AI sprite generation: portraits, idle loops, animated sheets via Codex/Nano Banana. Per-row generation, animation presets, video-to-sprite, identity lock. Use for generated character art.
AI sprite generation: portraits, idle loops, animated sheets via Codex/Nano Banana. Per-row generation, animation presets, video-to-sprite, identity lock. Use for generated character art.
agent
python-general-engineer
user-invocable
false
allowed-tools
["Read","Write","Bash","Glob","Grep","Edit"]
routing
{"triggers":["AI sprite","AI character art","generate character","generate sprite","generate wrestler","generate character portrait","spritesheet pipeline","animated spritesheet","animated sprite","portrait loop","idle loop","road-to-aew sprite","sprite for road-to-aew","wrestler portrait","character sheet","fighter spritesheet","RPG character spritesheet","platformer spritesheet","per-row sprite generation","video to sprite","animation preset","deterministic idle","breathing loop","sprite QA","contact sheet"],"complexity":"Complex","category":"game","pairs_with":["phaser-gamedev","threejs-builder","python-general-engineer","game-pipeline"]}
Game Sprite Pipeline
Local-first AI sprite generation. One skill, four modes behind --mode:
portrait — single full-body character PNG (road-to-aew wrestlers, card-game characters).
portrait-loop — 2×2 = 4-frame subtle idle (breathing + blink) at 200ms/frame in ONE Codex call. Animated portraits without re-prompting new poses.
spritesheet — animated multi-frame grid with connected-components detection, ground-line anchor alignment, and Phaser atlas output.
spritesheet --per-row — row-strip generation: each animation state generated as a separate strip with layout guide grounding and identity lock, then composited into a final sheet. Eliminates cross-row contamination and reduces retry scope from whole-sheet to single-row.
Backend chain (per ADR-198): Codex CLI imagegen is the default when installed and authed. When Codex is unavailable AND GEMINI_API_KEY (or GOOGLE_API_KEY) is set, the skill falls back to Nano Banana via nano-banana-builder's scripts. When neither is available, the skill fails loud with BackendUnavailableError listing both fix paths. Both paths use keys the user already holds — there is no third-party billing the toolkit hides. Reference implementation of the "Local-First, Deterministic Systems Over External APIs" principle in docs/PHILOSOPHY.md (user-owned-key clause).
Portrait is the road-to-aew immediate need; spritesheet is the forward-looking capability. Both share one backbone (prompt scaffolding, backend dispatch, bg removal, validation).
Reference Loading Table
Signal
Load
Why
picking a style preset
references/style-presets.md
9 era/hardware presets + custom slot
--target road-to-aew or --archetype
references/wrestler-archetypes.md
9 color archetypes + 10 gimmick types + tier
sizing --grid CxR / --cell-size
references/grid-shapes.md
allowed dims, direction conventions
building a prompt
references/prompt-rules.md
ART_STYLE, CHAR_STYLE, GRID_RULES slot contents
picking or troubleshooting a backend
references/backend-chain.md
Codex CLI invocation pattern + failure modes
spritesheet Phase D
references/frame-detection.md
connected-components algorithm
spritesheet Phase F
references/anchor-alignment.md
shared-scale + bottom-anchor math
any bg removal phase
references/bg-removal-local.md
magenta chroma + rembg
Phase H assembly / output shape
references/output-formats.md
PNG / GIF / WebP / atlas JSON
any phase errors
references/error-catalog.md
failure mode → fix mapping
user reports clipping / blank cells / cut effects
references/error-catalog.md (top section)
"Codex Regeneration as a Post-Processing Fix" failure mode; debug slicer, never raw
asset has effects (fire, projectile trails, auras, extended limbs)
references/error-catalog.md + use slice_with_content_awareness
content extending past cell boundaries needs centroid-ownership extraction. ADR-207 RC-1: on dense grids (cols * rows >= 16 AND both dims >= 4) --content-aware-extraction is silently downgraded to strict-pitch with a warning unless --effects-asset is also passed (orchestrator-side) or effects_asset: True is set (spec-side). Use --effects-asset only for genuine sparse-but-cross-boundary content (fire breath, plasma trails, projectile auras).
--target road-to-aew deploy
references/road-to-aew-integration.md
snake_case, paths, manifest regen
--per-row subagent orchestration
references/subagent-delegation.md
subagent boundary rules: who generates, who composes, who finalizes
VFX issues in per-row prompts
references/vfx-containment.md
full containment ruleset: allowed/forbidden effects, per-state rules
Read the references whose signals match the task — references load on demand, so the cost is paid once per execution, not once per routing decision.
