Arcs and Path of Action: Clean Trajectories in 3D Space

Arcs are the “default” shape of organic motion because most movement is driven by rotating joints, swinging limbs, and objects traveling around pivots. In 3D, the arc principle expands from a 2D curve on screen to a trajectory through space plus a matching pattern in rotation. Clean arcs are not about making everything curvy; they are about choosing the correct path of action (curved or linear), then keeping that path consistent from every viewing angle.

1) When motion should arc vs. move linearly

Use arcs when the motion is driven by rotation or a pivot

Look for a center of rotation (real or implied). If a part is swinging around a joint, the end of that part should trace an arc in world space.

• Character limbs: wrist and ankle paths usually arc because the shoulder/hip rotates and the elbow/knee folds.
• Head turns: the nose tip and ears trace arcs as the head rotates around the neck.
• Props attached to hands: a sword tip, a flashlight beam origin, or a handbag bottom will arc as the wrist rotates.
• Thrown or swung objects: a pendulum, a door, a bat, a tail, antennae—anything with a hinge or flexible chain.

Use linear motion when constrained by contact, mechanics, or intent

Some actions read best with straight segments, especially when there is a constraint, a track, or a deliberate “aim.” Linear does not mean robotic; it means the path is intentionally straight in space.

• Sliding contact: a hand sliding along a table edge, a foot planted then translating minimally, a box pushed across a floor.
• Mechanical guides: elevator up/down, piston travel, drawer pull, camera dolly on rails.
• Targeted reaches: a fingertip moving straight toward a button can be a clean line, then arc away after contact.
• Projectile segments: even a thrown ball has a parabolic arc overall, but between closely spaced frames it can appear nearly linear; don’t force extra curvature where none exists.

Mixing arc and line: the common “line-then-arc” pattern

Many actions combine both: a straight approach to a target, then an arc as the body rotates away, or an arc that straightens briefly during a constrained moment.

2) Evaluating arcs in the viewport and through motion trails

In 3D, an arc can look perfect from one view and be broken in depth. Evaluate in at least two orthographic views plus your shot camera. The goal is a trajectory that is clean in world space, not just “pretty” in a single projection.

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Step-by-step: check translation arcs with motion trails

• Step 1: Choose the right control. For a limb, check the end-effector control (wrist/ankle) and also a mid control if available (elbow/knee) to confirm the chain is behaving.
• Step 2: Turn on motion trail / editable motion path. Display enough frames to cover the full action plus a few frames before/after.
• Step 3: Inspect in multiple views. Check the shot camera, then a side view and top view (or any orthographic views that reveal depth changes).
• Step 4: Look for continuity. A clean arc has a smooth curvature with no sudden corners. Spacing markers on the trail should progress smoothly without erratic clustering that doesn’t match the intended motion.
• Step 5: Confirm contact constraints. If a hand is supposed to stay on a surface, the trail should lie on that plane (or along that edge) during the contact interval.

Step-by-step: check rotation arcs (orientation) separately from translation

Translation trails won’t reveal a “rotation kink” where the object’s position is fine but its orientation flips or wobbles. Evaluate rotation with visual guides:

• Step 1: Enable local axes display (or a custom axis/arrow control) so you can see orientation changes clearly.
• Step 2: Scrub frame-by-frame around key moments (direction changes, contacts, fast turns). Watch for sudden axis flips or micro-wobbles.
• Step 3: Use an orientation trail if available (some tools can draw a trail for a chosen axis, e.g., the forward vector). This makes rotational arcs visible as a path traced by the axis tip.
• Step 4: Compare against intent. A head turn should have a smooth rotational progression; a mechanical knob might rotate at a constant rate; a sword swing should have a clean leading edge direction.

What “bad arcs” look like in practice

• Kinks: a visible corner in the motion trail where the curve changes direction abruptly.
• Wobble/jitter: tiny zigzags in the trail, often caused by over-keying or mismatched tangents.
• Depth collapse: the arc is smooth in camera view but in top/side view it becomes a flat line or a sudden Z pop.
• Rotation pop: the object’s position path is clean, but the orientation snaps or spins unexpectedly.

3) Fixing arc issues by adjusting breakdowns and tangent handles (not random keys)

Clean arcs come from well-placed breakdowns and coherent curve shapes. Adding “fix keys” on every other frame often hides the problem in one view while creating jitter in another. Prefer fewer, smarter edits: adjust the breakdown pose and the curve tangents so the path becomes smooth in 3D space.

Workflow: fix a kinked translation arc

• Step 1: Identify the kink frame range. On the motion trail, locate where the path corners. Note the keys that surround that corner (usually a key pose, a breakdown, or an accidental extra key).
• Step 2: Check if the breakdown is doing the wrong job. A breakdown should define the path. If it is off-plane or drifting in depth, the arc will kink when projected into 3D.
• Step 3: Move the breakdown in 3D, not just in camera view. Adjust in orthographic views to place the control on the intended arc. If the action is a swing, imagine a circle around the pivot and place the breakdown accordingly.
• Step 4: Edit tangents on the translation curves. In the graph editor, smooth the tangents so the curve transitions are continuous. Avoid tangent shapes that create overshoot unless overshoot is intended.
• Step 5: Re-check the trail in multiple views. The arc should now be smooth from top/side and still read correctly in the shot camera.

