Blend trees in Unity - Step-by-Step Execution

Concept Flow - Blend trees

Start Animation State

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Get Input Parameters

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Evaluate Blend Tree

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Calculate Blend Weights

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Mix Animations Based on Weights

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Output Final Animation Frame

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Loop

Blend trees take input values, calculate weights for animations, mix them smoothly, and output the final animation frame.

Execution Sample

Unity

BlendTree blendTree = new BlendTree();
blendTree.AddMotion(walkAnimation, 0f);
blendTree.AddMotion(runAnimation, 1f);
float speed = 0.5f;
AnimationClip current = blendTree.Evaluate(speed);

This code creates a blend tree with walk and run animations and evaluates the blended animation at speed 0.5.

Execution Table

Step	Input Parameter (speed)	Blend Weights (walk, run)	Action	Output Animation
1	0.0	(1.0, 0.0)	Select walk animation fully	walkAnimation
2	0.5	(0.5, 0.5)	Blend walk and run equally	Blended walk/run animation
3	1.0	(0.0, 1.0)	Select run animation fully	runAnimation
4	1.2	(0.0, 1.0)	Clamp to max run animation	runAnimation
5	Exit	N/A	Input out of range or animation played	Stop or loop animation

💡 Input parameter exceeds blend range or animation ends, stopping evaluation.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
speed	undefined	0.0	0.5	1.0	1.2	1.2
blendWeights	undefined	(1.0, 0.0)	(0.5, 0.5)	(0.0, 1.0)	(0.0, 1.0)	(0.0, 1.0)
outputAnimation	undefined	walkAnimation	Blended walk/run animation	runAnimation	runAnimation	runAnimation

Key Moments - 2 Insights

Why does the blend weight for run animation stay at 1.0 even when speed is 1.2?

Because the blend tree clamps input values to the max defined motion (1.0), so speeds above 1.0 use full run animation as shown in execution_table step 4.

How does the blend tree decide the weights for walk and run at speed 0.5?

At speed 0.5, the blend tree calculates weights proportionally between walk (0.0) and run (1.0), resulting in equal weights (0.5, 0.5) as in step 2.

Visual Quiz - 3 Questions

Look at the execution table, what are the blend weights at step 2 when speed is 0.5?

A (1.0, 0.0)

B (0.5, 0.5)

C (0.0, 1.0)

D (0.25, 0.75)

💡 Hint

Check the 'Blend Weights' column at step 2 in the execution_table.

At which step does the blend tree use only the run animation without blending?

A Step 3

B Step 2

C Step 1

D Step 5

💡 Hint

Look for where blend weights are (0.0, 1.0) in the execution_table.

If the input speed was 0.25, how would the blend weights change compared to step 2?

A Weights would be (0.5, 0.5)

B Weights would be (0.25, 0.75)

C Weights would be (0.75, 0.25)

D Weights would be (1.0, 0.0)

💡 Hint

Weights are proportional to input between 0.0 and 1.0 as shown in variable_tracker for speed and blendWeights.

Concept Snapshot

Blend trees mix animations smoothly based on input parameters.
Define motions with parameter values.
Input controls blend weights between motions.
Weights sum to 1 for smooth transitions.
Clamp inputs outside defined range.
Output is the final blended animation frame.

Full Transcript

Blend trees in Unity take input values like speed to decide how much to mix different animations. The process starts by reading the input parameter, then calculating blend weights for each animation based on that input. For example, if speed is 0.5 between walk (0.0) and run (1.0), the blend weights are equal, mixing both animations evenly. If the input goes beyond the max defined value, the blend tree clamps it to the max animation. This way, the output animation smoothly changes as input changes, creating natural transitions. The execution table shows step-by-step how input affects weights and output animation. Variables like speed and blendWeights update each step, guiding the final animation frame output.