Abstract Hydrodynamics Project Files

Introduction:

So back on April 23rd, 2021, me and my friend David did a Houdini User Group presentation about abstract hydrodynamics in Houdini. This presentation also had a focus on cloudscapes and planetary atmospheres in Houdini. A lot of people were asking about the Houdini file and the build overall. So here are the files and iterations. We will update this page as the build progresses. So enjoy :)

Here is the link to the HUG event in case you missed it :) :

In case you don’t want to watch the HUG, here is some general information for general cloudscapes. But I highly recommend you watch it if you’d like a clear breakdown on how atmospheres work here on Earth and on other worlds. In the meantime, let’s talk about the build and clouds.

There are many reasons why artists may never get to simulate cloudscapes. Mostly being that when it comes to cloudscapes, the majority of the work can be done in comp and the matte painting department. As well as layout.

For example, it is also easier production wise to render and comp a matte painting. It takes a long time to simulate a planet, and even longer when you are trying to do it to scale. We’ll get into more of that later. So often when you see planets in movies such as Avengers Endgame, they are digital matte paintings with a 3D base.

However, paintings are great when you want to visualize something far away. They aren’t so great if you want to do a pan through an atmosphere, or see clouds move. So today we are going to try and do that, and incorporate some scientific ideas, and try to be efficient as possible. So maybe, just maybe, someone can try and use this in their own personal productions if they’d like.

Simulating and visualizing clouds is probably one of the most underrated things you can do in a production. Clouds are always somewhere in the background of a space shot, or through aircraft chases, and they also give a scene a lot of ambiance when it comes to the tone of a film.For example, if you working on a shot where some characters are walking through a bog or marshland at night, you’re probably going to want to fill the scene with some fog or some mist rolling off the water. So understanding how these cloud systems work is pretty important.

Science In Cloudscapes

There is a lot of science here on Earth that is also applicable to other planets in our solar system, and in interstellar space. It is a great starting point to understand the factors we are going to incorporate.It is also very important to talk about, because NASA continually studies life and environments here on Earth, to get a better sense of how life might operate on other worlds.

Here is a quick run down of the composition of Earth's atmosphere:

Earth’s atmosphere is made up of 78% nitrogen, 22% oxygen, and the remaining 1% percent made up of carbon dioxide, neon, and hydrogen. The reason we are alive on Earth is because of that lucky 22%.

It spans over 10,000 kilometers into space, and has many different layers. These layers behave differently the higher up they are, as the pressure decreases the higher up you go. This pressure pushes the elements of Earth’s atmosphere down, and this results in the majority of it being bulked around our planet’s surface. This is also important when it comes to clouds as they also behave differently depending how close to the ground they are. The higher up they are, the less air resistance there is for them to move, the farther down, the slower they become. Unless you are dealing with extreme pressures. Such on other worlds. There, cloud velocities may vary.

There are also names for the levels of Earth’s atmosphere. These names are also applicable to other planets as well. It is divided into 5 different main layers.

The closest layer to the ground is called the Troposphere.
The next layer is called the Stratosphere. This extends for over 50km.
Above that is the Mesosphere. This extends for another 85km. The coldest temperatures of the atmosphere reside in these levels.
The Thermosphere is next. This boundary extends for over 600km.
Finally, the uppermost layer is called the Exosphere.

The pressure of Earth’s atmosphere is also known as barometric pressure. It is measured with something called a barometer. And the units it measures are labeled as atm. One atm unit is equivalent to the average atmospheric pressure at sea-level. The barometric pressure of Earth is around 1 atm.

Earth’s Clouds

Clouds are classified according to their height above the ground. Depending on which category they are in, they have unique names that correspond with them. There are about 5 different categories of clouds. However, cloud species can change and overlap between categories. This isn’t a complete list of all the different types of clouds, but few that are very relevant to this presentation. But you’ll also see that a lot of creativity went into the names of these scientific cloud categories. (sarcasm)

Extreme Level Clouds: These are clouds that exist anywhere between 80-85 km in the atmosphere. The species of clouds that exist here are Noc-til-ucent clouds. As well as other wispy forms of stratiform and cir-rus.

Very-High Level Clouds: These are clouds that exist anywhere between 18-30 km in the atmosphere. The species of clouds that exist here are cir-rostratus, haze, or strato cumu-liform clouds.

High Level Clouds: These are clouds that exist anywhere between 5-13 km in the atmosphere. The species of clouds that exist here are cirro-cumulus, cir-rus, and cirrostratus clouds.

Mid Level Clouds: These are clouds that exist anywhere between 2-7 km in the atmosphere. The species of clouds that exist here are altocumulus, altostratus, and nimbostratus.

Low Level Clouds: These are clouds that exist anywhere between 0-2 km in the atmosphere.The species of clouds that exist here are stratus, cumulus, cumulonimbus, and stratocumulus.

So that is some useful information you can use in your cloudscape builds. :) Now onto the Planetary build.

