Some particles do not appear in the render when the 'Vector' pass is enabled #102761

New Issue

Adrian · 2022-11-25T10:47:39+01:00

Adrian commented

2022-11-25 10:47:39 +01:00

System Information
Operating system: Linux-5.19.17-2-MANJARO-x86_64-with-glibc2.36 64 Bits
Graphics card: NVIDIA GeForce RTX 2060 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 470.141.03

Blender Version
Broken: 3.2.2 - 3.3.1
Worked: 2.79

Short description of error
Particles in Render don't match particles in viewport. Some particles do not appear. When they move very fast the issue is more visible.
It happens when we enable the pass Vector

Exact steps for others to reproduce the error

Open attached file or:
In the default scene (with the Cube), add a Plane
Add a ParticleSettings toe the plane
In {nav Particle Properties > Render} set Render As to Object
For the Instance Object choose the Cube
In {nav Particle Properties > Field Weights} set Gravity to 0.0
Optionally bake for cache
Set Render Engine to Cycles
In {nav View Layer Properties > Passes > Data} enable Vector
Move Cube out of camera view
Choose a frame in the middle and render
Note that the particles closest to the plane disappear

particles displaced.blend

**System Information** Operating system: Linux-5.19.17-2-MANJARO-x86_64-with-glibc2.36 64 Bits Graphics card: NVIDIA GeForce RTX 2060 SUPER/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 470.141.03 **Blender Version** Broken: 3.2.2 - 3.3.1 Worked: 2.79 **Short description of error** Particles in Render don't match particles in viewport. Some particles do not appear. When they move very fast the issue is more visible. It happens when we enable the pass `Vector` **Exact steps for others to reproduce the error** - Open attached file or: - In the default scene (with the Cube), add a Plane - Add a ParticleSettings toe the plane - In {nav Particle Properties > Render} set `Render As` to `Object` - For the `Instance Object` choose the Cube - In {nav Particle Properties > Field Weights} set `Gravity` to `0.0` - Optionally bake for cache - Set `Render Engine` to `Cycles` - In {nav View Layer Properties > Passes > Data} enable `Vector` - Move Cube out of camera view - Choose a frame in the middle and render Note that the particles closest to the plane disappear [particles displaced.blend](https://archive.blender.org/developer/F13967983/particles_displaced.blend)

Adrian commented

2022-11-25 10:47:39 +01:00

Added subscriber: @slepy8

Germano Cavalcante commented

2022-11-25 14:12:50 +01:00

Added subscriber: @mano-wii

Germano Cavalcante commented

2022-11-25 14:12:50 +01:00

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

Germano Cavalcante commented

2022-11-25 14:12:50 +01:00

The particles don't have a delay, the newest ones just don't appear in the render.
I can't reproduce the problem from scratch.
This was the file I used:
particles.blend

Can you give us more clear instructions on how to reproduce it from scratch.

The particles don't have a delay, the newest ones just don't appear in the render. I can't reproduce the problem from scratch. This was the file I used: [particles.blend](https://archive.blender.org/developer/F13968190/particles.blend) Can you give us more clear instructions on how to reproduce it from scratch.

Adrian commented

2022-11-25 15:11:20 +01:00

Create a 'Plane'
Create a 'Cube'
Assign Particle system to the plane.
In this particle system go to Render->Render As -> select 'Cube'
Move 'Cube' away to see the 'Plane'
In particle system settings go to FieldWeights->Gravity -> change to '0'
In particles settings you can change Velocity->Normal to a higher value. This will show the problem even more.

Create 'Camera'

Play animation. Render it at any frame and compare the result.

You can bake particles. No difference in outcome (baked/unbaked).

Create a 'Plane' Create a 'Cube' Assign Particle system to the plane. In this particle system go to Render->Render As -> select 'Cube' Move 'Cube' away to see the 'Plane' In particle system settings go to FieldWeights->Gravity -> change to '0' In particles settings you can change Velocity->Normal to a higher value. This will show the problem even more. Create 'Camera' Play animation. Render it at any frame and compare the result. You can bake particles. No difference in outcome (baked/unbaked).

Germano Cavalcante commented

2022-11-25 19:20:33 +01:00

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

Germano Cavalcante commented

2022-11-25 19:20:33 +01:00

In #102761#1451290, @slepy8 wrote:
Create a 'Plane'
Create a 'Cube'
Assign Particle system to the plane.
In this particle system go to Render->Render As -> select 'Cube'
Move 'Cube' away to see the 'Plane'
In particle system settings go to FieldWeights->Gravity -> change to '0'
In particles settings you can change Velocity->Normal to a higher value. This will show the problem even more.

Create 'Camera'

Play animation. Render it at any frame and compare the result.

You can bake particles. No difference in outcome (baked/unbaked).

I followed these steps exactly, but I can't reproduce the problem.
Let's see if someone else can.

untitled.blend

> In #102761#1451290, @slepy8 wrote: > Create a 'Plane' > Create a 'Cube' > Assign Particle system to the plane. > In this particle system go to Render->Render As -> select 'Cube' > Move 'Cube' away to see the 'Plane' > In particle system settings go to FieldWeights->Gravity -> change to '0' > In particles settings you can change Velocity->Normal to a higher value. This will show the problem even more. > > Create 'Camera' > > Play animation. Render it at any frame and compare the result. > > You can bake particles. No difference in outcome (baked/unbaked). I followed these steps exactly, but I can't reproduce the problem. Let's see if someone else can. [untitled.blend](https://archive.blender.org/developer/F13968978/untitled.blend)

Richard Antalik commented

2022-11-25 20:45:10 +01:00

Added subscriber: @iss

Richard Antalik commented

2022-11-25 20:45:10 +01:00

In #102761#1451427, @mano-wii wrote:
I followed these steps exactly, but I can't reproduce the problem.

