Simulating Horse Leg Musculature Using Houdini Otis

11/05: Animation Set-up and Process

With the rig built it was time to start animating.

I needed to get the reference video into Houdini so that I could animate along it.

I did so by creating a grid and applying a principled shader with the video applied as an image sequence to the shader

**NOTE: Houdini doesn't like applying videos as textures, so I had to convert the .mov to a jpg sequence.

Fig 1. Rigging Setup With Animation

Below is the progress on blocking. I wanted it to loop, so the last frame and the first frame are identical.

Fig 2: Blocking Progress

TO DO:

• Fix foot-ground contact

• Spline Animation

• Clean Up Splines

• Add Muscle Model

11/02: Navigating APEX and Setting Up the Rig:

Order of Operations:

1. Rig Skeleton Model

2. Animate Skeleton Model

3. Sculpt Muscle/Modify Capybara Test Geo Muscle

4. Simulate Muscle

In order to begin rigging the hind legs of this horse, I needed to become familiar with the APEX environment. As many of the new additions occurred in recent versions of Houdini, it was crucial that the tutorials were recent.

Referenced SideFX Rigging Tutorials:

• DRACARD

• Stylized Crab

BUILDING SKELETON

APEX is an environment which handles a lot of the rigging through a "configure autorig" node. Although there are way to circumvent this node, its base structure requires the user adhere to some very strict rules in skeleton construction:

• ALL bones must be flagged with a "side" ie. L_Hip, R_hip, C_Root

• Bones and Skeleton must be packed into a rig container prior to autorig initialization

  • Hint: Save the skeleton and skin as separate .bgeo files before packing

• Rest pose must be saved prior to initializing the environment

• Set scale to 1 on all bones

• Make sure model is facing the Z axis

**NOTE: The node will NOT FUNCTION AT ALL if you skip any of these. For those coming from Maya or Unreal Engine, this is a big change, as often in those programs, the node will still work, it will just move strangely if configured incorrectly.

Skeleton is initially built entirely in the "Skeleton" node and modified by additional nodes.

Fig 1: Hind Leg Skeleton Structure in Viewport

Useful Nodes:

• Rig Doctor: flags and resolves errors with skeleton structure, naming, and orientation

• "Rig Tree" window (NOTE: you cannot rename your joints here)

• Joint Properties: (NOTE: Click "P" when selecting a joint in the skeleton node to access. Use for renaming and checking scale)

• Orient Joints: Orients your joints for you

• Joint Deform: Allows you to see your skin deformations without building the rig

SKINNING:

I opted to paint my skin weight using the "jointcapturepaint" node as although my geometry was divided into groups on import, the groups were not clearly labelled. Had they been labelled by body part, "jointcapturebiharmonic" may have been able to procedurally skin the bones to their corresponding joints by name.

I decided that renaming the groups would take about as long, if not longer than manually painting the weights on the bones, so I opted for the latter. Since I am skinning the horse skeleton mesh, one bone geo would be bound to one joint, making this process very straightforward.

Fig 2: Skeleton and Skinning Node Structure

RIGGING:

Quadruped legs can be done a variety of ways. For a good reference on different setups, check out This Video by antCGi.

1. First one that I wanted to try was "Multi IK"

Fig 3: Multi IK Result

Result:

• Not enough Control

• Erratic Movement

2. Second was Leg and Foot

Fig 4: Leg IK with Foot Result

Result:

• Difficult to control AutoRoll pivot placements

• Too many unneeded controls/functions clutter the scene

3. Third was Leg IK and two controls for ankle and toe

Fig 5: IK Leg with Two Extra Controls Result

Result:

• Most predictable movement

• Encompasses range of motion in reference

  • THIS IS THE ONE SELECTED

10/27: Gathering Models, Reference, and Tutorials

To begin this process, I needed to source a model and a video to reference. After watching a few overview tutorials covering Otis, it was clear that the model needed both a skeletal and a muscular form as well as the final "skin mesh".

This video on Adobe Stock was deemed a good reference, as it shows clear movement of the hind leg's skin and muscle deformations where it is unobscured by the watermark.

Possible Models:

• Horse Skeleton

• Horse Skin and Skeleton

Rigging Tutorial:

• Horse Overview

Otis Tutorial Series found on SideFX:

• Tutorial Series Muscle

10/25: Defining the Scope

Given the time constraints of this project as well as my incredibly limited knowledge of muslce simulation, I chose to focus this process around simulating one limb within a Horse.

I wanted to focus on a quadroped since many muscle tutorials are centered arond bipeds (and specifically the arms of a person), and I wanted to branch out away from that. Furthermore, the upper part of the leg featured many more muscles than the lower part, which would help narrow the scope if many obstacles were encountered, but also allow me to expand if the process went smoothely.

GOALS:

1. Create a functional hind leg rig and animate it

2. Simulate muscle movement on that hind leg

3. Texture/groom the limb if time allows

4.