Blend trees in Unity - Step-by-Step Execution

Concept Flow - Blend trees

Start Animation State

↓

Get Input Parameters

↓

Evaluate Blend Tree

↓

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.

Practice

1. What is the main purpose of a Blend Tree in Unity's animation system?

easy

A. To smoothly mix multiple animations based on parameters like speed or direction

B. To create 3D models for characters

C. To write scripts for controlling animations

D. To manage audio clips in a game

Solution

1. Step 1: Understand what blend trees do

   Blend trees combine animations smoothly based on input parameters.

2. Step 2: Identify the correct purpose

   They help create natural transitions by mixing animations like walking and running.

3. Final Answer:

   To smoothly mix multiple animations based on parameters like speed or direction -> Option A

4. Quick Check:

   Blend trees = smooth animation mixing [OK]

Hint: Blend trees mix animations smoothly using parameters [OK]

Common Mistakes:

• Confusing blend trees with 3D modeling tools
• Thinking blend trees are for scripting
• Mixing up audio management with animation blending

2. Which of the following is the correct way to create a 1D blend tree in Unity's Animator window?

easy

A. Right-click in Animator > Create Layer > Add Blend Tree > Set parameter to bool

B. Right-click in Animator > Create State > From New Animation Clip > Set parameter to int

C. Right-click in Animator > Create State > From New Script > Set parameter to string

D. Right-click in Animator > Create State > From New Blend Tree > Set parameter to float

Solution

1. Step 1: Recall how to create blend trees

   In Animator, you create a new state from a blend tree and assign a float parameter for blending.

2. Step 2: Match the correct steps

   Right-click in Animator > Create State > From New Blend Tree > Set parameter to float correctly describes creating a blend tree state and setting a float parameter.

3. Final Answer:

   Right-click in Animator > Create State > From New Blend Tree > Set parameter to float -> Option D

4. Quick Check:

   Create blend tree with float parameter = Right-click in Animator > Create State > From New Blend Tree > Set parameter to float [OK]

Hint: Blend trees need float parameters, not int or bool [OK]

Common Mistakes:

• Using int or bool parameters instead of float
• Creating animation clips instead of blend trees
• Trying to create blend trees in layers instead of states

3. Given a 2D blend tree with parameters Speed and Direction, what happens when Speed is 0 and Direction is 1?

medium

A. The character plays the idle animation because speed is zero

B. The character runs forward because direction is 1

C. The character blends between walk and run animations

D. The character plays a backward animation regardless of speed

Solution

1. Step 1: Understand parameter roles in blend tree

   Speed controls movement intensity; 0 means no movement (idle).

2. Step 2: Analyze given values

   Speed=0 means no movement, so direction does not cause running.

3. Final Answer:

   The character plays the idle animation because speed is zero -> Option A

4. Quick Check:

   Speed 0 means idle animation [OK]

Hint: Speed 0 means idle, direction alone doesn't move character [OK]

Common Mistakes:

• Assuming direction alone triggers movement
• Thinking blend tree blends walk and run at zero speed
• Ignoring that speed controls animation intensity

4. You created a blend tree but the character animation does not change when you adjust the parameter. What is the most likely cause?

medium

A. The blend tree has too many animations

B. The parameter used in the blend tree is not linked to the Animator Controller

C. The Animator window is closed

D. The character model is missing textures

Solution

1. Step 1: Check parameter linkage

   If the parameter controlling the blend tree is not connected, animations won't change.

2. Step 2: Evaluate other options

   Too many animations or closed Animator window don't stop blending; missing textures affect visuals, not animation.

3. Final Answer:

   The parameter used in the blend tree is not linked to the Animator Controller -> Option B

4. Quick Check:

   Unlinked parameter stops animation change [OK]

Hint: Ensure blend tree parameters are linked in Animator [OK]

Common Mistakes:

• Blaming number of animations for no change
• Not linking parameters properly
• Confusing visual issues with animation blending

5. You want to create a blend tree that smoothly blends between idle, walk, and run animations based on speed, but also changes direction smoothly. Which setup is best?

hard

A. Use multiple 1D blend trees, one for speed and one for direction, without combining them

B. Use a single 1D blend tree with only 'Speed' parameter and ignore direction

C. Use a 2D blend tree with 'Speed' and 'Direction' float parameters, assigning animations at correct positions

D. Use animation layers instead of blend trees to handle speed and direction

Solution

1. Step 1: Understand blending needs

   To blend speed and direction smoothly, a 2D blend tree with both parameters is ideal.

2. Step 2: Evaluate options

   Multiple 1D trees won't combine parameters smoothly; ignoring direction loses natural movement; layers handle different animations but not smooth blending.

3. Final Answer:

   Use a 2D blend tree with 'Speed' and 'Direction' float parameters, assigning animations at correct positions -> Option C

4. Quick Check:

   2D blend tree with speed and direction = best setup [OK]

Hint: Use 2D blend tree for smooth speed and direction blending [OK]

Common Mistakes:

• Using separate 1D blend trees without combining
• Ignoring direction parameter
• Relying on layers instead of blend trees for smooth blending