Rain

[Joelaff]

Regarding sparse sims - they are about 30% slower than a dense grid of the same size. We measured this with VDBs compared to AUR, and also with AUR compared to our own prototype sparse implementation. So in order for a sparse simulation to make sense, it would need at least 30% of the grid volume to be unused, which is rarely the case because even if you don't see the velocities, they make a hell of a difference to the end result of the simulation. So a good example of a practical case where sparse sims would help would be a T-shaped simulation, such as a rocket launch.

Interesting. That is using some sort of tree structure I take it (octree or whatever)? According to some articles I saw supposedly that is faster than pointer de-referencing, which I thought was interesting, but I guess it makes sense. I guess the VDB folks are still trying to find ways to optimize further. But RAM is pretty cheap these days; so more is probably the better solution.

[glorybound]

I am getting somewhere. Working on blur is a headache. Too much blur and it looks bad, too little, and it looks almost as bad.

Here is it frozen in time.

[Svetlin.Nikolov]

Interesting. That is using some sort of tree structure I take it (octree or whatever)? According to some articles I saw supposedly that is faster than pointer de-referencing, which I thought was interesting, but I guess it makes sense. I guess the VDB folks are still trying to find ways to optimize further. But RAM is pretty cheap these days; so more is probably the better solution.

With a dense grid, you just know the next voxel is always 1 index ahead or behind, but with a dense grid, yeah, it could be a tree or some other sort of spatial arrangement, but calculating the neighbor index is inevitably slower...

[glorybound]

How do I decrease the velocity of the falling rain? My client wants the water to run down and over the gutter closely.

The water "material" looks 100% real. What's not working is that it doesn't appear to follow the contour or surface of the gutter. It almost looks like it's on the lease. I did a little real-life experiment by spraying water on a white surface to compare. I'll post that image for reference.

[glorybound]

basically fewer drops - any help?

I think if we pull back the “number” of droplets altogether, we’ll be good to go.

[Svetlin.Nikolov]

Hey,

If there are large and small droplets, then you might use the Surface Level option in the rendering rollout - raising it would shrink the liquid droplets, lowering it will make the droplets larger. A similar effect might be achieved using the Grid Chanel Smoothing in the Input rollout if you enable it for the Liquid channel - this would remove the smaller droplets and keep the larger ones.

You might also try adding some Mesh Smoothing in the Rendering rollout and then enable Use Liquid Particles and change the particle radius.

These are the methods I can think of for modifying the liquid at render time, without needing to simulate again.

If I understand correctly, if you want to slow down the liquid, you could use a Drag force during the simulation, but again there is a way to do this at render time - you can slow down the entire animation from the Input rollout's Play Speed.

Cheers!

[glorybound]

I am still struggling. They just want the water droplets to be spread out more. Right now there is a drop every square inch, and they want to triple that distance. Maybe it isn't what rain does, but it is what they want. A downpour on your windshield... it covered your windshield with running water. When it drizzles, you can see the drop hit the windshield and run down it.

[Joelaff]

Try using a map to control the emission. If you use a noise map with the default white and black you can adjust the low cutoff value to make more of the area of the texture black thereby spacing out the emission more.

[glorybound]

I went through all of the settings and I am seeing no way to use a map to drive the particles. Where do I look?

[Joelaff]

Are you using Phoenix for the rain particles? If so you would have a PHX Liquid Src. In there you can set a map for the outgoing velocity (which would stop particles from appear if zero/black).

Yours will of course have something under "Emitter Nodes."

[glorybound]

This is so time consuming! When I try a Noise or Cellular, it doesn't seem to tile. Only part of my emitter is being used. What am I doing wrong?

[Joelaff]

Maybe the scale of the noise is way too big and it is causing emission in one spot. I find applying the noise to the object to view in the viewport helps (give the emitting object a UV map and use Explicit Map channel for the noise rather than the default Object Scale so you can see if properly). Depending on your object and scene scale etc. this might end up with a scale of something like 0.0001 to 1000. Don't be afraid to make big changes in scale until you can see the noise in the viewport.

If you don't want to take the moment to set it up with the UVs so you can see the scale in the viewport (highly recommended) then you will have to play with the scale until you can tell it is working. I usually alter the scale of the noise by a factor of 10 or 0.1 until I get it to start showing results. Then you can tune it from there.

[glorybound]

I didn't know you can apply a UV map to the emitting object. I'll start there. Thank you!

[Joelaff]

I guess it depends on the object, but for geometry you can of course. I think you can even see the texture on some helpers.