Animation Blend Tree: UE5 and Unity Setup Guide

What an Animation Blend Tree Does

An animation blend tree is the system inside Unreal Engine and Unity that smoothly interpolates between multiple animation clips based on runtime parameters — character speed, movement direction, weapon state, or any value your game logic produces. Where a state machine manages discrete transitions between animation states, blend trees manage continuous blending within a state. Together they form the core of every production character animation system.

Blend Trees vs. State Machines

The distinction matters in practice. An animator state machine handles transitions between mutually exclusive states — you cannot be walking and jumping at the same time, so those are separate states with a transition between them. An animation blend tree handles continuous blending within a single logical state — a character can be at any speed between idle and sprint simultaneously, blending between the corresponding clips in real time.

Every production locomotion system uses both: a state machine at the outer level (idle, locomotion, airborne, combat) and a blend tree inside the locomotion state (idle → walk → run → sprint based on speed). See the animation state machine guide for how the two systems connect.

1D vs. 2D Blend Trees

1D Blend Tree (Blend Space 1D in UE5)

Uses a single axis — typically Speed — to blend between clips. At Speed 0 the idle plays; at Speed 150 the walk plays; at Speed 400 the run plays. The engine interpolates between the nearest clips at any intermediate value. Use 1D when your character always moves forward and direction is handled by the character's rotation — common in third-person games where camera-facing direction drives rotation.

2D Blend Tree (Blend Space 2D in UE5)

Uses two axes — typically Speed and Direction — to blend between a grid of clips. This allows a character to walk left, right, forward, or backward while maintaining correct body posture without rotating to face the direction of movement. A 2D locomotion blend space typically uses 8 clips at the compass positions at walk speed and repeated at run speed — 16 clips total for full directional coverage. Essential for over-the-shoulder shooters, cover systems, and any game where the camera and character facing direction can be decoupled.

Setting Up a Blend Tree in UE5

1. In the Content Browser, right-click → Animation → Blend Space 1D (or Blend Space for 2D). Assign your skeleton.
2. Configure the axis: name it Speed, set the range (0 to 500 for walk-run-sprint).
3. Drag your idle animation to position 0, walk to ~150, run to ~300, sprint to 500.
4. Preview the blend by scrubbing the axis. Look for foot sliding or pop at blend transition points — these indicate clips need better alignment in timing or root position.
5. In your Animation Blueprint's AnimGraph, drag in the Blend Space asset and connect it to the Output Pose node.
6. In the Event Graph, calculate Speed from the character's velocity: GetVelocity → VectorLength → Speed variable → Blend Space input.

Setting Up a Blend Tree in Unity

1. In your Animator Controller, right-click in the Animator window → Create State → From New Blend Tree.
2. Select Blend Type: 1D for speed-only, 2D Simple Directional for speed + direction.
3. Set the parameter (Speed, or Speed + Direction for 2D).
4. Add Motion fields: idle at 0, walk at 0.3, run at 1.0 for 1D. For 2D, position each directional clip on the grid.
5. Enable Loop Time and Loop Pose on each locomotion clip in the clip import settings.
6. Drive from code: animator.SetFloat("Speed", agent.velocity.magnitude, 0.1f, Time.deltaTime) — the smoothing parameters prevent jitter from frame-by-frame velocity changes.

Advanced Blend Tree Techniques

Additive Animations

Additive animations overlay a secondary pose on top of any base state without requiring a unique clip for every combination. An additive lean animation records only the difference between the neutral run cycle and the leaned run position. At full weight the character leans; at half weight the lean is half as pronounced; at zero weight the base run plays unmodified. This single additive clip covers the full lean range, versus requiring 5–7 directional lean clips at fixed angles to achieve the same result with standard blending animation.

Aim Offsets

Aim offsets in Unreal Engine are a specialized form of additive blend tree that drives upper-body pose based on camera pitch and yaw, allowing a character to aim at any point in a hemisphere without a unique clip per angle. A high-quality aim offset requires clips at the extremes of the aim range — full up, full down, full left, full right, and center — captured at the same energy level as the locomotion base pose.

Layered Blending

Layered blending (Layered Blend Per Bone in UE5, Avatar Masks in Unity) resolves the problem of combining upper and lower body animation independently. The blend node's bone mask isolates which bones each layer drives. The quality of the blend depends heavily on the transition zone at the spine — a gradual blend across spine-01 through spine-03, applying the upper body layer at 30%, 60%, and 100% weight respectively, produces a natural result rather than a mechanical split.

Common Blend Tree Problems and Fixes

• Foot sliding at blend points — clips have different stride lengths. Resample clips to match stride length, or use Motion Matching in UE5.
• Pop when transitioning to/from blend tree — state transition blend time too short. Increase blend time on the transition arrow.
• Character snapping direction on strafe — using 1D blend tree but needing 2D. Upgrade to a 2D blend space with directional clips.
• Speed parameter jittering — raw velocity fluctuates frame to frame. Smooth the input parameter before passing it to the blend space.

Getting Quality Clips for Your Blend Tree

A well-built animation blend tree is invisible to the player — the character simply moves with natural fluidity. Getting there requires quality animation clips that match in timing and energy, configured so the blend space finds the right blend at every parameter value.

MoCap Online's motion capture animation packs are captured as matched sets — walk cycle, run cycle, idle, and direction variants all at the same energy level and timing — so they blend together cleanly without artifacts. Browse locomotion packs or download a free sample pack to test blending animation quality in your engine before purchasing.