Clouds and Atmospheric Effects
So after spending the last 2 years doing a lot of atmospheric effects, I figured it was time for another cloud article. Also the beta release of my Houdini Atmospheric toolkit is here. So feel free to check it out.
Also, I never want to look at a cloud again after this………….. May the VFX gods show me mercy.
Also, if you haven’t read the three other cloud articles on the site, I strongly recommend it.
An Efficient Way of Building Clouds
So what is the best emitter you could build for a cloud? Well it really depends on where your cloud is located in the atmosphere.
If you are creating a high level cloud, then you really want to focus on creating the thin wispy sheets. So you’ll need a thin emitter, lots of velocity packed into it, and ramped densities throughout it. Ramped densities are important, because you want to keep the rasterization of the density thinner than usual, and emit from a noisy shape. Make sure the pyro simulation that you emit from this inherits the velocity. Don’t be afraid to multiply the value.
Mid level clouds are more based around sky coverage. They can extend your scene into the distance. They usually look like uniformed layers of poofy shapes. For example, altostratus and altocumulus clouds. They are much denser than high level clouds, and have less velocity than them. For these clouds, you generally want to build your emitter in a wave-like pattern. Or on a grid that has spaces in between. Then scatter some points, add a mountain SOP and you’re good to get started.
Low level clouds are your storm clouds. They need to be big and massive. They need to have lots of detail, have wispy edges, velocity shearing, and loads of density.Make your emitter big, add velocity to it, and the more complex the shape the better. Don’t be afraid to layer spheres over each other, add noise to the shape, and rescale it to fit your scene.
Quick tip: You can think of low, mid, and high level clouds in terms of BG, MG, and FG. The high level clouds would be your background clouds, the mid level clouds would be for the midground, and low level as your foreground. But it really depends where your camera is , and where it is facing.
Atmospheric Effects for Moving Objects
It's easy to create static volumetric objects. It's another to create them to move with other moving objects. As well as get their movement right to match.
90% of the battle is how you inject velocity into the emitter for your volume. If you can get this velocity off the object the volume is interacting with in the scene, then the more accurate the movement will be.
If you have an object passing through a volume or cloud, then you’ll need to see how the cloud wraps around the object, if the advected cloud needs to linger in the air, or if it needs to be pushed away. Usually you can do this with the new pyro solver as it has presets for collision objects. You can just plug in your collision object, and then switch over the collision mode to SDF+Volume Velocity.
You can create a volumetric object, inject velocity in it, and then attach it to the object you need. For example, if you were creating a thruster for an aircraft. Thrusters are easy in concept. However, if you were to try and simulate one on a fast moving aircraft, you would have a lot of stepping issues. If you do this you will have to increase the global substeps, and animate the dissipation over time. Just so the thruster has a more animated length.
Sometimes the best way to create them is out the center of worldspace in a static volume shape. Then advect or animate noise through them. Then translate and attach the thruster to the aircraft.
Thrusters also require you to taper the density. So you will need to create a falloff in order to make your thruster look believable and realistic. A lot of volumes that work with moving objects have this same falloff. Mainly because, the faster the object, the faster the movement and dissipation.
Tips and Tricks for Clouds in Production
Clouds can be annoying to make in production for a number of reasons. Firstly they eat up a lot of disk space. They also can get rather big, eat up lots of memory file wise, cause resolution issues. So let’s cover some basics.
The best storm clouds you can make in a production is with Pyro. Pyro can give you a great base shape. As well as let you easily advect certain areas of the sim, add turbulence and disturbance for a better shape. You’ll want to simulate the emitter until you find the best shape, and then save out one frame. Or an offsetted frame range if you’d like to add some movement.
You can always create a cloud from a static volume or from the other range of VDB tools in Houdini. However, this can be a bit unreliable when you are dealing with large objects that you need to rasterize. If you want a really detailed cloud the Cloud SOP and Cloud Noise SOP will make your cloud super heavy. The Cloud Noise SOP is also very temperamental based on spatial size.
Saving clouds, or caching clouds can be tricky. It's easy to save something to your disk space. But voxels take up a lot of space depending on how high res they are. So if you are creating a static cloud, you only need to save one frame. Don’t save more than you need.
You need to also factor in where your clouds are being used in the scene. If they are far away, then you probably don’t need too much resolution. If they are close to the camera, crank it up. By creating clouds for your Foreground , Midground, and Background, you can decide which clouds need the highest resolution. This in turn saves on disk space as not every cloud cache is high res.
Animating clouds is taxing. Sometimes you can get a better result by animating the noise within a cloud than the cloud itself. It can save you some time when it comes to resiming. But placing down a VOP and animating noise onto the base shape of your cloud, you can create movement within the structures that really aren't there.
Finally, what do supervisors want from clouds? Well it really depends on the scene.
If your supervisor wants a storm, you need to make a lot of low level clouds. Such as clouds with a lot of detail, density, and turbulence. Don’t give them cotton ball shaped clouds. They don’t want them to be whimsical, or happy. They need a dark and broody storm cloud.
However, from personal experience, I do find that sometimes clients don’t know what they want from clouds. They might always ask for dense cloudscapes, however you do need variations in densities throughout them. So my general rule of thumb is to present a version that has 15% of the density they ask for. Then you can work your way up from there.