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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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Art Directing Gore: Timing is Everything:

When you work on a gore shot at a studio, most of the time you will have to match the effect to the timing of the shot, as well as your supervisor's vision. Here are some tips that can help you do that.

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Should You Cover The Character’s Face?

This is a hard take when it comes to gore effects. Depending on where the blood and various other fluids are coming from.

Usually, for any effect it’s a good rule of thumb for any effect to never cover the character’s face. Covering a face or the front of a character can draw attention away from the main focus of the scene. For example, if a conversation is taking place, you probably don’t want to have a dust cloud pass in front of the character while they are talking. It can hide what the character is saying, and muddle the expression of the dialogue being delivered.

But what should you do if you are simulating a head explosion, or a character vomiting blood?

The direction of the splatter matters. If it’s coming towards the camera, then at some point it will obscure the character. You just have to choose where it should be. Generally, it's good to keep the splatter away from the character’s eyes. Eyes are where most of the emotion of the face is conveyed, besides the mouth. Since the mouth is one of the many holes of the body where blood can come from, you can probably count that as an emitter of fluid.

Another reason for avoiding covering a character’s eyes is that the lights from the scene can reflect off of them, which will assist in adding a deeper emotion to the character’s expressions. If your gore effect is leading up to a character passing away, you probably also want their last expression to add to the shock factor of it all.

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Body Placement of Blood Splatter

Should you make the gore look artistic? Well that’s really up to your supervisor.

Placement matters with gore effects. If a character’s shoulder is emitting blood, then you probably want the blood dripping down the character’s shoulder. However, the majority of the notes you receive might be towards how the blood drips down the character’s arm. Depending on the theme of your production, a general random smear might be alright, or perhaps you might need some artistically controlled blood streams around the arm muscles.

If the purpose of the shot and the sequence is to show how the injury will impact the character overtime then you’ll probably have to keep that in mind while creating your blood setup. You’ll need to make sure you can have controls in your setup so you can control the emission of the blood, and the continuity in the overall sequence. The more injured the character is, the longer the blood trail down the arm is going to be.

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If a character is talking with fluid leaking out of its mouth, you probably want to be careful with how the fluid impacts the character’s words, and how it flows over the mouth. You will have to be careful on meshing issues with particle fluids. Meshing issues can include uv, normal, uneven thickness in geometry, and incorrect motion blur. These issues can distract from the words being spoken in the scene.

You probably also need the fluid being emitted to flow in relation to the amount of force that is causing it to be emitted. More force and action means more globular splatter, as well as larger amounts. Less force will equate to a smaller stream trickling and hugging the character’s facial features. This concept can also be applied to other features of the body as well.

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Holes.

How should I add holes to something?

Let’s say you have a character that has an item blasted through it, or is impacted by bullet holes. Where should those holes be, and how should they look?

Well, sometimes bullet holes in characters are added in comp as a 2D effect, or as stock 2D elements that the department might have on hand. However, if you have to tackle them in 3D, you’ll have to keep note of how many objects are impacting the character, and how and where the geometry collides with the character. You also need to think about how lighting will approach lighting the now exposed internal sections of the body.

If you are working with two sets of animation. One containing the character anim, and the other the colliding object, you’ll most likely need to create a color visualizer to show the location of impact. You can do this by making one object white, the other another color, and then using an Attribute Transfer SOP between the two. The radius of impact is always going to be slightly larger than the size of the object due to the speed of impact.
Wherever a character is impacted by an object, their body will always be forced away in the opposite direction. After the impact has slowed down, the body of the character will start to react to the injury. This might mean collapsing due to the size of it, tightening or stiffening to hold the injury in place, or skin flaps dropping down to cover the hole.

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Rule of thumb for holes:

Don’t over think them: Assess your shot. Check to see if it’s a close up shot of a wound or far away. You don’t need to rebuild the human internal system organ for organ within the wound. You can just create a basic crumbled mesh, and layer some blood over it. Or anything else that is needed.

Keep them away from the edges/outline of your character: Adding bullet holes that hit the edges of your character’s shoulders or legs is pretty inconvenient. Only do this unless explicitly told to do so. If you do this you are adding more work to your setup, and you’ll have to account that blasting away part of the character outline will cause more bone, tendons, and muscle flaps to appear.

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Timing is Everything:

Especially in Dark Comedic Effects

Depending on the production you are working on, you might have to create a system for the fluid splatter or gory elements to emit and move on specific shot beats.

Blood generally times its release from the body based on the heart rate of the person. Every second or so artery blood can spurt out. Blood being released from the veins will flow out in a slower manner.

In productions such as The Boys, gore is used in a comedic manner. Several gory explosions through the seasons are timed against character reactions and moments that lean into more relaxing or juxtaposing tones. Such as the infamous first episode scene of season three, and the head explosions in season two.

The head explosion scene is a great example of dark comedy. The first explosion goes off, and then the rest slowly ramp up as the surrounding characters become more panicked. The splatter also has a great dynamic timing of how it interacts with the other elements and characters in the scene. The effects were clearly timed with the plate, and art directed in a way where it still maintains the gravity of the situation and adds an element of comedy.


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References:

How could one do gore and dismemberment in Houdini: https://www.sidefx.com/forum/topic/69085/?page=1#post-293799

Gore monster in Houdini: https://www.youtube.com/watch?v=_YwxkNBjwUw

Mortal Kombat 11: Blood Production in Houdini: https://80.lv/articles/mortal-combat-11-blood-production-in-houdini/

Good Houdini Blood Tutorials: https://www.reddit.com/r/Houdini/comments/j8skvg/good_houdini_blood_tutorials/

VFX Gore (Hole in Face) Version 2: https://www.reddit.com/r/vfx/comments/cj871b/vfx_gore_hole_in_face_version_2/

VFX Assets 003: Blood Hits: https://www.youtube.com/watch?v=TJfIFpqOSwI

Blood effects: https://www.sidefx.com/forum/topic/47348/

Best practices for Bloody VFX?: https://realtimevfx.com/t/best-practices-for-bloody-vfx/13937

Day 15 | Animals: Drool: https://www.sidefx.com/forum/topic/74680/?page=4#post-317572

How to make a drool???: https://www.sidefx.com/forum/topic/10435/?page=1#post-50049

simulating flesh- vellum: https://www.sidefx.com/forum/topic/84424/?page=1#post-365268

Skin geometry node: https://www.sidefx.com/docs/houdini/nodes/sop/skin.html

How to make Vellum tissue more smooth and flesh-like?: https://www.reddit.com/r/Houdini/comments/u8z9wf/how_to_make_vellum_tissue_more_smooth_and/

Zombie Skin Generator Made With Houdini: https://80.lv/articles/zombie-skin-generator-made-with-houdini/

head explosion: https://sajadalavi.gumroad.com/l/iliza

Procedural Bone and Flesh Shader: https://www.sidefx.com/gallery/procedural-bone-and-flesh-shader/

Meat Tearing | Houdini Effect: https://www.youtube.com/watch?v=3A1JISCogbU

Vellum fluids setups: https://www.sidefx.com/docs/houdini/vellum/fluidsetups.html

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