ink in water

[bandini]

Hi,

I am trying to create an ink-in-water effect with particles. I am pretty happy with the motion of the particles - I am advecting them with a smoke simulation.

My main question is about texturing/rendering solutions. What is working (somewhat) for me is to render millions of particles with low opacity - when they overlap I get nice wisps and details (If I get permission, I'll post a test movie). But, the results are still pretty grainy, like a Krakatoa render - not smooth enough for me, and the render time is quite high.

I'm wondering what other solutions there are for this type of effect - in particular how to handle rendering and shading. Any help or example .hip files would be appreciated.

Thanks!

[bandini]

I was able to get permission to post two tests so far.

Test 1

Test 2

I'm really trying to get this to look smoother - less grain, but still have an inky feel to it. No large blobby surfaces.

Edited January 8, 2009 by bandini

[Nico D.]

This is looking very nice !!!

How many particles did you use ? And what about simulation times ?

[johner]

My main question is about texturing/rendering solutions. What is working (somewhat) for me is to render millions of particles with low opacity - when they overlap I get nice wisps and details (If I get permission, I'll post a test movie). But, the results are still pretty grainy, like a Krakatoa render - not smooth enough for me, and the render time is quite high.

Hopefully you're using velocity motion blur on the particles? What sometimes helps is to artifically scale the velocity attribute up on the points before rendering, which has the effect of "streaking" them more and can cut down on the graininess. You can do this with the Point SOP, but with a lot of points it should be much faster (and multithreaded) with a simple VOP SOP to do the scaling.

For the render time, it might help to play with the "Opacity Limit" parameter on the Render Tab of the Mantra ROP. If you can set it lower without introducing artifacts that might help speed things up a bit.

[tayga]

i dont get... Why are you using particles. Why dont you render volumes from smoke simulation?

[bandini]

Hi,

I didn't realize there were responses to my post - I guess I forgot to turn on e-mail notification...

Gaucho,

Not too many particles right now - about 50,000 per frame. Sim time is quick - about 3-4 minutes. Render time is about 10 minutes a frame at a low resolution.

Johner,

I never quite figured out how to get motion blur to work with particles rendered as points. Still learning the program... Any quick tips? I have played with the opacity limit, but I really can't push it too far without artifacts.

Tayga,

I need my volume to inherit the colors of the emitter. Is there a way to do this with smoke volumes that will retain the amount of detail that I am getting with particles? An example file would be really helpful here.

[goldleaf]

That's amazing!

Are the particles only being "advected" through a fluids domain, or are there other things on top of it?

I can't imagine it would happen, but if you were ever able to post a scene file, that would be amazing to learn from. In any case, well done!

[johner]

Johner,

I never quite figured out how to get motion blur to work with particles rendered as points. Still learning the program... Any quick tips? I have played with the opacity limit, but I really can't push it too far without artifacts.

It's pretty straightforward. On your particle object, under the "Render" tab, "Sampling" sub-tab, toggle "Geometry Velocity Blur". Then on the "Geometry" sub-tab, toggle "Render as Points". Turn on motion blur on your Mantra ROP node, and that should do it.

I've attached a simple example. The POP network here is just driven by a simple wind POP with some noise. At the SOP Level there's a VOP Sop that scales the velocity by a parameter. Since it's done at the SOP level after the POPNet cooks, you can play with the velocity scale without re-running the sim. If you have particles displayed as lines (in the scenes "Display Options" dialog) you'll get some idea of the velocity scale. The final SOP creates a pscale attribute at the Detail level, which is much faster than creating one for each point assuming they all have the same pscale.

Edit: tweaked this a bit. So here's two renders: one with velocity motion blur as the velocity comes out of the POPnet, one with the velocity scaled aftewards by 5.

Hope that helps.

velocity_blur_scale.hip

Edited January 25, 2009 by johner

[bandini]

Thank you very much for the help johner. I figured it was probably something easy I was overlooking.

Goldleaf, it is only advection. I can't post the file, unfortunately, but the setup is actually pretty easy. The rendering is the hard part. My latest tests I've been able to render 12 million particles, which looks great at D1 resolution, but will never transfer to 2K or 4K. For that, I'll need somewhere in the range of 100-200 Million, I would think...

I've heard of renderman solutions that will up-res the particle count at render-time, as well as do some kind of segmented rendering so the computer can handle it. Something like what is demonstrated here:

http://www.hosok2.com/project/helper/helper.htm

I don't have time to look into this at the moment, but it seems like a solution like it is required.

[goldleaf]

Interesting. Thanks!