecht ?

Dont we have to fill in at least some sort of data for each step?

128eb6cbe928e58dfee1c64f340fd8d663134c26 states that when using the velocity attribute >/* Only export previous and next frame, we don't have any in between data. */ Other parts of code will still go over all steps and might end up accessing garbage data because of that @brecht ? Dont we have to fill in at least some sort of data for each step?
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Brecht Van Lommel commented 2024-01-10 19:26:09 +01:00
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What it's supposed to do is ignore the motion steps for deformation blur, as there is nothing more than 1 step we can get out of a velocity attribute. For object transform it can still use more steps.

What it's supposed to do is ignore the motion steps for deformation blur, as there is nothing more than 1 step we can get out of a velocity attribute. For object transform it can still use more steps.
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