Gore Effects: Part Two
Intro:
Since I had the chance to write my previous gore article, times have changed. I’ve also had the chance to work on various other gore effects. Plus, some other amazing other Houdini gore tutorials have been released. The various techniques have enlightened me, and I figured it was time to revisit this topic.
Link to part one: https://www.katexagoraris.com/creating-bodily-fuilds-in-houdini
Disclaimer: if you are squeamish, this probably isn’t the article for you.
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Tutorials:
Before we get started on our deep dive, I want to point out the amazing tutorial artists out there who have taken their time to touch on this topic. I recommend using these tutorials if you want to be a gore pro.
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Simulating Blood Splatters in Houdini: https://www.youtube.com/watch?v=TcWy4ig0BEE
Houdini Redshift - Vellum softbody tearing tutorial - Part I - Softbody sim: https://www.youtube.com/watch?v=2EqLi2r1Ccw
This is how you create Bloodbaths in Houdini and Redshift!: https://www.youtube.com/watch?v=i2Zk_u1Af1s
Building and Tearing a Flesh Portal for Diablo IV: https://www.youtube.com/watch?v=XauHIveIVuA
Houdini Redshift - Vellum softbody tearing tutorial - Part IV - Blood sim: https://www.youtube.com/watch?v=jFrv1V-wcyU
MK11 Blood Splatter From Houdini to Unreal Engine Niagara | VAT | RBD to FBX |: https://www.youtube.com/watch?v=NgW6ukL5g_4
Boiling Blood in Houdini Tutorial: https://www.youtube.com/watch?v=zLMV7cty2P4
Compositing Blood Tutorial - ActionVFX Quick Tips: https://www.youtube.com/watch?v=YJl_dWeOLlI
Blood VFX Pack - TUTORIAL 01 : How to implement blood spatter [NIAGARA / Unreal Engine]: https://www.youtube.com/watch?v=fwMe7KV_wck
UE4 VFX For Games - How to create blood textures, shaders and particles!: https://www.youtube.com/watch?v=J3pVQrElBcE
The Flippy Murders: https://www.rohandalvi.net/flippy
Houdini Tutorial | Yolk Albumen Simulation: https://www.youtube.com/watch?v=SDMgQ9HfVaE
Houdini 19.5 Tutorial: Melting Goo (Viscosity Fluid sim): https://www.youtube.com/watch?v=BMvIPA1ehfg
Creating Realistic Saliva in Houdini FX: A Step-by-Step Guide: https://www.youtube.com/watch?v=SyG3djUaUUc
Let's Make Some Drool || Houdini Tutorial: https://www.youtube.com/watch?v=toi4Y3bmerE
Houdini | Procedural Drool and Saliva (Part 2): https://www.youtube.com/watch?v=7j75qESxyUY
Houdini Muscle System Tutorial - Muscles - Part 01 - Wild VFX: https://www.youtube.com/watch?v=UU7jiX5TNVM
Patreon Course Preview - Tutorial #9 - Skin Shading & Sub-Surface Scattering: https://www.youtube.com/watch?v=di-AsfgQuG4
Stylized Skin Growth in Houdini: https://www.cgcircuit.com/course/stylized-skin-growth-in-houdini
Houdini 19 Tutorial: Creating Wrinkles & Rendering Them In Karma XPU: https://www.youtube.com/watch?v=zY8ahnF73A4
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Deep Cuts: Emitters:
-- Where and how should you emit something gory? - It really depends on what you are simulating.
Never emit blood, mucus, or any other liquid from anything other than the character or wound itself. I often see people attempting to place a sphere or a grid inside the object that has the fluid emitting from it. If you isolate the area on the character or object that the fluid should be emitted from , you’ll have smoother interactions and a more realistic emission.Emitting from an object that does not match the wound will cause fluid to be emitted in unusual places, and may cause the fluid to interact with the collision objects weirdly.
You might have to layer emitters.if you are dealing with a wound that has blood emitting from arteries and veins. Blood coming from arteries moves faster, and probably will have a larger starting velocity than the rest of the blood. This means you can do one of two things. Make two separate emitters, one with a larger velocity than the other. Or you can randomize or localize the velocity in certain areas of the emitter.
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Attributes:
Attribute control is some of the most important things you can do in gore sims.
If you are working with FLIP simulations, or POP Fluids, you most likely will be working with various attributes. Mass, velocity, density, etc. A lot of these attributes can be controlled and set up pre-simulation.
For example, let’s say your goal is to create an accurate simulation of a head explosion. There is going to be the initial pop of the skull cracking open, the blood and flying debris, the blood pouring out from the neck, slower blood trails flowing over the neck, etc. You'll have a lot of simulations to layer, and need to set that up extremely efficiently. Not to mention, control the overall timing of everything.
