Blend Trees
A common task in game animation is to blend between two or more similar motions. Perhaps the best known example is the blending of walking and running animations according to the character’s speed. Another example is a character leaning to the left or right as he turns during a run.
It is important to distinguish between Transitions and Blend Trees. While both are used for creating smooth animation, they are used for different kinds of situations.
- Transitions are used for transitioning smoothly from one Animation State to another over a given amount of time. Transitions are specified as part of an Animation State Machine. A transition from one motion to a completely different motion is usually fine if the transition is quick.
- Blend Trees are used for allowing multiple animations to be blended smoothly by incorporating parts of them all to varying degrees. The amount that each of the motions contributes to the final effect is controlled using a blending parameter, which is just one of the numeric animation parameters associated with the Animator Controller. In order for the blended motion to make sense, the motions that are blended must be of similar nature and timing. Blend Trees are a special type of state in an Animation State Machine.
Examples of similar motions could be various walk and run animations. In order for the blend to work well, the movements in the clips must take place at the same points in normalized time. For example, walking and running animations can be aligned so that the moments of contact of foot to the floor take place at the same points in normalized time (e.g. the left foot hits at 0.0 and the right foot at 0.5). Since normalized time is used, it doesn't matter if the clips are of different length.
Using Blend Trees
To start working with a new Blend Tree, you need to:
- Right-click on empty space on the Animator Controller Window.
- Select from the context menu that appears.
- Double-click on the Blend Tree to enter the Blend Tree Graph.
The Animator Window now shows a graph of the entire Blend Tree while the Inspector shows the currently selected node and its immediate children.
The Animator Window shows a graph of the entire Blend Tree. To the left is a Blend Tree with only the root Blend Node. To the right is a Blend Tree with a root Blend Node and three Animation Clips as child nodes.
This gives a graphical visualization of how the animations are combined as the parameter value changes (as you drag the slider, the arrows from the tree root change their shading to show the dominant animation clip).
You can select any of the nodes in the Blend Tree graph to inspect it in the Inspector. If the selected node is an Animation Clip the Inspector for that Animation Clip will be shown. The settings will be read-only if the animation is imported from a model. If the node is a Blend Node, the Inspector for Blend Nodes will be shown.
You can choose either 1D or 2D blending from the Blend Type menu; the differences between the two types are described on their own pages in this section.
Blend Trees and Root Motion
The blending between animations is handled using linear interpolation (ie, the amount of each animation is an average of the separate animations weighted by the blending parameter). However, you should note that root motion is not interpolated in the same way. See the page about root motion for further details about how this might affect your characters.
1-D Blending
The first option in the Inspector of a Blend Node is the The Blend Type. This drop-down is used to select one of the different blend types that can blend according to one or two parameters. 1D Blending blends the child motions according to a single parameter.
After setting the Blend Type, the first thing you need is to select the Animation Parameter that will control this Blend Tree. In this example, the parameter is direction which varies between –1.0 (left) and +1.0 (right), with 0.0 denoting a straight run without leaning.
Then you can add individual animations by clicking the small “+” button and selecting from the popup menu. When you’re done, it should look something like this:
The diagram at the top of the Inspector shows the influence of each of the child motions as the parameter varies between its minimum and maximum values. Each motion is shown as a little blue pyramid (the first and last are only shown in half), and if you click and hold down the left mouse button on one them, the corresponding motion is highlighted in the motion list below. The peak of each pyramid defines the parameter value where the motion has full influence, meaning that its animation weight is 1 and the other animations have a weight of 0. This is also called thethreshold of the motion.
The diagram at the top of the Blend Node Inspector visualizes the weights of the child motions over the range of the parameter values.
The red vertical bar indicates the value of the Parameter. If you press in the Preview at the bottom of the Inspector and drag the red bar in the diagram left and right, you can see how the value of the parameter is controlling the blending of the different motions.
