Tech Notes For Short Film Project

Short Film Previsualisation

Film Previsualisation Created in Motion Builder
Soundtrack “Reflection” Silver Bone Tone

This is more a side-project where some important processes could be practiced
The piece is not far from a completed set of film shots. This one remains as a several weeks ‘practice’.
Motion Capture for robotic type character
Customisation of Human skeleton, to something slightly different with regards to rigging and retargeting
Creation of visual storyboard, script, cinematography
Organising a reasonably complex scene and character into coherent set of data and visualising, pretty close to render-ready

Tech Notes Automated ‘Two Elbows’ Rig Extension

Details on construction of the Arms Rig Extension to handle the structure of creatures arms.

The key thing here is that the rig works within a motion capture pipeline.

Motion Capture drives the character, the rig extension is ‘packaged’ alongside the character and solves the non-human bits, at all times.

1. Description of a technical solution to automatically generate solve for a 3 bone arm chain (shoulder to wrist).

The Seraph character has three articulations from shoulder to wrist. All the character animation for the project is driven from motion capture data, where the regular biped (and me the actor) has two bones in the chain from shoulder to wrist. Shots are built using Motion Builders Story Editor, using a Character track. Since animation clips are brought into Story as externally referenced files, and these files can be updated at any time – example for cleaning data – the cleaning of data was able to be done after most of the shots were built in Story. Ideal that animation data remains within a Character for all steps of the project. Character Extension is the solution. I’ve built a system in Motion Builder consisting of an IKChain constraint which solves the 3 bone chain, based on the nodes in the 2 bone chain, and minimal extra fussing. Animation of pole vector node is all that’s required (Quite easy as they are parented to upper spine anyhow and generally positionally fine for motion of the motion coming through).

The IKConstaints parameters are the skeleton start and end node, a Pole Vector node and the End Effector node. Three extra Parent/Child constraints are made. 1) End Effector Child/Parent links to the regular Character Wrist Effector on the Control Rig. 2) The First chain of the 3 bone skeletons Parent/Child links to the regular Shoulder Effector in the Character Control rig. It only links translation though – rotation is handles by the IKChain constraint for this bone. 3) A Parent/Child constraint to bind the Pole vector node to a suitable part of the body such that it has to be hand-animated only in a few instances where arm pose is extreme.

This system, consisting of the IKChainConstraint, the three extra constraints, the the 3-bone chain ‘extra’ skeleton are packaged into a Motion Builder Character Extension, and hence can be available as part of the character – including handing in Story mode, and inclusion of it’s data for import / export fbx Character data. (Option exists to include Character Extensions as part of Character data). Also Preferred angles are set in the skeleton chain also since Tpose and the character model itself meant that the initial chain (Stance Pose) was not nicely curved ready for a 3 bone chain solve. Attention was given to make sure the new 3-bone chain has optimal rotational axis (joint rotation), as well as optimal axis solve order for each joint, for purpose of the chain solver. Rotational axis consistent, Y axis pointing bone direction. Axis order Euler XYZ for each (except last node which isn’t important for the solve). The regular two-bone chain remains in project. Motion Builder recognises it as input data for Characterisation. However skinning binds the model to the 3-bone chain instead. After setup, the system remains stable, and solves nicely, and works in Story.

Red = Conventional 2-bone structure
Green = Character Extension 3-bone structure. Sky’s the limit really on which extensions can be added to conventional control rig.


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