Believable 3D Motion: Timing, Spacing, and What the Viewer Reads

Timing vs. Spacing: Two Separate Controls

Timing is how long an action takes: the number of frames (or seconds) between key poses. If two poses are 12 frames apart, the timing is 12 frames—regardless of how the object travels between them.

Spacing is how the distance is distributed across those frames: the pattern of position changes from frame to frame. Spacing is what creates acceleration, deceleration, and changes in perceived effort, even when the timing stays the same.

In 3D, these are independently controllable: you can keep the same keyframe times (timing) and change the curve shape (spacing), or keep the same curve shape and move keys closer/farther in time.

Simple Example 1: Ball Drop

Pose A: ball at the top. Pose B: ball on the ground.

• Timing change: If A→B is 24 frames, the drop reads slower than if A→B is 12 frames.
• Spacing change: With the same 24 frames, you can make the ball feel like it accelerates (small gaps early, big gaps late) or like it floats (even gaps throughout).

Simple Example 2: Hand Wave

Think of a wave as alternating left/right extremes (Pose L and Pose R) with a passing pose between them.

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• Timing: 8 frames per side-to-side feels brisk; 16 frames per side-to-side feels casual.
• Spacing: If the hand spends more frames near the extremes (small spacing near L and R) and moves quickly through the middle (large spacing through the passing pose), it reads like a friendly, relaxed wave with clear beats. If spacing is even, it reads robotic.

What the Viewer “Reads” From Frame Count and Spacing

Speed: Duration + Distance

Viewers infer speed from distance traveled and time taken. In practice, notes like “too fast” often mean one of two things:

• Timing issue: the action completes in too few frames.
• Spacing issue: the action covers too much distance early (big spacing at the start) or lacks a readable slow-in/slow-out.

Intent: Where the Motion “Holds” vs. “Moves”

Intent is communicated by where the motion lingers. If spacing is small near a pose, the viewer feels a hold/decision. If spacing is large, the viewer feels commitment and follow-through.

• Small spacing near a pose = thinking, aiming, resisting, carefulness.
• Large spacing through the middle = confidence, throw, swing, release.

Effort and Weight: Acceleration Patterns

Effort shows up as how quickly something accelerates and how it comes to rest.

• Heavy/effortful: slower to start (tiny spacing early), then builds speed; often needs more frames to stop (longer decel).
• Light/easy: can start quickly (bigger spacing early) and stop quickly without looking strained.
• Powered motion: sudden burst (big spacing right after a pose) followed by controlled settle (tight spacing into the end).

Turning “Fast/Slow” Notes Into Measurable Edits

Vague feedback becomes actionable when you decide whether to change timing (frame count) or spacing (curve/spacing pattern). Use this translation workflow.

Step 1: Identify the Readable Poses and the Action Span

Pick the two poses that define the note.

• Example (ball drop): Top pose → ground contact.
• Example (hand wave): Left extreme → right extreme.

Measure the current duration: duration = endFrame - startFrame.

Step 2: Decide Whether the Note Is About Duration or Acceleration

• If the action feels globally too fast/slow everywhere, change timing first.
• If the action feels okay overall but “rushes the start,” “floats,” “slams,” or “drifts,” change spacing first.

Step 3A: Timing Edits (Frame Count) You Can Actually Do

Method: scale keys in time for the selected action span.

• To slow down: increase duration by adding frames between key poses (e.g., 12f → 16f or 18f).
• To speed up: reduce duration (e.g., 16f → 12f).

Practical increments that are easy to evaluate:

• ±2 frames for subtle changes (especially at 24 fps).
• ±4 frames for clearly noticeable changes.
• ±25% scaling when you need a decisive shift (12f → 9f, 16f → 12f, 24f → 18f).

Step 3B: Spacing Edits (Curve Shape) You Can Actually Do

Spacing edits are about where the motion spends frames. You can do this without changing key times.

• To reduce floatiness: add more contrast: tighter spacing near poses, larger spacing through the middle. In graph terms, strengthen ease-in/ease-out rather than staying linear.
• To reduce snappiness (too sharp): reduce the sudden jump in spacing near the start or end. In graph terms, soften tangents so acceleration ramps more gradually.
• To reduce sluggishness (never gets going): increase mid-arc spacing (make it travel more per frame in the middle) while keeping the same start/end timing.

Concrete Translations of Common Notes

Checklist: Diagnosing Floaty, Snappy, and Sluggish Motion

Before You Touch Anything: Verify the Problem Scope

• Is the start pose readable? If not, you may be missing a clear hold (timing) or you’re drifting (spacing).
• Is the end pose readable? If it never settles, you may need more decel spacing or more frames at the end.
• Is the path correct? A weird arc can look like timing/spacing issues. Confirm the motion path is intentional before over-editing curves.

