nParticle Mesh "Popping" Render

#1

Hey All,
I have this problem where my render shows these strange artifacts, kind of a sporadic popping. I think it might be attributed to motion blur, mental ray might be having a difficult time with the mesh. But as you can see in the video, the particles are pretty contained.

Breakdown Problem

I’m not done with the particle simulation yet. Velocity Per Vertex is turned on in the Output Mesh tab in the particle shape node. My max triangle resolution is set to 100 for faster simulation rates (I don’t think that is the problem).
My max triangle size is set to 0.5, and I have a feeling this could be the problem. The doc reads:
“[The Max Triangle Size] limits how many voxels are used to created the output triangle mesh. This can be viewed as the resolution on the side of a lattice, so the maximum resolution of the voxel grid used in generating triangles will be the cube of this value. This limit will be hit when the bounds of the particle system combined with the mesh triangle size create a voxel grid with more voxels than this value cubed. After the limit is hit the triangles will start to grow in size as the particle system bounds get larger( rather than, for example, dropping out triangles )”

Any ideas what’s causing this ‘popping’ effect?

Best,
Colby

#2

Any tips on this popping/artifacts? It has something to do with the motion blur.
Any suggestions would be greatly appreciated.

Best,
Colby

#3

Have you cached the output mesh?

#4

Before rendering the particles should be cached(velocity needs to be cached on particles, but it is by default). The particle mesh limit attributes should not be causing problems. Do you have any construction history(like a smooth) added to the particle to poly mesh? The motion blur vectors are defines as a color set on the mesh:velocityPV. This is set on the mesh shape as the default “motionVectorColorSet”. Some construction history propagates the color set correctly but some doesn’t. Also I’m assuming you are rendering with Mental Ray. (the motionblur is not supported on particle meshes in MayaSW).

One other point is that the motion blur vectors will be a bit smoother if the blobby threshold > 0.0.

#5

On other thought is perhaps your units or scaling might be resulting in large motion blur vectors somehow. (possibly a units related bug)

#6

Is there a particle goal at play here? if so I have seen this when a birth particle moves to its goal position.

The work around for me was to birth the particle at near zero radius then after a frame or so have it be the radius you require.

its a cool looking effect
good luck

#7

Hi guys,
thanks for the response! I figured I could get around without caching the particles because I am using a script so I don’t have to batch render. It runs the sim and renders each frame, saving the image.

As far as scale, this could definitely be the culprit. My sphere was very large. My threshold was set above 0 for the output mesh. I’ve been playing quite a bit with the output mesh and haven’t found it to be working for what I want. I’ve realized I’m going to need lots and lots of particles (it was inevitable I suppose).

Here’s a link to the latest test - it’s getting there!

Water Sphere RnD

Best,
Colby

#8

One other thought on that blur problem… if particles are dying then it could be a problem where on emission a particle is getting the previous position from a particle that just died, causing a long streak. I’m not sure if this is happening but I’ve seen similar bugs in the past. Doing things like making the radius zero for the first frame, as suggested, might help.

Note that your sim time might be large due to the meshing… it is better to keep the bounding box for the particles as small as possible, because of the voxelation step involved in the meshing. Either create a colliding plane to catch particles and keep them from falling too far or have them die after they go out of frame.

#9

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