USD Anim Project Status

USD Anim is IN DEVELOPMENT. It is not yet ready for general use.

Mostly Complete

The following are believed close to final:

• The Spline / KnotMap / Knot API.
• USD serialization formats (usda, usdc).
• Bezier evaluation.
• Anti-regression.

That said, these aspects are still subect to change.

Still to Come

UNIMPLEMENTED API

Hermite Evaluation

Currently Ts only supports Bezier curves. It will be expanded to support Hermites as well, matching Maya behavior. Hermites have fewer degrees of freedom than Beziers - tangent lengths are fixed - and, in return, they are faster to evaluate, requiring only a cubic evaluation rather than an iterative parametric solve.

Evaluation Variations

• Sample(): fast curve approximation for drawing.
• A simple evaluation cache class.
• Find the time regions where two splines differ.

Spline Editing

• Split(): add knots without affecting the curve shape.
• RemoveKnot() with affectedIntervalOut.
• ClearRedundantKnots().
• Simplify and resample.

Looping

Ts supports both inner loops and extrapolating loops. These are currently handled correctly in Bezier evaluation.

There will also be support for baking, in which copies of the knots are returned with duplicates made to represent loops. This is often convenient for interactive spline editing.

Loops are not yet correctly supported by the anti-regression system.

Queries

• GetValueRange()
• IsLinear()
• IsC*Continuous()
• IsSegment{Flat,Monotonic}()
• IsKnotRedundant()

ADDITIONAL FEATURES

Automatic Tangents

Ts will support one of Maya's automatic tangent algorithms, allowing Bezier knots to assume "nice" tangents based only on their values. The specific algorithm has yet to be chosen.

Reduction

Ts will not require clients to implement every feature. A simple client, for example, may permit series authoring, but support only Bezier curves. We want all clients to be able to read all series, so we will allow clients to transform Ts series into other Ts series that use only a specified set of features. Most such conversions will be exact. Examples include the emulation of Hermites, looping, and dual-valued knots.

USD INTEGRATION

Attribute Value Resolution

This will be the main purpose of USD Anim: to have a USD attribute's value be determined by a Ts spline or series. This work will be done last, after everything else is finished.

In addition to evaluating exactly at a given time, UsdAttribute::Get() will be able to evaluate at a pre-time, a "limit from the left". This will allow introspection of discontinuous value functions, including from held interpolation and dual-valued knots.

usdview

Basic visualization of splines will be added to usdview.

Scalar xformOps

Currently, USD's xformOps support scalar-valued rotations, but for translation and scaling, only vector values (three scalars, one for each spatial axis). We will expand the xformOp schema to allow scalar values for translations and scales also. This will make USD Anim more useful in its first release: all basic transforms will be animatable with splines. (Ts will not support splines of vectors, only linearly interpolating TsLerpSeries of them.)

TESTS AND DOCUMENTATION

There will be extensive additional:

• Correctness tests.
• Performance tests.
• Documentation.