Same here. Can't repro from scratch.

> In #102761#1451427, @mano-wii wrote: > I followed these steps exactly, but I can't reproduce the problem. Same here. Can't repro from scratch.

Adrian commented

2022-11-28 09:16:45 +01:00

I checked your blend file. Indeed - looks correct.
Did you look into mine? Does it work fine for you?

If in my blend file an error is visible, than it must have something to do with personal settings of Blender on my side. But which one?

I checked your blend file. Indeed - looks correct. Did you look into mine? Does it work fine for you? If in my blend file an error is visible, than it must have something to do with personal settings of Blender on my side. But which one?

Germano Cavalcante commented

2022-11-28 14:02:36 +01:00

In #102761#1452371, @slepy8 wrote:
(...)
If in my blend file an error is visible, than it must have something to do with personal settings of Blender on my side. But which one?

I also tried to locate this setting.
The only thing I noticed is that the problem is only seen in Cycles.
For a bug report to be complete, it is important to identify what caused the bug.

> In #102761#1452371, @slepy8 wrote: > (...) > If in my blend file an error is visible, than it must have something to do with personal settings of Blender on my side. But which one? I also tried to locate this setting. The only thing I noticed is that the problem is only seen in Cycles. For a bug report to be complete, it is important to identify what caused the bug.

Adrian commented

2022-11-28 14:38:11 +01:00

I figured it out.
It happens when you have "Vector" pass enabled in View Layer Properties.

I figured it out. It happens when you have "Vector" pass enabled in View Layer Properties.

Germano Cavalcante changed title from ~~Particles delayed in render view vs viewport.~~ to Some particles do not appear in the render when the 'Vector' pass is enabled

2022-11-28 15:22:25 +01:00

Germano Cavalcante commented

2022-11-28 15:22:25 +01:00

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

Lukas Stockner commented

2023-01-11 22:09:28 +01:00

Added subscribers: @brecht, @LukasStockner

Lukas Stockner commented

2023-01-11 22:09:28 +01:00

As far as I can tell, this is because the Vector pass needs motion data - in particular, it needs to know where the particle was in the previous and next frame. If it was just spawned, it doesn't have a previous position, so I assume that Cycles just skips it in that case.

You can reproduce this by enabling motion blur and setting the shutter time to 2 frames, it causes them to disappear as well.

@brecht, do you think this is something worth improving (e.g. assuming zero motion for particles that just spawned for the purpose of the Vector pass) or is this just a known limitation?

As far as I can tell, this is because the Vector pass needs motion data - in particular, it needs to know where the particle was in the previous and next frame. If it was just spawned, it doesn't have a previous position, so I assume that Cycles just skips it in that case. You can reproduce this by enabling motion blur and setting the shutter time to 2 frames, it causes them to disappear as well. @brecht, do you think this is something worth improving (e.g. assuming zero motion for particles that just spawned for the purpose of the Vector pass) or is this just a known limitation?

Brecht Van Lommel commented

2023-01-16 21:53:53 +01:00

I've considered this a known limitation up to now. The simple alternatives seem bad too. Making the particle stay still would look bad when all other particle are moving fast, and extrapolating motion can lead to a particle appearing on the other side of a surface it is supposed to collide with.

The better solution I think is to hide the particle only on specific timesteps, but it's unclear how best to represent that in the BVH, if it's possible to do it at all in Embree and OptiX. Maybe it's possible with a NaN transform or something but obviously that could cause other issues.

I've considered this a known limitation up to now. The simple alternatives seem bad too. Making the particle stay still would look bad when all other particle are moving fast, and extrapolating motion can lead to a particle appearing on the other side of a surface it is supposed to collide with. The better solution I think is to hide the particle only on specific timesteps, but it's unclear how best to represent that in the BVH, if it's possible to do it at all in Embree and OptiX. Maybe it's possible with a NaN transform or something but obviously that could cause other issues.

Lukas Stockner commented

2023-01-24 03:45:00 +01:00

In #102761#1475162, @brecht wrote:
The better solution I think is to hide the particle only on specific timesteps, but it's unclear how best to represent that in the BVH, if it's possible to do it at all in Embree and OptiX. Maybe it's possible with a NaN transform or something but obviously that could cause other issues.

To avoid it being visible, I guess a zero-scale transform might be enough? That still leaves the question of what to use for the "previous frame" motion vector, but there I guess using the next-frame vector would be okay.

> In #102761#1475162, @brecht wrote: > The better solution I think is to hide the particle only on specific timesteps, but it's unclear how best to represent that in the BVH, if it's possible to do it at all in Embree and OptiX. Maybe it's possible with a NaN transform or something but obviously that could cause other issues. To avoid it being visible, I guess a zero-scale transform might be enough? That still leaves the question of what to use for the "previous frame" motion vector, but there I guess using the next-frame vector would be okay.

Brecht Van Lommel commented

2023-02-03 18:55:59 +01:00

In #102761#1478977, @LukasStockner wrote:
To avoid it being visible, I guess a zero-scale transform might be enough? That still leaves the question of what to use for the "previous frame" motion vector, but there I guess using the next-frame vector would be okay.

I don't think it's enough? It would interpolate from zero scale to full scale, not be invisible.

> In #102761#1478977, @LukasStockner wrote: > To avoid it being visible, I guess a zero-scale transform might be enough? That still leaves the question of what to use for the "previous frame" motion vector, but there I guess using the next-frame vector would be okay. I don't think it's enough? It would interpolate from zero scale to full scale, not be invisible.

Philipp Oeser removed the

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Some particles do not appear in the render when the 'Vector' pass is enabled #102761