H3 → Game Sprites

Mortal Kombat filmed actors and digitized the footage into sprites. Do the same with H3 as the actor: one 5 s clip per move (~$0.64), cut down to 8–16 pose extremes, keyed to transparency, validated with numbers rather than vibes.

Read the h3-video skill for H3 API mechanics (submitting, polling, frame pinning, the parallel runner). This skill is the sprite layer on top of it.

Why a 5-second clip becomes a quarter-second move

The video is source footage, not gameplay. 5 s at 24 fps is ~120 frames; a game move needs 8–16. H3's slow, deliberate performance is an advantage — more in-betweens to choose from, less blur per frame. Game timing (startup, active, recovery) is authored separately in frame data and never inherited from the clip.

1. The character still

Generate with scripts/gen_still.py (Nano Banana, ~$0.07, --batch for parallel style probes). The prompt must nail four things:

• Side view facing right, full body including feet, centered, feet planted on an invisible ground line. Cropped feet make a character you cannot anchor.
• Flat solid magenta #FF00FF, edge to edge, no gradient, no cast shadow, no ground line. This is your chroma key.
• Clean cel: no film grain, no dust, no scratches, no paper texture, no vignette. Beautiful grain destroys the key — say so explicitly, because style prompts tend to add it.
• Bold outlines and flat fills survive downscaling to sprite size.

Style choice is a motion question, not a taste question: some styles H3 simply won't animate. Probe several looks, then spend one clip motion-testing the finalist before committing to a full move set. Ink wash, claymation, rubber-hose and photoreal move well; flat vector and paper cutouts risk freezing.

Extra end-pose stills (KO, downed) are generated from the idle as a reference image. Say "EXACTLY ONE character, completely alone, no second figure, no duplicate" — asking for "knocked out" while showing a standing reference reliably returns both a standing and a lying character in one frame.

2. Clip briefs, one per move

The idle pin. Every move clip sets first_frame = last_frame = the idle still. This buys three things at once: the character cannot drift off-model (H3 keeps identity only when it is anchored in a locked frame), the animation returns exactly to neutral so it loops, and moves chain cleanly in a state machine. Moves that end elsewhere (KO) pin their own end frame instead.

Use the beat structure from h3-video, plus these sprite-specific guards — each one was learned by getting the opposite:

The character's hands and feet stay their normal size at all times - NO
ballooning, NO growing fists; limbs may stretch but hands and feet keep
constant scale.
Camera locked on a tripod, no zoom, no pan, no cut.
The background stays a perfectly flat uniform solid magenta at all times - no
gradient, no cast shadow, no ground line, nothing else enters the frame.
The character stays centered on the same ground level and does not travel
across the frame; everything not described stays static.
Audio: SFX only. NEVER generate music, melody, score or musical tones.

• Walks are briefed in place ("as if on a treadmill") — the engine moves the transform, so a sprite that travels fights the engine.
• Hit-stun and KO react to "an invisible blow", since no opponent is in frame.
• Cartoon models love exaggeration. Unforbidden, fists balloon on impact frames.

A basic set — idle, walk forward, walk back, jump, crouch, punch, kick, block, hit-stun, KO — is ~$6.50 and submits as one parallel batch. Consider skipping the back-walk clip and playing the forward cycle in reverse (a classic 2D fighter trick): asked for a backward stride, H3 tends to give a wary sway.

3. Decompose and validate

python3 scripts/sprite_cut.py clips/v_punch.mp4 sprites/punch --frames 12
python3 scripts/sprite_cut.py clips/v_walk.mp4  sprites/walk  --frames 8 --loop

Writes keyed/*.png (RGBA), strip.png, anim.gif (pose preview at source pacing) and report.json.

Keyframes are picked by cumulative motion arc-length — equal steps of accumulated frame-to-frame change, so frames concentrate where the action is. Uniform sampling looks mushy, and picking local motion maxima over-samples the idle bounce at the head of the clip.

--loop for cycles. Whole-clip picks include the start-up and the settle back to idle, which is exactly why a walk visibly resets instead of looping. Loop mode finds the closest-matching frame pair inside the sustained middle of the clip and samples between them, giving a true seam.

Read the summary before trusting anything:

metric	what it tells you
worst_bg_purity	~1.0 means every pixel is confidently background or character; lower means key leak
loop_diff	move returns to its start pose (or, in loop mode, the cycle seam)
first_vs_last_baseline	feet end where they began, in px
baseline_drift_px	feet wandering — large is right for a jump, suspicious for a stance

Despill note: do not blend the fringe toward the off-channel, which visibly tints the character. Clamp the key channels against the off-channel instead, as the script does.

4. Atlas: one global scale

The trap. Scaling each frame to a fixed on-screen height is the obvious thing and it is wrong: a crouch or jump tuck has a smaller bounding box, so per-frame normalization renders it bigger than standing. Derive one scale from the idle frame and apply it to every frame of every move.

python3 scripts/build_atlas.py sprites/ atlas.json \
    --moves idle,walk,jump,crouch,punch,kick,block --ref idle --height 340

Frames are stored trimmed, at global scale, with offsets to a shared anchor: footOffset (feet vs the ground line), anchorOffset (stance-center column inside the crop), driftX (this frame's drift from the canonical center).

The anchor X comes from the ankle-region centroid, not the bbox center — a big arm or tail swing moves the bbox, and centering on it makes the character slide sideways between poses.

The script prints per-move on-screen heights. Crouch and jump-tuck must be shorter than idle; if they are taller, per-frame scaling has crept back in.

5. Render

ctx.translate(x + f.driftX, GROUND_Y + jumpY);
ctx.scale(facing, 1);              // player 2 is a flip, never a generative mirror
const foot = state === 'jump' ? 0 : f.footOffset;   // engine arc supplies the lift
ctx.drawImage(img, -f.anchorOffset, foot - img.height, f.w, f.h);

Two real bugs live in those four lines. drawImage's y is the top edge while footOffset positions the feet, so dropping - img.height sinks the character through the floor. And if the engine authors a jump arc, adding the frame's baked-in air offset on top double-lifts it.

Frame data stays separate from pixels — durations per frame, a loop flag, and for held states an enter/hold/release split so a key can be held:

punch:  { frames:[0..11], durations:[35,35,45,50,...], loop:false }
crouch: { enterFrames:[0,1,2], holdFrame:3, releaseFrames:[4..7], hold:true }

Roughly 35–70 ms per frame for attacks (fast startup, slower recovery) against ~150 ms for idle. Tune feel here; never regenerate video for timing.

6. Verify with pixels, in a browser

Asserting on atlas metadata is not verification — it passes happily while the render is broken. Measure the character on the actual canvas:

// drive the loop deterministically: requestAnimationFrame freezes in unfocused tabs
const drive = (n, codes) => {
  for (const c in keys) delete keys[c];
  for (const c of codes) keys[c] = true;
  for (let i = 0; i < n; i++) tick(g, keys, 16.7);
  draw();
};
// then getImageData, find the non-background bbox, compare against GROUND_Y

Confirm: idle feet sit on the ground line, crouch is shorter and still planted, jump lifts without resizing, landing returns to exactly idle.

Chrome extensions cannot open file:// URLs — serve the page with python3 -m http.server and drive localhost.

Costs and habits

Still ~$0.07, 5 s clip $0.64, a 10-move fighter ~$7, failed jobs free. Retakes are per-move, never the whole set, so diagnose before rerolling: a wrong-looking character means the locked frames were wrong, not the prompt. Keep superseded assets rather than deleting them — an earlier take is often the only surviving copy of a pose you end up wanting back.