Page MenuHome

Hair particle system duplicates objects on some faces and don't fill the rest
Confirmed, NormalPublicBUG


System Information
Operating system: Ubuntu 19.04
Graphics card: AMD Radeon RX580

Blender Version
Broken: (2.81 release compiled from source)
Worked: (optional)

Short description of error
Even with even distribution enabled, 1 particle per face and a low count of particles to spawn, the particle system put two particles on the same face but with different sizes.

Exact steps for others to reproduce the error

  1. Create a cylinder with 32 vertices.
  2. Create a small (scale 0.05) cylinder to use as a reder for particle.
  3. Delete the top and bottom faces of the small cylinder so it becomes like an empty tube.
  4. Enable particle system hair on first cylinder and, choose 30 as number, 1 per face source and disable random order, select as render object the small cylinder, the scale 1.0 and choose to randomize the scale a few (0.5 is ok).
  5. Play with the number of particles and the seed and more than sometimes you will notice two or more small cylinders rendered on the same place overlaping (specially on the top and bottom faces).

What I expect:
If you count the number of faces of the cylinder (added with default parameters) it will have near 30. If I choose "even distribution" of particles and "1 for each face" without random, what I expect is that all the faces of the cylinder have at least 1 particle, no more, no less.
If there are 30 faces and if I limit the number of particles to exactly 30, there must be exactly one particle for each face.
What happens actualy is that there are 4 particles lying around on the top and bottom faces and I don't know why. And there are no particles on some other faces. And sometimes two spawns on the same place on the bottom or top face of the cylinder.

With complex meshes it makes a mess, specially if I need to paint the weight to affect density. In that case there are more particles on some places and there are no particles on places with a high value of weight.

Event Timeline

I was just thinking about this. There are an option called "volume". And it does some like the "face" option. "Volume" is OK as actually works because it is clear that only the volume is taking in account.
But actually choosing "face" is working like a "surface". Because it is something similar to "volume" but per face (or, surface as the term fits better).

So, I suggest to change the actual name of "face" to "surface" and create a new one using the name "face".

Because when I choose "vertex" it is working as I expect. It is just creating one particle per vertex if I specify exactly the same ammount of particles than vertex.
But, for a "face" it must have the same behaviour as "vertex". It must respect the amount of particles expected per face.

And there can be another option called "auto" for the "number" editbox. So it means that the particle hair will try to fill all as the parameters instructs. And, if we use as density weight paint the "auto" mode for number of particles can try to fill exactly the needed amount for that situation.
I mean, "auto" can be a smart behaviour to try to fill only limited by the options and nothing more.

With all that modifications we can use the particle system of hair to "paint" surfaces with objects. Or, better to understand, it will be useful to put objects over a terrain. Or to put beard to a character or to put just hair. In a easy way of only texture or vertex painting.

I hope this is taked in account because I need to do that things a lot of times and the particle system of hair is the only procedural method to place and distribute objects over other objects. So it is an important tool to take in account.

thank you

Germano Cavalcante (mano-wii) changed the task status from Needs Triage to Needs Information from User.Jan 9 2020, 8:52 PM

Thanks for the report, but I'm confused by the steps to reproduce the problem.
What do you mean with "at least 3 faces per view"?
Could you provide a simple file demonstrating the problem?

Sorry, 3 faces per side (right and left sides). What I mean is that there must be enought faces but you need some special case to discover it better. The default cylinder is OK with 32 vertex.

Richard Antalik (ISS) changed the task status from Needs Information from User to Confirmed.Jan 17 2020, 3:31 PM
Germano Cavalcante (mano-wii) changed the subtype of this task from "Report" to "Bug".

Apparently internally the n-gons are transformed into quads and tris. And the center of these is that is computed.