Houdini Clouds

This cloud article will be an in-depth breakdown on clouds. I will go over the types of clouds, the structure, physics, and movement. Then we will explore the Houdini aspects of clouds, and how to create them from there. Clouds, besides vellum, are my favorite study of focus. There are so many applications of clouds in film and TV, that they can be very simple to understand, to somewhat complex.

One recommendation that I would give anyone before reading this article, is to understand what a VDB is. This is important, as later our simulated clouds in Houdini will be made from VDBs.

Let's start with the different types of clouds we see in the sky everyday.

Cloud Types and Categories

There are a ton of different cloud types, but for now, lets break down the ten most basic ones. Out of these ten cloud types , they all fit into four different cloud categories(or Genera/Genus).

High Clouds: Clouds that exist at 20,000 feet or higher.

Cirrus: These clouds look like thin streamers stretching across the sky. They are an indication that warmer weather is approaching

Here is an example of this cloud: Here.

Cirrostratus: These clouds appear as sheets over the entire sky. They are thin enough that moonlight, and sunlight will bounce directly through them. These clouds usually appear before a rain storm.

Here is an example of this cloud: Here.

Cirrocumulus: These types of clouds look like scaly puffs in the sky. The exist mostly in cold weather, and in winter.

Here is an example of this cloud: Here.

Middle Clouds: Clouds that exist at 6,500 feet or higher.

Altostratus: These are blue-grayish clouds that blanket the sky. When light passes through them, it creates a fuzzy look. This cloud signals rain or snow is coming.

Here is an example of this cloud: Here.

Altocumulus: These clouds also signal that a storm might be on the way. They are much darker than their other cloud counterparts, and extend for many KM.

Here is an example of this cloud: Here.

Low Clouds: Clouds that exist below 6,500 feet.

Stratus: These are very dark grey clouds. They are often mistaken for fog. However, they do not touch the ground. Light rain can fall from these clouds.

Here is an example of this cloud: Here.

Stratocumulus: These are your lumpy bumpy sheet clouds. They appear grey, however usually no rain falls from them.

Here is an example of this cloud: Here.

Nimbostratus: These clouds are huge grey sheets that can cover the entire sky. They can cause continuous rain or snow. Here is an example of this cloud: Here.

Clouds with vertical development: Clouds that exist at any elevation, and create towering puffy structures.

Cumulus: These are your default clouds, that appear almost everywhere. These are your cartoon poofs of cotton clouds. When the weather is either stormy or nice out, these clouds will be there.

Here is an example of this cloud: Here.

Cumulonimbus: These clouds are your billowy thunderstorm clouds. They can also help cause tornadoes. Here is an example of this cloud: Here.

What Exactly is a Cloud?

Scientifically, a cloud is frozen water particles that are adrift in the air. There is a basic three step process of how clouds are formed:

#1: Water evaporates into the air

#2: Water condenses to form a cloud.

#3: The water particles are either reheated by warm air/condense into rain and are diffused into the ground or air.

Clouds are formed by air densities rising and falling. The high the air is, the colder it will be. Therefore, clouds will form faster. The warmer and closer the air is to the ground, the heavier the cloud will be. As well as the likeliness that the cloud will emit rain.

Clouds can also be formed when air is blown across terrain such as hills or mountains. This forces the warm air to be pushed upwards into the atmosphere. The air will then cool, and form a cloud.

You may also notice that a lot of clouds form over bodies of water. Such as the ocean. This is because the water creates an area of low pressure, and most commonly cold fronts. Cold fronts happen when cold air pushes warm air upwards. This can also cause Thunderstorms.

All of these weather systems mentioned. You can produce in Houdini as a procedural weather system.

Read more up on Clouds: Here.

What are Clouds in Houdini?

Clouds in Houdini are classified as VDBs, or Volumes.

VDB:  an acronym for a Sparse Voxel Storage Format.

If you would like further reading on VDBs : HERE.  

or HERE.

Volumes store voxels, which are three dimensional pixels. Volumes also control fire,smoke, and anything that has a velocity field. Keep in mind, there different types of volume types for different types of simulations. For example, if you would like to create water, you would be using a Sign Distance Field(SDF) instead. 