The loop must be SUBTLE: viewers should barely notice the animation (just feels alive). New poses defeat the purpose — that's spritesheet mode. See references/prompt-rules.md for the loop prompt template.
Row-strip generation eliminates the monolithic-sheet failure mode. Instead of one image-generation call for the entire sheet (where frames drift, cross-row contamination occurs, and the slicer misroutes content), each animation row is generated as a separate strip image.
How it works:
Phase A generates a canonical base character image (identity lock anchor). All subsequent rows receive this base as --reference with "match silhouette, face, materials, palette, props exactly" -- prevents cross-row palette shifts, proportion changes, accessory drift.
sprite_canvas.py make-layout-guide generates per-row reference PNGs (frame boundaries with numbered cells on transparent background), passed as --reference alongside the canonical base to prevent frame drift at generation time.
Each row gets its own prompt with state-specific action/pose + VFX containment rules (see references/vfx-containment.md).
Strips are composited into the final sheet after all rows pass QA.
Retry granularity: re-generate a single failed row, not the whole sheet. Cost drops from O(sheet) to O(row).
--preset expands to --grid + per-row metadata + timing arrays. Users say --preset rpg-character instead of computing grid dimensions manually.
Video-to-sprite pipeline
New input modality: extract spritesheet rows from video clips when text prompting fails for complex animations (multi-hit combos, spell effects, motion-captured references).
Script
What
video_extract_frames.py
FFmpeg frame extraction from video clips at configurable FPS
Script: qa_artifacts.py generates all three artifact types after assembly.
Per-frame variable timing
Presets include per-frame millisecond timing arrays instead of uniform timing.
Example (idle): [280, 110, 110, 140, 140, 320] ms — longer hold on first/last frame creates natural breathing rhythm.
Output integration:
GIF/WebP output uses per-frame durations via Pillow's duration parameter.
Phaser atlas JSON includes frameDurations array and animationTimings map.
Custom timing override via --timing-json <path> for fine-tuning.
Backend (Codex default with Nano Banana fallback, per ADR-198)
Per generation call, evaluated in order:
codex in PATH and codex --version exits 0 → use Codex CLI imagegen. Codex CLI 0.125+ no longer exposes --output-image/--aspect-ratio/--reference/--seed as direct flags; image generation goes through the agent's internal image_gen tool, invoked from prompt text. Canonical invocation: codex exec --dangerously-bypass-approvals-and-sandbox --skip-git-repo-check [-i <ref>... --] "<wrapped prompt>". The wrapped prompt instructs the agent to call image_gen and save to an absolute path; aspect ratio, seed, and reference semantics are encoded into the prompt text. Reference list (when used) MUST be terminated with -- before the positional prompt or it consumes the prompt as another image filename. Subprocess timeout: 360s.
Else if GEMINI_API_KEY or GOOGLE_API_KEY is set → fall back to Nano Banana via nano-banana-builder's scripts (scripts/nano-banana-generate.py with-reference for reference-guided, batch for multi-variant). The skill never imports the Gemini SDK directly — composition through the existing skill is the contract.
Else fail loudly with BackendUnavailableError listing BOTH fix paths:
BackendUnavailableError: No image-generation backend available.
Fix path 1 (Codex CLI, recommended):
Install Codex CLI and run `codex auth` to authenticate against your existing subscription.
Fix path 2 (Nano Banana fallback):
Set GEMINI_API_KEY (or GOOGLE_API_KEY) in your environment to enable the Nano Banana fallback.
Both paths use keys the user already holds (Codex subscription, Gemini API key). There is no third-party billing the toolkit hides. The skill never calls api.openai.com directly, remove.bg, or any service the user did not authorize. See references/backend-chain.md for the locked-in invocation contract and failure modes.
--max-frames: pack the canvas
One Codex imagegen call produces ONE image. To get N frames, pack as many cells as possible into that one image rather than firing N calls.
--max-frames (on sprite_canvas.py make-template and sprite_pipeline.py) auto-computes the largest square grid that fits --max-canvas (default 1024x1024) at the given --cell-size:
# 4x4 = 16 frames at 256px on a 1024 canvas (default)
python3 sprite_pipeline.py --description "knight walk cycle" \
--style snes-16bit-jrpg --cell-size 256 --max-frames --action walking
# 8x8 = 64 frames at 128px on a 1024 canvas
python3 sprite_pipeline.py --description "tiny adventurer, attack cycle" \
--style nes-8bit --cell-size 128 --max-frames --action attack-punch
# 16x16 = 256 frames at 64px on a 1024 canvas (extreme density)
python3 sprite_pipeline.py --description "GB-era hero, 4-direction walk" \
--style gameboy-4color --cell-size 64 --max-frames --action walking
--max-frames overrides --grid; the chosen grid is logged to stderr. See references/grid-shapes.md for the cell-size → max-grid → total-frames table at canvas sizes 1024, 1536, and 2048.
