MR: Particles with opacity render black

#1

Hi there!

I stumbled upon a problem with strange black spots using particles for a fiery animation and was able to narrow down the problem in a simplified test scene: When there are more than 1022 facing particles with a material with opacity at zero (or cutout as black, the material type seems to be irrelevant) overlapping, these particles render black and even cut a hole in the alpha channel. Increasing the trace depth (sum and refraction to e.g. 4000) made no change. What has to be set to make mental ray render more than 1022 overlapping transparencies?
I have attached the simple test scene. I use 3ds Max 2011.

Thanks,
Bob

#2

It seems opacity can’t be zero. If you set to just 1, it works fine!

#3

Thanks for testing that. Unfortunately this is not an option, since I want parts of the particle to be completely invisible, so I do not see the square shape of the facing. Even with very little opacity it adds up to being visible again. Scanline and Quicksilver seem to have no problem with that, only mental ray. And it seems to me quite peculiar, that it is nearly 1K (1024) of opacity layers, which can be rendered. So it could be some kind of software issue, I guess.

#4

It seems something wrong with mental relating to pflow. I have experienced most time that if you want to render particles with mentalray , bake it to geometry.

Now if you create mesher object and take snapshot mesh, with that material, it renders fine.

I would suggest you to bake up particles as geometry whenever rendering with mentalray.

#5

Thank you for that interesting approach but it does not seem to work either. When I take a snapshot of 100% of the particles in the mesher, the same problem occurs. Actually it also happens independently from particles, when I just put 1023 planes in a row.

#6

No, actually i meant after creating mesher object and loading pflow in mesher, go to tools menu and use “Snapshot” utility to create single mesh from mesher.

EDIT : As I said earlier, mentalray needs particles to be bake as geometry. Ok try just adding Cache operator set to viewport/render and cache the particles, it will work as usual you want with particles.

#7

Thanks for the different attempt, unfortunately it has actually nothing to do with the objects being particles. I made another test scene with just plane objects and put 1023 in a row. The same problem occurs (see attachment), the 1023rd plane renders black. Even with an opaque background object there also is a black spot in the alpha as well.

#8

That black spot seems last plane in the row. and it’s even showing black in alpha. lol

Well in A&D material you used, you can set IOR to 1.0 and BRDF to by IOR and set transparency to 1.0. That can work!

#9

Oh, you are right. That is an option I have not tried, I thought this would make the render time much longer having the “real” transparency instead of just a cutout. But at least in the test scene it does not make much difference. I will try it with my actual particle system.
But the way mental ray handles this is still odd.

Thanks,
Bob

#10

Unfortunately this does not work either, it just shifts the problem a little: It is then the 1029th plane (and all above) that renders black. I have also put up the number of Max. Refractions and Max. Trace Depth to a test value of 5000 but that does not change the problem. But at least the alpha problem disappears that way.

#11

You are right. At 1030 planes, it started happening again. Alpha problem went away becoz you had “Transparency propagates Alpha” option was turned off.

So this 1.0 IOR trick works, if you render it with “Fast Rasterizer”.

I hope that helps! )

#12

Thanks for staying with me and your testing effort. I had tested the fast rasterizer with the particle system before without success. Passing on to the test scene with the planes it needs just one more plane to make the fast rasterizer do the same as well.
I start to believe that this is not the expected outcome of mental ray but some kind of bug.

#13

Just in case somebody else gets this problem, here is now a possible solution:
It works with using an Arch&Design material, transparency 1.0, IOR 1.0 and increasing the Max Trace Depth globally in the render setup AND in the material under Advanced Rendering Options. It still does not work with using the cutout channel though.

#14

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