Atmospheric Build


Link to build_v1 here:

So David was responsible for building this file for the presentation. And he did his best to try and incorporate the data I gave him for how planetary atmospheres operate, especially gas giants. He chose to create a more proceduralized build for VFX productions over science, as he is more comfortable working on visual effects in the film industry. But feel free to take this build file and modify it as you will.

One cool thing about this build though, is that if you turn on the lens shader/camera distortion options in Houdini you can warp this entire build into a sphere. And it looks pretty cool. You can even parent different parts of the build to the camera to further edit their movement that way.

There are also two static atmospheric layers in this build. These are mostly to make the build look better. But if you want to add minor glows, or light refraction through the planet they work pretty good with that.

There is also a layer of static instanced clouds in the build, which again are there for most detail in the upper atmosphere. You can animate them through the point velocity SOP. Or just turn off the timeshift SOP :).

Considering that gas giants are failed stars, and are mainly made up of gases. We really didn’t need to consider any collisions, or physical ground planes for this simulation. Just the clouds.

These planetary atmospheres are layered similarly to Earths, but are different in their own unique way. They have some of the same names for the levels of their atmosphere as we’d give our own.

One defining feature of a gas giant is that it has a layer of metallic hydrogen. This hydrogen is referred to as metallic, because it is so compressed in the atmosphere that it turns itself into an electrical conductor. If you are creating a build that displays the polar regions of your planet, you might want to consider adding this little detail in. As Northern and Southern Lights have been observed on these planets. In our build we were only doing side views of the planet, so we didn’t add this.

In our build we have three layers of clouds. As Jupiter, our main reference has three main levels of its atmosphere. So you will notice that there are three individual FLIP/rasterization setups.

Another thing that varies per layer/setup, is the opacity of each cloud system. Jupiter’s clouds have some main differences between it’s cloud systems. These overlaps and distinction between belts and lateral banding is caused by the differences in the opacity of its clouds.

Ammonia concentration is higher in zones, which leads to the appearance of denser clouds of ammonia ice at higher altitudes, which in turn leads to their lighter color. So coloring the clouds was based around this as well. Jupiter and other gas giants are mostly made up of helium and hydrogen. These elements also have their own unique colors. Let’s take Jupiter for example.

It’s atmosphere is very interesting because of its compounded atmospheric pressure. The upper layer of it consists of about 75% hydrogen, 24% helium, and random elements that contain the remaining 1%.

The lower levels of the atmosphere make up about 71% hydrogen, 24% helium, and 5% other elements. These trace elements are methane, H2O, silicon, and ammonia. As well as carbon, and ethane in both gas and liquid form, hydrogen sulfide, neon, oxygen, and sulfur.

When combined together, these elements produce pigments of white, orange, brown and red and blue. The white clouds are from ammonia crystals, the blue tints are from hydrogen, the orange and light pink from helium. As well as even more blue from the methane crystals in the clouds. Also, an important footnote in the color of Jupiter, because the elements in its atmosphere are ionized, the frequency of color in it might change.

Further Gas Giant Facts

Jupiter and other gas giants have something called Jovian bands, which are controlled by zonal atmospheric flows or winds. These winds are called jets. The eastward jets are found at the transition from zones heading away from the equator. And the westward jets mark the transition from belts to zones. The flow velocity patterns mean that the zonal winds decrease in lateral belts and increase in zones from the equator to the pole. Wind shear in belts is cyclonic, while in the transition zones it is anticyclonic. These in turn create anti-cyclonic storms.

The jet speeds are high on Jupiter, reaching more than 100 m/s These speeds correspond to ammonia clouds, which are high in the atmosphere and move according to their mass and current pressurization.

Atmospheric Build Part Two

Link to Build_v2:

So after our presentation we decided to revisit the build, because we wanted to include more storms across the different layers of clouds, have a better movement overall, and just generally improve things. So time to talk more about wind forces and velocity fields!

For phase two of this build we’ve started to create some custom velocity fields for our setup, to add the storms into our planet. We’ve created some custom velocities based on what type of storms and wind systems we saw on gas giants, particularly jupiter. So in my spare time, I grabbed a beer, and started setting these up.

(Thanks so much for the help Jermey :) )

Link to “Great Storm” velocity field file:

Link to Jovian bands velocity file:

Link to Transition Zone Velocity file:

Link to “Small Storm” Velocity file:

So using these different velocity fields we were able to mimic a few different storm types you see in gas giants. We’ve also named them according to their corresponding types. Feel free to check them out individually.

The “Great Storm” file is mostly for large storms. I guess you could use it for things like The Great Red Spot, but ours currently is just for large eye catching storms.

Project Thanks

-Jeffrey Mathew Phlip
-Alasgar Hasanov
-Emilis Baltrusaitis
-Ludovic Iochem
-James Albiez
-Countless Artists in Both The Keep Calm I’m in VFX and WVFX discord servers.
-Christian Schmidbauer
- Jeremy Jozwik
-The VFX Team at Mr X Toronto
-The VFX Team at Pixomondo Toronto