When creating simulations, 90% of the problem is always something to do with the emitter. 90% of your time will be spent modifying it, and the incoming attributes. Simulations just take the information they are given, and act accordingly on them.
By controlling attributes such as viscosity, mass, velocity pre-simulation, you can move faster through your simulation. Plus, if you edit attributes pre-simulation you don’t have to wait for the simulation to playback to see the changes, switch houdini over to manual, and have more confidence that the changes in the attributes are correct even before they are simulated.
You can easily create sliders and fit statements in Houdini with attribute wrangles for attributes to accurately animate timing of attributes to your shot. For example, let’s say your supervisor wants the velocity of the blood from your character’s neck (post head explosion) to slow down with each spurt over time. You could use this wrangle method to slowly decrease the velocity vectors over time. Which in turn will lower the starting velocity per frame.
This method is also great if you have multiple holes that gore needs to be emitted from, but need different areas to emit liquid with different speeds.
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Simulations:
You don’t always need to simulate bodily fluids. You can model them. There is a great tool called The Draw Curve SOP that allows you to draw lines across any piece of geometry in Houdini. If you need to draw trails of fluid coming away from a source, this is the node for you.
You can also use the Attribute From MAP SOP to bring in pictures of fluids into Houdini, then manipulate the picture through the color attributes to give it some 3D features.
VEX is also a very useful option for non-sim simulations.
There is a great tutorial series from a colleague of mine David Torno. He’s created this amazing Liquid Sops series which teaches you how to simulate fluids without needing DOPs.
Check it out here: https://www.houdini.school/courses/hs-224-liquid-sops
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Vellum Fluids:
Since Houdini 19 came out, vellum fluids have introduced a new way of simulating fluids. The reason I mention them for gore simulations is because fluids are heavy. Simulating them can be a pain, caching them even more so. Vellum fluids are lighter than traditional particle fluids. These fluid particles can interact with grains, cloth, and soft bodies easily.
Physical attributes like Density, Viscosity, and Surface Tension can be used to simulate different types of fluids. Surface tension controls a fluid’s tendency to contract and create drops or tendrils. This can be great if you need tendrilish oozes.
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Some fun facts about the human body.
-- Rate of blood flow varies greatly between different organs. Liver has the most abundant blood supply with an approximate flow of 1350 ml/min. Kidney and brain are the second and the third most supplied organs, with 1100 ml/min and ~700 ml/min.Rates of blood flow per tissue vary.
-- Blood loss through bleeding can be traumatic for anyone. A healthy adult can lose almost 20% of blood volume (1 L) due to a large injury and 40% of volume (2 L) before shock sets in.
-- Experts debate the amount of saliva that a healthy person produces. Production is estimated at 1500 ml per day and researchers generally accept that during sleep the amount drops significantly.
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Shading and Rendering:
Rendering gore is always a challenge. A lighting artist once told me a story of trying to light the inside of a CGI body that had been torn apart through simulations. The biggest struggle was trying to figure out how to light the skin and surrounding internal structure while there was fluid streaming out of the wound. The aim was to make sure the emission point of the blood held the focus through the shot, and the skin it was passing by was also visible.
This posed a few challenges. The first being that the light rig had to be placed behind the simulations, but this in turn created shadows that were projected towards the camera. Which hid the detail in the shot. So more lights were added in the surrounding front tissue to diffuse the shadows. Suddenly, everything became a bit too bright, and it was suggested that they just dial the brightness of the render down in comp. Which ended up working.
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A lot of bodily fluids have the following attributes in their appearance:
Matte-Like: Blood and mucus is not as reflective or refractive as you think. They have a matte-like appearance, and few areas where the light reflects off of it.
Transparency/Opacity: Opacity is the extent to which something blocks light. Transparency is the quality of being able to see through (or partially see through) an object. Blood, Mucus, Snot, and other fluids coming from mammals have an interesting texture to them where they can look transparent or opaque depending on the amount. The larger the amount, and the more crusty, vicious the fluid is, the more likely it is going to be opaque. It can also be less transparent. But this is dependent on the type of fluid you are creating.
Color: Bodily fluids almost always have very solid colors. Red, green, white, etc. But depending on the amount of the fluid, the darker the color is going to be. For example, deeper wounds are going to have darker amounts of blood due to the amount. Venous blood will be darker and arterial will be brighter.
Stains: Most disgusting fluids will stain anything they come into contact with. This means the stain will need to be rendered with its own mask to make sure it has the correct shininess, wetness, and other features that the untouched areas do not have. Contact detail will make sure the fluids in your scene appear realistic. The wet areas underneath it will also grow alongside the amount of fluid that it is associated with.
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Rendering wise…There really isn’t a preferred render engine for gore. But lately Karma is proving to be extremely helpful when rendering fluids. The realtime viewport is very helpful for previewing fluids, which will help you adjust your material settings before you commit to your final render.
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