Qt 3D Animation C++ Classes

Key frame animation data can either be created programmatically via the Qt 3D Animation APIs such as Qt3DAnimation::QKeyFrameData or it can come from digital content creation (DCC) tools such as Blender, Maya or 3D Studio Max. Qt 3D provides an example export script for animation data for Blender. The format consumed by Qt 3D Animation at present is a simple JSON based format. This allows both developers and artists to easily work with animation data. More formats optimised for runtime consumption will be added later.

The key frame animation data can be loaded from file using the Qt3DAnimation::QAnimationClipLoader class. To specify animation data programmatically use the Qt3DAnimation::QAnimationClip class.

By default, the key frame data is specified using cubic bezier curves. This allows smooth animations to be created from a small number of key frame data points. Other interpolation types will be added later.

In addition to the animation data containing the key frames, Qt 3D Animation also provides APIs for playing the animations and mapping the resulting property values onto properties of objects in your simulation. There are currently two ways of playing the animations:

• Qt3DAnimation::QClipAnimator

• Qt3DAnimation::QBlendedClipAnimator

Both of these are implemented as subclasses of Qt3DCore::QComponent meaning that objects of these types can be aggregated by Qt3DCore::QEntity objects to add animation capabilities to your simulated entities.

The Qt3DAnimation::QClipAnimator class allows the playback of a single Qt3DAnimation::QAbstractAnimationClip at a time. To add an animation to an entity, simply add an instance of the Qt3DAnimation::QClipAnimator class to your entity's components property.

The Qt 3D Animation module takes a slightly different approach to QPropertyAnimation and AbstractAnimation. With those animation frameworks, the animation specifies both the animation values and the target objects and properties. The animation components in Qt 3D separate these two orthogonal concepts. For example, the Qt3DAnimation::QClipAnimator component has a clip property for specifying the animation data (Qt3DAnimation::QAnimationClip or Qt3DAnimation::QAnimationClipLoader).

This allows calculation of the animated values, but more information is needed in order to map these values onto properties of objects. This is accomplished with the a Qt3DAnimation::QChannelMapper which contains a list of Qt3DAnimation::QChannelMapping objects. A Qt3DAnimation::QChannelMapping is used to map a specific channel from an animation clip onto a named property of a target object. By separating the animation data and property mappings like this, the same animation can be applied to many objects without needing to have multiple copies of the animation data or objects. It also allows animation data to easily be retargeted to other objects.

The Qt3DAnimation::QBlendedClipAnimator component allows to go beyond what is possible with Qt3DAnimation::QClipAnimator by blending several animation clips together before applying the property changes to the target properties. The animator component still takes a channel mapper just like the standard Qt3DAnimation::QClipAnimator component. However, instead of specifying a single animation clip, it is necessary to set the blendTree property to point to the root node of a blend tree.

A blend tree is a data structure representing how animation clips get aggregated or blended together as the function of properties on the blend tree nodes. The currently supported set of blend tree nodes are:

• Qt3DAnimation::QClipBlendValue

• Qt3DAnimation::QLerpClipBlend

• Qt3DAnimation::QAdditiveClipBlend

The source animation clip inputs are specified as leaf nodes in the blend tree using instances of the Qt3DAnimation::QClipBlendValue class. These animation clips can be combined in a large number of ways. For now the Qt3D Animation module provides linear interpolation (LERP) and additive blend operations. More blend node types will be added over time. These are expected to include at least a generalised LERP node and a barycentric LERP node.

As an example consider the following blend tree:

Clip0----
        |
        Lerp Node----
        |           |
Clip1----           Additive Node
                    |
            Clip2----

Let's assume that Clip0 represents a walk animation cycle with a duration of 3 seconds and that Clip1 is a run animation cycle with a duration of 2 seconds. These are both inputs (and dependencies) of the Lerp blend node. The result of evaluating the Lerp node depends upon the blendFactor property of the Lerp node. This could be bound to the speed of a humanoid character entity for example. As the speed of the character increases the animation gradually cross-fades from the walk animation in Clip0 to the run animation in Clip1.

Furthermore, let's assume that Clip2 represents some variation animation that can be added on (waving arms or shaking head for e.g.). The amount of this additive clip that is added can be controlled by the additiveFactor property on the Additive blend node.

When evaluating a blend tree, normalized time (or phase) is used so that clips of different durations can be blended together without problems. For example, even though the walk and run animation clips are of different lengths, as long as they are created by the animator such that the foot-falls line up at the same phase these can be nicely interpolated.

The implication of this is that the duration of the blended clip is actually a function of the blend factors of the nodes in the tree. Considering only the Lerp node in the above example, when the blend factor of the Lerp node is 0, only the walk animation in Clip0 is used resulting in a duration of 3 seconds. With a blend factor of 1 the resulting duration will be 2 seconds. For intermediate blend factors, the duration will be linearly interpolated between 3 and 2 seconds.

By definining your own blend trees, you have complete control over how to combine your collection of input animation clips. The blend tree can be configured by the properties on the blend nodes. Note also that the properties on the blend nodes themselves are just standard properties, so these could in turn be driven by other animations if desired.