Heavy Storm clouds?

[jtk77]

Has anyone been able to pull off heavy storms using maya fluids? I need to create a situation of sever weather rolling in.
I've gotten of course to the point where I can turn the shading down, make the coulds thicker and moving. But you know...it still looks like puffy clouds with no weight. No rolling storm clouds and I'm really not sure how to go about this.

[jtk77]

I think I'm having a lot better effect using Billow instead of perlin noise. But still, anyone have any ideas feel free.

[david-schoneveld]

You're going to notice a HUGE render time with Billow. In some cases it makes fluids an impossibility due to render times.

I got some cool clouds but the method is a bit of a pain. Save the shading settings for the cloud you like then apply them the 3dnuke example, and a few stratigic volume emitters. That should give you a cool look. it takes a while to get the right sort of shape so tips on that are:

Emit a lot of temp and have temp turbulens set high (5+) and tweak the fluid turbulence yet still keep the value of the turb low (like .15ish). Set the Density bouancy to 0 and temp to something like 3.

Then when you get a good looking cloud do the same for a bunch more scaling the container up a bit as you go further away. I did say it's a pain. You can't realistically render them all together so you have to render each cloud as a layer and comp them all together. I know this method is painful. If you jsut want the stock clouds you can make the double the rez and scale of the Y on the container. but you'll never get the real (As in moving and big storm clouds) clouds that way.

Good ref for clouds is the making of the Aviator. Have you seen that? I can't remember off hand where it is but they have a shot break down and their clouds are very good.

[jtk77]

Definitly a good movie reference...
Yea, billow seems painful, but I really have somthing I might be able to use and just let it render in layers overnight.
I'm not using selfshadowing, that turns it into an impossible render.
So far , I'm looking at 15 mins or so a frame. Wich, is acceptable since I only need 8 seconds and have a renderfarm. Your methoud sounds pretty good however, I might have to look into it.
I'm having a hardtime finding really great advanced tips on anything fluid related. Most of what I've found I've already figured out with the docs.
I wonder if that alias dvd on fluids that just came out is any good? I often stay away from Alias dvds though. I find them to be lacking if you already know a thing or two.

[duncanbrinsmead]

You might find that perlin with inflection on (can't remember if you need invert on as well) gives a close approximation to the look of billow, but renders faster.

I did the new Alias fluid DVD, and the goal was to provide detailed info for advanced fluids use. It goes into mel scripting and has a large number of example scenes and scripts. The old fluids DVD was just a masterclass that was filmed when we first released fluids, so that one was more demo than tutorial.

The new DVD is mostly technical explanations and rough examples. I just don't have time to develop fully polished examples with "paint by number" tutorials, if that is what you are after.

Duncan

[jtk77]

Oh not at all, I really enjoy the Dvds that go over workflow, implamentation. Rather than showing either the tool set, basic stuff, or showing a lot of highend examples you can't figure out how to use in an everyday workflow situation.
I don't know, I have a couple Alias Dvds way back, didn't care for them. I havn;t seen the new Fluid Dvd yet, so I can't judge itself, just not sure with how Alias sets up thier dvds. So far, some of the gnomon ones have been outstanding. The mental ray ones and most of the dynamics ones from them are just fabulous to me.
Anyway,
I guess for now I got billow down to the point where it works for me, gets me what I want, Perlin is pretty good, it just doesn't seem to thicken enough. I might have to try what you suggested though.

But here a question...
with Billiow, I'm getting what I want over all, except is there a way to smooth out the large clumps?
Right now I have large thunderhead looking clouds , but inside of that is little cotton balls, I've been trying to smooth them more but still getting some segments.
Hope I made sense, is there a setting that controls this?

[duncanbrinsmead]

If you are doing thick cumulus clouds, they are typically so opaque as to be like a translucent surface. If you can see the small clumps inside the larger ones then it is probably too transparent. Lower the transparency nearly all the way and move the left indice of the opacity graph to the right a bit to threshold out the lowest densities.
You may also need to increase the depth max and adjust the ratio and amplitude of the noise. You also might try the smooth falloff, although this would then be starting to look more like a perlin noise. Keep the billow density at 1.0.
For the best looks you really need self shadowing. This may not be too slow if you turn off real lights on the fluid. A good place to start, if you haven't checked it, is the afternoonClouds.ma preset fluid file.

Duncan

[jtk77]

Yea changing to Smooth falloff helped a bit getting rid of those little cotton balls inside the larger clouds.
I'm using the shading method shown in the stormcloud example in the fluid library. The selfshadowing has some advantages, but honestly, its way to expensive with the settings I have at the moment. We're talking a difference of 3 mins a frame to over an hour a frame without real lights being on.

Thanks for the tips, I'm playing with depth max and the noise to see if I can get it even smoother but retain the heaviness.
When I animate the texture, I would like to get the movement to be larger, as in,billow effect to animte on a larger scale than animating those little cotton balls I'm trying to get rid of when it flows. Again, messing with it now to see what I can do, it just takes a while to see if what you're messing with it doing what you want or not.

[jtk77]

Alright, I got clouds all done, they look fine, everythings good..
However , I have a quick question or a suggestion.
I almost needed, when doing this project, some way of mixing the Billow and perlin noise texture.
Like if I could add billiow to the bottom of a Y direction ramp and Perlin on top. When you look at some storm clouds, they are thick and heavy at the bottom then they wisp out on lop. like cake icing almost. I couldn't get that effect using the texture methoud, I tried mixing a fluid with perlin into one with billow and it really looked like what I wanted, but you could really see the speration when I aniamted them together.
Anyway, anyone have any idea what I'm talking about, I'd love to hear any suggestions.

[duncanbrinsmead]

One thing you might find useful. Make the opacity graph input a y gradient, if is isn't already. One then textures the y gradient and the fluid becomes more transparent at the top( the various cloud bank files use this technique, as well as the cloudBank fluid attribute preset ).

Another option is to create a second fluid overlapping the first. You could turn off self shadowing in this second fluid so it does impact render times to badly. Also if has a more wispy and soft texture then the shading quality on it can also be set lower.

However when combining two fluids you may need to increase the volume subsamples(on the fluid nodes), particularily in the case of fluids that have thick opaque chunks. By default the render gets a single volume span from each fluid per ray( the span has a start , end and cmputed rgb+transparency). The renderer then blends these together based on degree of overlap in depth, assuming that they are solid blocks of uniform density. In some cases the assumption of uniform density works OK, but for two fluids with textured clouds it won't work very well, so you will likely prefer the result with more volume samples.

Duncan

[jtk77]

Yea I was definitly working along those lines when I was attempting to get that look. The way I was able to get something that worked was thinning out the top of the clouds with opacity.
I'll look into those subsamples, I never thought about it working that way thanks!
Might explain why I was getting a lot better results with mental ray than I was with maya renderer.

[duncanbrinsmead]

Keep in mind with the subsamples... they only make sense to use if
1. you are overlapping two fluids( or particles with a fluid.. or in general overlapping non-uniform volumes).
OR
2. you have textured fluid attributes with a shading network (as opposed to built in) texture( also must be a 3d or projective texture).

Otherwise you would just be slowing the fluid render for no reason.

Duncan

[jtk77]

thanks for the tips Duncan!