Houdini has multiple tutorials on how to build clouds from geometry. Feel free to visit their website, or YouTube instead.

How can I Animate Clouds?

 

Animating clouds can be hard, that animating it may result in unnatural noise, jerky animation, or long sim times. The approach to animating a cloud is based on how you created it.

If you've created some low clouds with the Billowy Smoke shelf tool. you'll be able to have a lot of control over the sim. Such as the speed,dissipation,cooling, and buoyancy. You can access these controls in the DOP network of the simulation.

Unfortunately, the shelf cloud tools in Houdini are not set up to react to heating and cooling of the cloud. The tool itself only creates a static cloud. In Houdini 17, there are some new add-ons to the cloud system, however they are mostly just used to fade the edges of the clouds, and adjust the volume voxels accordingly. You can create other simulations, and cluster them on the clouds to give them the appearance of movement. 

Another point to remember, is that sometimes all you need to do is animate how the light is coming through a cloud, rather than to move the cloud itself. You can use a Volume Scatter SOP to generate a Cd field that mantra will use to animate the way the light is refracted.

 A Volume VOP will probably be your best case in animating a cloud.

Rendering Clouds

When rendering clouds, remember they are considered volumes.

So they need to be rendered in a different way than poly-models.

or particles.

Some tips for rendering:

  • Because clouds are slightly transparent, you'll need to crank up the transparent samples. Try increasing them in increments of two. For example:  2, 4,6, 8, 10, or 12 . Keep doing this until you can no longer see any noise.

  • In this case, adding a GI light to the scene will help speed up the rendering process. This will help mantra render out the light bounces in the clouds faster. 

  • If you would like a high quality cloud. Adjust the amount of the pixel samples. The pixel samples multiples the overall quality of volumes. However, this will lead to longer render times. The same result can be achieved by increasing the Volume Step Rate. Increasing this parameter instead will also allow you to have a faster render time.

  • Decreasing the Volume Step Rate and the Volume Shadow Step Rate, will speed up your render times, but also decrease the quality of your clouds. 

  • If you overlap your clouds, mantra will render the scene faster. This is because, there are less light bounces to calculate because of the overlapping volumes. The less transparent the clouds, the less light bounces, and the more solid they will appear. 

  • Try if possible using a linear color-space for clouds, as this will also reduce noise in your clouds.

  • The Volume Limit is exactly the same as the Diffuse Limit, except it is meant for volumes. The default setting for this parameter is zero, but increasing this parameter will make your clouds more lighter as they are transmitting more light. The lower the parameter, the less light bouncing around in your cloud.

  • The volume filter is used for reducing anti-aliasing ( the box setting), and preventing banding(the Gaussian setting).

    Rendering and Incorporating Clouds with the Sky

 When building cloudscape, or a scene where there are clouds in the sky, there are some important things to be aware of.

  • Make sure you sky and scene are not over saturated. A lot of the time is sounds like brightening the blue of the sky, or pulling out the color makes sense, as you might want a bright summer sky. However, this is one of the worst things you can do. This will overpower anything else in the scene. Make sure your Hue Saturation is under control.

  • Sometimes you can make the sky of a scene look Photoshoped and unnatural by adjusting the colors too much, the saturation, or the placement of the light. Keep an eye on your scene for this effect. Sometimes it's better to lower the saturation of a sky than to raise it.

  • Don't cram your scene or sky with too many clouds. Too many clouds in the wrong placement can make your scene disorienting. Try focusing your clouds around an area of focus, and either space them out or layer them accordingly. 

  • Always remember the direction where your sunlight is coming from. Keep in mind whether or not your clouds are covering your light source, or the way your light is shining through them. 

  • Perspective of your clouds are very important. We don't really think about this too much in our day to day life, but clouds do follow the same perspective rules. The farther clouds are from the viewer the smaller they are, and the bigger the cloud, the closer it is.

After breaking all of these facts down, I thought it might be fun if next we could discuss some cloud physics, and subcategories of clouds. I will link the page down below. Feel free to read. :)