If It Feels Floaty

• Symptom: constant-speed drift; no sense of gravity/force; poses don’t “land.”
• Timing check: Are there enough frames to see a change in speed? (If it’s extremely short, you may not perceive easing.)
• Spacing check: Are frame-to-frame gaps too even?
• Fix steps:
  1. Keep key times.
  2. Make spacing tighter near the start and end (stronger ease-in/ease-out).
  3. If needed, add a breakdown that defines the mid-speed (place it so the middle travels farther per frame).

If It Feels Snappy

• Symptom: motion “pops” into movement or “slams” into the stop; acceleration feels abrupt.
• Timing check: Is the action span too short for the size of the move?
• Spacing check: Is there a sudden spacing jump right after the first key or right before the last key?
• Fix steps:
  1. First try spacing: soften the curve near the start/end so acceleration ramps.
  2. If still snappy, add 2–4 frames to the span.
  3. Consider adding a small anticipation/settle pose (even a subtle one) to distribute spacing more gradually.

If It Feels Sluggish

• Symptom: it takes too long to get going; the middle feels underpowered; the action lacks punch.
• Timing check: Is the duration simply too long for the intent?
• Spacing check: Is too much of the motion happening near the start/end with not enough travel in the middle?
• Fix steps:
  1. Keep timing and increase mid spacing: move the breakdown closer to the middle (or adjust tangents) so the object covers more distance per frame mid-action.
  2. If the intent is energetic, reduce duration by 10–25%.
  3. Ensure the stop isn’t over-eased (too many frames with tiny spacing at the end can read as mushy).

Exercises: Isolate Timing From Spacing (Stepped Keys + Constant Tangents)

These drills are designed so you can change one variable while holding the other constant. Use a simple object (ball or cube) and a single control channel (Translate Y for drop, Translate X for wave). Work at a fixed frame rate (e.g., 24 fps) and keep notes on what you changed.

Exercise 1: Timing Only (Stepped Keys)

Goal: Change perceived speed using only frame count, with no interpolation influence.

Setup (ball drop poses):

• Frame 1: ball at Y=10 (top)
• Frame 25: ball at Y=0 (ground)

Steps:

1. Set the animation curve to stepped (so there is no in-between motion; only pose changes at keys).
2. Playblast/scrub: you should see the ball “teleport” from top to bottom at frame 25.
3. Now change only the end key time: try frame 13, 19, 31.
4. Observe: even with stepped keys (no spacing), the action reads faster/slower purely from duration.

What to learn: timing is measurable and independent: durationFrames is a direct control knob.

Exercise 2: Spacing Only (Constant Timing, Constant Tangents)

Goal: Keep the same duration but change the acceleration feel using spacing.

Setup (ball drop):

• Frame 1: Y=10
• Frame 25: Y=0

Steps:

1. Switch interpolation from stepped to spline (or your default smooth interpolation).
2. Set tangents to linear/constant (often called Linear tangents). This produces even spacing (constant velocity).
3. Playblast: it should feel mechanical/floaty for a “drop.”
4. Without moving key times, change tangents to create easing (e.g., Auto/Clamped, or manually adjust handles): make the curve start flatter and end steeper (or vice versa depending on your graph orientation) so spacing increases over time.
5. Playblast again: same 24-frame duration, different feel—now it accelerates.

What to learn: spacing is the distribution of motion across frames; you changed the read without changing the frame count.

Exercise 3: Hand Wave—Same Timing, Two Spacing Styles

Goal: Make two waves with identical key times but different intent.

Setup: Animate Translate X (or Rotate Z) of a hand control.

• Frame 1: Left extreme
• Frame 9: Right extreme
• Frame 17: Left extreme
• Frame 25: Right extreme

Version A (robotic):

1. Set tangents to Linear for all keys.
2. Result: even spacing, constant speed, mechanical wave.

Version B (friendly):

1. Keep the same key times.
2. Adjust tangents so the hand eases into each extreme (tight spacing near extremes) and moves faster through the middle (open spacing).
3. Result: clearer beats and more personality without changing timing.

Exercise 4: Diagnose a “Snappy Stop” With a Single Breakdown

Goal: Use one breakdown key to control spacing near the end without changing total timing.

Setup (move from A to B):

• Frame 1: X=0
• Frame 13: X=10

Steps:

1. Playblast with smooth tangents; if it snaps into the stop, continue.
2. Add a breakdown at frame 11 (keep end at 13).
3. Try breakdown values: X=8, then X=9.5.
4. Observe: with the same timing (1→13), you can allocate more frames to deceleration by pulling the breakdown closer to the end value, tightening spacing near the stop.

What to learn: breakdown placement and value are spacing tools; they reshape how motion is distributed across frames.