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Particle Created at Object's Origin Even If No Face Is There
Needs Information from User, NormalPublic

Description

System Information
Operating system: Both Windows and Linux
Graphics card: Various nVidia cards

Blender Version
Broken: 2.90
Worked: (newest version of Blender that worked as expected)

Create an object with two separate components. Set its origin to somewhere strictly between the two components, so that the origin is not part of any face. Generate particles on that object. Then a particle can appear at the origin.

Exact steps for others to reproduce the error
Create two planes. Space them apart. Join them. Set the joined object's origin to geometry. Put a particle system on it. Change end time to 1. Observe a particle is generated at the object's origin even though there is no face there.


Event Timeline

I cannot reproduce the problem from scratch.
It only appears in the attached file.

Has it been fixed in the daily build?
https://builder.blender.org/download/

I installed the latest build: blender-2.91.0-6624c4c22514-linux64
The following steps will recreate the problem.

  1. Delete cube.
  2. Add a plane.
  3. Go to edit mode.
  4. Add a plane.
  5. Grab it and move it away from the first plane (no overlap).
  6. Go back to object mode.
  7. Object > Set Origin > Origin to Geometry
  8. Confirm the origin is not touching either plane.
  9. Add a particle system and modify its End time to 1 (down from 200).
  10. Confirm there is a particle (white halo) at the origin of the plane. In greater detail, there are lots of particles covering both planes, but there is a lone particle halfway between the two planes that should not be there.

I can't reproduce this from scratch.

In provided file if I go to edit mode and back, or touch any setting of particle system the particle at origin dissappears.

There was similar report recently though so there is probably something to it, but I can't find it anymore

In case you missed it, please see my comment above where I show how to recreate the problem from scratch. The comment starts with "I installed the latest build: blender-2.91.0-6624c4c22514-linux64. The following steps will recreate the problem.".

(Note too that this first occurred in an actual project: there was a very noticeable bright object in the middle of the rendered animation when there shouldn't have been. So presumably if you extend the above step-by-step instructions to render particles as objects then render the animation, you will see an unwanted particle/object in the rendered animation.)

In case you missed it, please see my comment above where I show how to recreate the problem from scratch

Thanks, I din not miss that. I followed that procedure as well.

Germano Cavalcante (mano-wii) changed the task status from Needs Triage to Needs Information from User.Fri, Sep 18, 8:57 PM

I also can't reproduce form scratch.
Unfortunately unless there are clear instructions on how to reproduce it from scratch we cannot debug this any further.
GIF:

I watched the GIF and the steps are correct. My first suggestion is to try increasing the number of particles generated and also changing the seed. I don't know what influences the random generation of particles, so to be on the conservative side, the following steps now include precise distances and seed values. Although I haven't tested the following steps on different setups, in the past I have found the problem to be easy to reproduce on two different blender versions and two different computers, one Windows and one Linux.

I am using the standard Blender 2.90 for this.


[ I used the daily build of Blender last time. ]

  1. Select "General" on splash screen.
  2. Delete cube.
  3. Press shift-A M P to add a plane.
  4. Press shift-D X 4 to duplicate then place 4 units away on the X axis.
  5. Select both planes then type Ctrl J to join.
  6. Object > Set Origin > Origin to Geometry
  7. Particle Properties > "+" to create a particle system.
  8. Change End to 1 (down from 200).
  9. I already get a particle at the origin of the planes:
  1. Changing seed to any value between 1 and 20 seems to alter the distribution (as it should) but keep the particle at the origin.
  1. Set the Seed to 555.
  2. Set the Number to 51. There is no particle at the origin:
  3. Set the Number to 52. A particle appears at the origin:
  1. Place the 3D cursor anywhere on the screen.
  2. Object > Set Origin > Origin to 3D Cursor
  3. The particle follows it, still appearing at the origin:

This is a mystery, as it seems very robust to recreate.

ps. Here is the Blend file at the end of the last step.

A faster way to recreate the problem is to use the "Particle At Origin Bug.blend" file attached to the previous comment.

  1. Download and run, say, blender-2.91.0-6624c4c22514-linux64
  2. Open Particle At Origin Bug.blend
  3. In particle system settings, change Emission > Number to 51 (down from 52). Observe the particle at the origin disappears (where the 3D cursor is).
  4. Change Emission > Number back to 52. Observe the particle at the origin appears again (where the 3D cursor is).

If this does not work, then somehow there is a dependence on the hardware/OS/compiler....

ps. The above recreated the bug with blender-2.91.0-6624c4c22514-linux64 and I have now also confirmed it recreates the bug with Windows 2.90: