Creating Bodily Fluids in Houdini
So working between shots in the studio, I started to read The Body by Bill Bryson. Which is a great read if you'd like to learn about the history of medicine, and the overall understanding of the human body. In the end it got me thinking about simulating the human body, and all the fluids it contains. This article is going to cover how to simulate some of the fluids inside our bodies, and maybe a few weirder organs. Heads up, this article may also get a bit graphic as I'll be describing how these fluids work, so be prepared if you don't like squeamish stuff.
This fluid is well understood across the board. This is the substance that keep us moving, and our limbs alive and healthy. The blood is made up of red and white blood cells, as well as plasma. The red ones carry oxygen to different parts of the body, and the white ones are designed to catch infected cells, and prevent viruses from spreading. Blood is also designed to clot around any bleeding area of the body. Minus the internal organs as the blood flow is too rampant there for it to stick. (Keep this in mind for later.)
Blood is a bit thicker than other fluids. The general viscosity of blood is normally 3 × 10−3 to 4 × 10−3 per centimeter per gram, per second. Which sounds pretty confusing. The best way to describe the viscosity of blood is by the study of Hematology. This is the study of blood flow, as well as the overall study of the way blood cells and plasma move through the body.
A general rule of thumb is that the faster moving the blood flow is, the lower the viscosity. The slower the blood is, the higher it will be. This is something most artists forget when creating their simulation. You don't want the blood to appear like water, rather you want it to appear as if it is between a melted jello like substance, and an orange juice level of runniness depending on your scene.
Around our bodies there is something called arteries. Arteries pump blood upwards and out of your heart. They usually carry their own pulse, and this is why people sometimes measure their pulse on the sides of their necks, or wrists. Veins take old blood back to the heart to be replenished with oxygen. When you puncture a vein comparative to an artery, you will bleed differently. Because arteries have a pulse, blood will burst out of the body where the rupture is, and in time with your heartbeat. It will also fly farther away from the body as it is carrying more velocity. And since blood fails to clot around punctured organs or fast moving areas, it does not stop. Remember to add motion blur. ;)
When in doubt, a good particle fluid simulation will almost always work for blood.
Keep your particle separation and scale low.
For a blood material, blood is less shiny than you may think. But depending on where the blood is emitting from, and the lighting you may want to range the color for human blood between a moderate red, to a deep one. If the blood is emitting from an artery it will be darker.
Snot and Mucus
One cool thing about the nose is that it has a whole range of fluids that condense out of it. We call these fluids snot, boogers, and sometimes mucus. Depending on our feelings, time of day, sense of unwell, or weather, the fluids in our nose react differently. For example, if you come inside after a cold day, you may have to blow your nose. This is because your nose is reacting similarly to a cold coke can being pulled from the fridge. It is creating condensation, and the only way for the condensation to escape is to go down.
Snot can also contain dead skin cells from the inside of your nose as well as blood cells from nose bleeds. These chunks of mucus will be thicker and more viscous than other types of mucus. They may also appear more flaky, and stick to the inside of the nose more.
Nose mucus also helps capture any dust we may breath in. In a regular day we breath 2,000 gallons of air. Depending where we work, it varies on how much dust we breath in. So overall our nose has a hard time catching all the dirt. Fortunately for us, the mucus in our nose also lines our esophagus, lungs, and intestines. So there are plenty of time for our body to catch any allergens, or other dusty blockages.
Mucus also tends to be the lubricant for our lungs and organs. it keeps everything moving in the direction it should go. Plus, it reduces inflammation in the bowls, and protects the acid in your stomach from destroying itself. The overall study of mucus is called macrorheology. So go digging if you'd like to explore this topic more.
Creating Nasal Mucus
With any fluid, adding motion blur is key. Especially if the fluid is dripping, or flying through the air.
Particle fluids will work well for any liquid and jelly-like mucus.
For any chunky, flaky boogers, try modeling them instead. This will save you a lot of simulation time. Plus this type of mucus tends to move less.
A principle shader should work well for shading snot. it will give you more control over the color.
if your mucus is more sticky. Try playing with the friction and constraints of your simulation. Or maybe tell the particles to stick to the surface of it's collision object.
