Free-form sketching of poses and movements for expressive character animation

Keywords :

• character animation
• expressive motion

Free-form animation allows for exaggerated and artistic styles of motions such as stretching character limbs and animating imaginary creatures such as dragons. Creating these animations requires tools flexible enough to shape characters into arbitrary poses, and control motion at any instant in time. The current approach to free-form animation is keyframing: a manual task in which animators deform characters at individual instants in time by clicking-and-dragging individual body parts one at a time. While this approach is flexible, it is challenging to create quality animations that follow high-level artistic principles — as keyframing tools only provide localized control both spatially and temporally.
When drawing poses and motions, artists rely on different sketch-based abstractions that help fulfill high-level aesthetic and artistic principles. For instance, animators will draw lines of action to create more readable and expressive poses. To coordinate movements, animators will sketch motion abstractions such as semi-circles and loops to coordinate a bouncing and rolling motions. Unfortunately, these drawing tools are not part of the free-form animation tool set today.
The fact that we cannot use the same artistic tools for drawing when animating 3D characters has an important consequence: 3D animation tools are not involved in the creative process. Instead, animators create by first drawing on paper, and only later are 3D animation tools used to fulfill the pose or animation. The reason we do not have these artistic tools (the line of action, and motion abstractions) in the current animation tool set is because we lack a formal understanding relating the character's shape — possible over time — to the drawn abstraction's shape.
Hence the main contribution of this thesis is a formal understanding of pose and motion abstractions (line of action and motion abstractions) together with a set of algorithms that allow using these tools in a free-form setting. As a result, the techniques described in this thesis allow exaggerated poses and movements that may include squash and stretch, and can be used with various character morphologies. These pose and animation drafting tools can be extended. For instance, an animator can sketch and compose different layers of motion on top of one another, add twist around strokes, or turning the strokes into elastic ribbons. The Figure shows a summary of the content and contributions of this thesis, also summarized as follows:
•	The line of action facilitating expressive posing by directly sketching the overall flow of the character's pose.
•	The space-time curve allowing to draft full coordinated movements with a single stroke---applicable to arbitrary characters.
•	A fast and robust skeletal line matching algorithm that supports squash-and-stretch.
•	Elastic lines of action with dynamically constrained bones for driving the motion of a multi-legged character with a single moving 2D line.

Directors:

• Mr Rémi Ronfard (Chargé de Recherche - INRIA )
• Mme Marie-Paule Cani (Professeure - Grenoble INP )

Raporteurs:

• Mr Robert Sumner (Adjunct Professor - ETH-Zurich )
• Mr Michiel Van de Pann (Professor - University of British Columbia )

Examinators:

• Mme Joëlle Thollot (Professeure - Grenoble INP )
• Mr Paul Kry (Professor - McGill University )
• Mr Franck Multon (Professeur - Université Rennes 2 )