Parameter Range
The range of the parameter used by the Blend Node is shown below the diagram as two numbers to the left and right. Either one of them can be changed by clicking on the number and dragging left or right with the mouse. Note that the values correspond to the threshold of the first and last motion in the motion list.
Thresholds
You can change the threshold value of a motion by clicking on its corresponding blue pyramid in the diagram and dragging it left or right. If the “Automate Thresholds” toggle is not enabled, you can also edit the threshold value of a motion in the motion list by typing in a number in the number field in the Threshold column.
Below the motion list is the checkbox Automate Thresholds. Enabling it will distribute the thresholds of the motions evenly across the parameter range. For example, if there are five clips and the parameter ranges from –90 to +90, the thresholds will be set to –90, –45, 0, +45 and +90 in order.
The drop-down will set the thresholds from data of your choice obtained from the root motions in the Animation Clips. The data that is available to choose from is speed, velocity x, y, or z, and angular speed in degrees or radians. If your corresponds to one of these properties, you can compute the thresholds using the Compute Thresholds drop-down.
| Property: | Function: |
|---|---|
| Speed | Sets the threshold of each motion according to its speed (the magnitude of the velocity). |
| Velocity X | Sets the threshold of each motion according to its velocity.x. |
| Velocity Y | Sets the threshold of each motion according to its velocity.y. |
| Velocity Z | Sets the threshold of each motion according to its velocity.z. |
| Angular Speed (Rad) | Sets the threshold of each motion according to its angular speed in radians per second. |
| Angular Speed (Deg) | Sets the threshold of each motion according to its angular speed in degrees per second. |
Say, for example, you had a walk animation that covered 1.5 units per second, a jog at 2.3 units per second, and a run at 4 units per second, choosing the option from the drop-down would set the parameter range and thresholds for the three animations based on these values. So, if you set the speed parameter to 3.0, it would blend the jog and run with a slight bias toward the jog.
2D Blending
The first option in the Inspector of a Blend Node is the The Blend Type. This drop-down is used to select one of the different blend types that can blend according to one or two parameters. The 2D blending types blends the child motions according to two parameters.
The different 2D Blend Types have different uses that they are suitable for. They differ in how the influence of each motion is calculated.
2D Simple Directional: Best used when your motions represent different directions, such as “walk forward”, “walk backward”, “walk left”, and “walk right”, or “aim up”, “aim down”, “aim left”, and “aim right”. Optionally a single motion at position (0, 0) can be included, such as “idle” or “aim straight”. In the Simple Directional type there should not be multiple motions in the same direction, such as “walk forward” and “run forward”.
2D Freeform Directional: This blend type is also used when your motions represent different directions, however you can have multiple motions in the same direction, for example “walk forward” and “run forward”. In the Freeform Directional type the set of motions should always include a single motion at position (0, 0), such as “idle”.
2D Freeform Cartesian: Best used when your motions do not represent different directions. With Freeform Cartesian your X parameter and Y parameter can represent different concepts, such as angular speed and linear speed. An example would be motions such as “walk forward no turn”, “run forward no turn”, “walk forward turn right”, “run forward turn right” etc.
After setting the Blend Type, the first thing you need is to select the two Animation Parameters that will control this Blend Tree. In this example, the parameters are velocityX (strafing) and velocityZ (forward speed).
Then you can add individual animations by clicking to add an Animation Clip to the blend tree. When you’re done, it should look something like this:
The positions in 2D blending are like the thresholds in 1D blending, except that there are two values instead of one, corresponding to each of the two parameters. Their positions along the horizontal X axis correspond to the first parameter, and their positions along the vertical Y axis correspond to the second parameter. A walking forward animation might have a velocityX of 0 and a velocityZ of 1.5, so those values should be typed into the Pos X and Pos Y number fields for the motion.