This one of the body fluids that can be one of the easier effects to create. Once again you'd use a particle fluid to create the saliva.
Saliva is 98% water, and contains enzymes to break down your food before it reaches your stomach. It also helps tell your throat when to swallow when you are eating, and helps prevent your teeth from decaying in your mouth from sugary substances and bacteria. On top of that, it helps lubricate your mouth and throat, maintain the Ph level of your body, and carry chemicals from food to your taste receptors on your touge. This is one of the reasons why people tend to salivate more when they eat. Your body needs to taste what it is ea
A fun fact about saliva is that it contains anti-bacterial agents. So if you were to lick any open sores on your body, you would be able to disinfect them.
How can we build intestines? One way is that we could use vellum. Vellum provides a multitude of ways to create soft bodies, and balloon-like objects. You could theoretically create a tube-like object and use vellum balloon from there. Vellum's soft body solver could also work if you'd like to make them more squishy than balloon like. You could also randomize the constraints on the vellum sim so some of the intestines are more loose than others. Or if you would like, turn the constraints on and off at certain times so they can fall in and out of place. Remember, combining different effects can get better results.
When asking a co-worker(David Ertsinian) how he did the dolphin car crash scene from The Boy's season one; he mentioned how he didn't even focus too much on the constraints of the simulation. Rather he focused on the emission time where the car ran over the dolphin on the pavement. It was a quicker way for creating a fast splotch of dolphin.
If you were having a monster tear apart a character or feast upon a body, than that would be a slightly different approach altogether. The insides would need to be able to stay in one place, and then slowly move over the the motion it is being carried by. Therefore, more constraints would be applied in this simulation than something like the dolphin above.
FEM could also be an option for this effect. The updates to the Houdini 18 solver allow for more viable way to squish things, and also allow for a smooth control on skin like surfaces. However, FEM is sometimes more complex than vellum, and can lead to longer setup and cache out times.
Another cool feature to mention about human intestines is that they are pretty darn large. The small intestine is about 20 feet long, and the large intestine is about 5 feet long.
I would consider this to be the second most easiest effect on this list. Sweat is the condensation of the skin. It will develop in little droplets, and when there is too much in one place or it is moved off the skin, the sweat will drip away. Sweat is probably one of the closest bodily fluids to water. Most of the time sweat is made of a combination of salt and water. If you've ever come back from a run, and had your pet lick your arm uncontrollably afterwards, they are doing this to eat the salt off your skin.
For this effect you could possibly create a POP simulation, and or use the shelf tool for condensation in Houdini. However, the shelf tool is very dependent on the size of your object. if it is too small or too big, then it will not work properly. You could also model droplets on the skin instead of simulating them.
Out of all of these fluids, this effect might be the easiest to create. When you cry, your muscles tense up and your eye ducts seal shut. With nowhere for the fluid around your eyes to go they begin to pool out of your tear ducts.
You can build tears with a simple particle system. From there, using your model, you can decide how they will drip down the person's face. You can change that particle system into a particle fluid. Or in a cartoon way, copy a tear drop model to your POP simulation.
As we've discussed above, tears exist all over your eyes to keep them from drying out. So in the case that you would like to make a simulation of someone's eyes tearing up, you'd need to consider the surrounding geometry. A tearing-up effect is incredibly subtle. So instead of creating a complete pop sim, you might want to consider an animated modeling approach. Therefore you could move the water build-up, up and over the eyelid without having to waste any simulation time.
Here's something that could be applied to more than just a human character. Almost every creature on the planet sheds it's skin. It's one of the more creepy aspects of our bodies. Depending on the placement on the body, some skin cells die faster than others. For example, skin cells in your mouth divide once every 24 hours, compared to the skin on your feet. Our skin tends to hang and turn into a white flaky layer when it dies. However, if you have a creature such as a snake, then the skin is going to peal all at once, or slowly over time in one piece.
Dead skin has the appearance of a crumpling, peeling, or flaky effect. These effects range from if it is dandruff, vs if you are peeling a scab off your body. So creating a dead skin simulation would range from a POP simulation for slow falling pieces, modeling a white layer of skin, or a soft body simulation of skin crumpling and being pushed off.