Sprite Animation Motion¶

Added in the next release (Unreleased).

SpriteAnimationHelpers reads a clip's sprite keyframes and finds the last bounds movement larger than a threshold you choose. Use it to author a landing sound or another beat against the picture: a clip can continue settling long after the subject lands. The helper is in WallstopStudios.UnityHelpers.Editor.Sprites and is available only in the editor. Call it on Unity's main thread.

Read the authored frames¶

SpriteKeyframesOf(clip, bindingPath) returns an IReadOnlyList<SpriteAnimationKeyframe>. Each entry contains Time in seconds, Sprite, and BindingPath relative to the animated root. Only SpriteRenderer sprite assignments are included; other object-reference curves and UI images are excluded.

Entries are sorted by time, then binding path using ordinal comparison. Repeated sprites and null assignments stay in the result. Pass null for all renderer paths, string.Empty for the root, or an exact path such as "Body". Path matching is case-sensitive. Null or destroyed clips return an empty list. Every call reads a fresh snapshot, so a redraw or a clip edit is visible immediately.

Find the last movement¶

LastMovingFrame(clip, settledUnits, edge, bindingPath) returns the zero-based index of the last keyframe whose selected bounds edge differs from its previous assignment by strictly more than settledUnits. Index the result of SpriteKeyframesOf with the same path filter to get the authored time. It finds the last qualifying change, so a later bounce wins even if an earlier impact was larger. Equality with the threshold counts as settled.

Bounds are in sprite-local Unity units, including the sprite's pivot and pixels per unit. For tightly cropped frames whose pivots preserve their drawn positions, the bottom edge can reveal a landing. These are not scene world coordinates: animated transforms, scale, and renderer flips are not evaluated. Artwork moving inside identical bounds cannot be detected this way.

Each renderer is compared independently. When all paths are selected, the return value indexes the combined sorted list; it is not a renderer's private frame number. A null, destroyed, or invalid sprite assignment resets that renderer's comparison. Disappearance and reappearance do not imply movement across the gap. Non-finite edge coordinates also reset comparison.

The method returns -1 for no movement, empty or single-frame clips, null or destroyed clips, negative or non-finite thresholds, and unspecified or invalid edges. Zero is a valid threshold. No clip, sprite, or asset is modified.

Author a landing time¶

Place this code in an editor assembly. Use the result in your own authored constant or validation test; there is no runtime dependency on this helper.

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namespace MyGame.Editor
{
    using System.Collections.Generic;
    using UnityEngine;
    using WallstopStudios.UnityHelpers.Editor.Sprites;

    public static class LandingTiming
    {
        public static bool TryGetLandingTime(AnimationClip clip, out float seconds)
        {
            const string bodyPath = "Body";
            int index = SpriteAnimationHelpers.LastMovingFrame(
                clip, 0.1f, SpriteBoundsEdge.Bottom, bodyPath);
            seconds = 0f;
            if (index < 0)
            {
                return false;
            }

            IReadOnlyList<SpriteAnimationKeyframe> frames =
                SpriteAnimationHelpers.SpriteKeyframesOf(clip, bodyPath);
            seconds = frames[index].Time;
            return true;
        }
    }
}

For a 63-frame entrance at 12 fps, a landing at the 46^th keyframe has index 45 and time 3.75 seconds. A subsequent settle of up to 0.06 units per frame does not move the result when the threshold is 0.1. If the authored clip length is 5.25 seconds, the normalized landing time is 3.75 / 5.25, about 0.7143. Use the actual keyframe time and actual clip length, not an assumed frame rate; clips can have uneven timing.

Related: Animation tools.