The 2D Blending Diagram
The diagram at the top of the Inspector shows the positions of the child motions in the 2D blend space. The motions are shown as blue dots. Motions with no Animation Clip or Blend Tree assigned have no influence on the blend and are shown as gray dots. You can select a motion by clicking on its dot in the diagram. Once selected, the influence of that motion for each point in the blending space is visualized as a blue field. The field is strongest right under the position of the motion, where the motion has full influence, meaning that its animation weight is 1 and the other animations have a weight of 0. Further away the influence decreases as the influence of other motions take over.
The diagram at the top of the Blend Node Inspector visualizes the weights of the child motions over the extends of the parameter values.
The red dot indicates the values of the two Parameters. If you press in the Preview at the bottom of the Inspector and drag the red dot in the diagram around, you can see how the values of the parameters are controlling the blending of the different motions. In the diagram you can also see the influence of each motion represented as circles around each motion. You will see that if you move the red dot on top of one of the blue dots representing a motion, the circle for that motion gains its maximum radius and the circles for all other motions shrink down to nothing. At positions that are in between several motions, multiple of the nearby motions will have an influence on the blend. If you select one of the motions in order to see the blue influence field of that motion, you can see that as you move the red dot around, the circle size of the motion corresponds exactly with how strong the influence field is at various positions.
When no motion is selected, the diagram shows a mix of all the influence fields that is more blue where a single motion dominates and less blue where many motions contribute to the blend.
Positions
You can change the positions of a motion by clicking on its corresponding blue dot in the diagram and dragging it around. You can also edit position coordinates of a motion in the motion list by typing in numbers in the number fields in the Pos X and Pos Y columns.
The drop-down will set the positions from data of your choice obtained from the root motions in the Animation Clips. The data that is available to choose from is speed, velocity x, y, or z, and angular speed in degrees or radians. If one or both of your correspond to one of these properties, you can compute the Pos X and/or Pos Y using the Compute Positions drop-down.
| Property: | Function: |
|---|---|
| Velocity XZ | Sets the Pos X of each motion according to its velocity.x and the Pos Y according to its velocity.z. |
| Speed And Angular Speed | Sets the Pos X of each motion according to its angular speed (in radians per second) and the Pos Y according to its speed. |
Furthermore you can mix and match by choosing and/or to only auto-compute one of them at a time, leaving the other unchanged.
| Property: | Function: |
|---|---|
| Speed | Sets the Pos X or Pos Y of each motion according to its speed (the magnitude of the velocity). |
| Velocity X | Sets the Pos X or Pos Y of each motion according to its velocity.x. |
| Velocity Y | Sets the Pos X or Pos Y of each motion according to its velocity.y. |
| Velocity Z | Sets the Pos X or Pos Y of each motion according to its velocity.z. |
| Angular Speed (Rad) | Sets the Pos X or Pos Y of each motion according to its angular speed in radians per second. |
| Angular Speed (Deg) | Sets the Pos X or Pos Y of each motion according to its angular speed in degrees per second. |
Say, for example, that your parameters correspond to sideways velocity and forward velocity, and that you have an idle animation with an average velocity (0, 0, 0), a walk animation with (0, 0, 1.5), and two strafe animations with velocities of (–1.5, 0, 0) and (1.5, 0, 0) respectively. Choosing theVelocity XZ option from the drop-down would set the positions of the motions according to the X and Z coordinates of those velocities.
Additional Blend Tree Options
The options below are common to both 1D and 2D blending.
Time Scale
You can alter the “natural” speed of the animation clips using the animation speed number fields (the columns with a clock icon at the top), so you could make the walk twice as fast by using a value of 2.0 as its speed. The button rescales the speeds of the clips so that they correspond with the chosen minimum and maximum values of the parameter but keep the same relative speeds they initially had.
Note that the drop-down is only available if all the motions are Animation Clips and not child Blend Trees.
Mirroring
You can mirror any humanoid Animation Clip in the motions list by enabling the mirror toggle at the far right. This feature enables you to use the same animation in its original form and in a mirrored version without needing twice the memory and space.
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