Workflow: fix jitter without adding keys

Jitter is often a symptom of conflicting curve shapes across X/Y/Z channels or tiny “correction keys” fighting each other.

• Step 1: Find dense key clusters. If there are many keys close together, consider whether they are all necessary.
• Step 2: Remove redundant keys strategically. Delete keys that were added as patches, keeping only the keys that define intent (poses and true breakdowns). Then reshape tangents to recover the motion.
• Step 3: Unify the curve intent across axes. If X is easing in while Z is easing out, the combined 3D path can wobble. Adjust tangents so the axes cooperate to form one clean spatial curve.
• Step 4: Use weighted tangents (if available) to control curvature. Small tangent length changes can remove a bump without introducing new keys.

Workflow: fix rotation artifacts by editing rotation curves and choosing stable representations

Rotation problems often come from Euler angle discontinuities (values jumping) or from an axis order that becomes unstable near certain orientations.

• Step 1: Identify whether the issue is a curve discontinuity or a gimbal-like axis interaction. If a channel suddenly jumps from, for example, 179 to -179, you likely have a discontinuity that needs curve continuity (often called “unroll” or “filter”).
• Step 2: Apply an Euler filter / unroll operation to make the rotation curves continuous over time (tool names vary by software).
• Step 3: Change rotation order if needed. Choose an order that puts the dominant rotation on the last axis (common practice) to reduce coupling. Test by scrubbing through the problematic range.
• Step 4: Adjust breakdown orientations. A rotation arc is defined by the in-between orientation. If the breakdown is “twisted” relative to the intended swing, the interpolation may take a strange route.
• Step 5: Smooth tangents on rotation curves. Avoid sharp tangent angles that cause sudden angular acceleration unless it’s a deliberate snap.

Practical example: cleaning a wrist arc in a swing

Scenario: a character swings a hammer. In camera view the hammer head looks fine, but in top view the wrist path has a corner halfway through.

• Check the wrist control motion trail: the corner aligns with the mid-swing breakdown.
• In top view, move the breakdown so the wrist stays on a consistent curve around the shoulder pivot (or the implied arc of the swing).
• In the graph editor, smooth the translation tangents around that breakdown; ensure X and Z curves don’t fight (one overshooting while the other undershoots).
• Re-check the hammer head (a child of the wrist): its tip should now trace a cleaner, broader arc without a mid-swing “hiccup.”

4) Avoiding common problems

Kinked paths: corners caused by mismatched breakdowns

Cause: a breakdown placed off the intended arc, or tangents that change direction abruptly.

Prevention checklist:

• Place breakdowns to define the path, not just the pose silhouette.
• Check arcs in orthographic views early, before polishing.
• Prefer tangent edits over adding mid-frame keys.

Jitter: micro-zigzags from over-keying and noisy curves

Cause: too many keys, tiny value changes, or conflicting easing across axes.

Fix pattern: reduce keys → smooth tangents → verify trail smoothness. If you must add a key, add it as a purposeful breakdown that clarifies the path, not as a patch.

Gimbal-like rotation artifacts: flips, wobbles, and unexpected spins

Cause: Euler angle interpolation taking an unintended route, discontinuities, or an unstable rotation order near a problematic orientation.

• Use continuity tools: Euler filter/unroll to remove jumps.
• Choose a better rotation order: match the dominant rotation axis of the action.
• Animate with clear breakdown orientations: don’t rely on interpolation to “guess” the twist.
• Consider isolating twist: if your rig supports separate twist controls (e.g., forearm twist), keep twist distributed rather than dumping it into one control.

Arcs that look correct in one camera angle but collapse in another

Cause: animating to the shot camera only, creating a 2D-looking arc that is actually a flat or broken path in depth.

Step-by-step safeguard:

• Step 1: Block and refine while regularly checking at least one orthographic view that reveals depth (top or side).
• Step 2: Use motion trails in world space, not just screen space.
• Step 3: If the arc collapses, adjust the breakdown’s depth (Z) first; then refine tangents.
• Step 4: Re-check parallax: scrub the camera move (if any) to ensure the path reads consistently throughout the shot.

Quick diagnostic table: symptom → likely cause → best first move

Polish rule: one clean idea per control

When a control’s path is unclear, it’s usually because multiple intentions are layered into the same channel set (translation and rotation both trying to “steer” the motion). Decide what creates the arc: translation around a pivot, rotation around a joint, or a constraint-driven line. Then simplify the curves so the motion trail shows one readable trajectory in 3D space.