Dimension and cell policy
Portrait mode — variable output, validated after trim:
width 350-850, height 900-1100, aspect 1:1.5 to 1:2.5 (h:w).
--force-dimensions skips the gate (emergency only; logged loudly).
Spritesheet mode — fixed cells:
cell-size in {64, 128, 192, 256, 384, 512}; default 256.
grid CxR via --grid; total canvas <= 2048x2048.
--grid 4xR or --grid 8xR triggers per-direction strip output.
See references/grid-shapes.md for direction-to-row conventions.
Tiebreak: smallest shared-scale variance (visually consistent height across frames).
Tiebreak: lowest seed number (reproducible tie resolution).
--curate writes a contact sheet and opens a manual review gate. Off by default; interactive gates block automation.
Shared constraints
User-owned keys only. Codex CLI (default) and Nano Banana via GEMINI_API_KEY/GOOGLE_API_KEY (fallback) are the two authorized backends — both billed under the user's existing accounts. The skill never calls api.openai.com directly, remove.bg, or any service the user did not authorize. See ADR-198 and the user-owned-key clause in docs/PHILOSOPHY.md.
Magenta background (#FF00FF) is the chroma-key default because it never appears in realistic character skin or wrestling gear. Backend prompts include explicit "solid magenta background" instruction; post-processing validates the dominant corner color.
Fixed seed per run for reproducibility. Re-running with the same --seed should produce identical output given identical backend output (best-effort: Codex CLI does not currently expose a public seed flag, so the seed travels in the prompt body as a comment).
Verification
Before shipping any change to this skill, run:
cd /home/feedgen/vexjoy-agent
python3 scripts/generate-skill-index.py
python3 scripts/validate-references.py --check-do-framing
ruff check . --config pyproject.toml
ruff format --check . --config pyproject.toml
Plus the two smoke tests:
# Portrait (no backend required in dry-run)
python3 skills/game/game-sprite-pipeline/scripts/portrait_pipeline.py \
--prompt "veteran wrestler, indie circuit, 35yo, scarred face, leather jacket" \
--style slay-the-spire-painted --target road-to-aew --dry-run
# Spritesheet (no backend required in dry-run; --allow-frame-duplication# because the synthetic fixture has 4 near-identical figures that trip the# verify_frames_distinct gate by construction at the new 70% threshold;# see "Verifier gates" below). Default-on verify; exits 0 on success,# 2 on gate failure, 3 on --no-verify (ADR-207 Rule 4).
python3 skills/game/game-sprite-pipeline/scripts/sprite_pipeline.py \
--prompt "wrestler walk cycle, 4 frames" \
--grid 4x1 --cell-size 256 --dry-run --allow-frame-duplication
Both dry-run modes skip the backend call, generate a synthetic fixture, and exercise every post-processing phase. Pass criteria: exit 0, expected output files present, dimension gates satisfied.
Verifier gates (default-on, ADR-199)
Both pipelines run a verifier suite as the LAST step (after assemble). Default-on; opt out with --no-verify.
Flag
Default
Effect
--verify
ON
Run the applicable verifier gate suite after assembly. Print structured JSON to stdout; exit 2 on any failure.
--no-verify
—
Skip all gates. Logs WARNING: --no-verify opted out; output not validated to stderr so silent skips remain visible. Spritesheet mode (ADR-207 Rule 4): returns exit code 3 (VERIFIER_SKIPPED_EXIT_CODE) instead of 0 so orchestrators cannot silently mask spritesheet failures with the same exit status as success. Portrait + portrait-loop modes: retain exit 0 because their verifier surface is small enough (verify_no_magenta only) that an explicit skip is plausibly intentional.
--allow-frame-duplication
OFF
Relax verify_frames_distinct from 70% to 100% duplicate-pct (ADR-208 RC-3). Use for spec-known sheets with legitimate frame repetition: idle loops where 8 frames repeat to fill 64 cells, taunt poses where the character holds a stance, or any animation where >70% duplicate-pct is the artist's intent. Without this flag the gate fires at 70% to catch the layout-drift signature where centroid mis-routing lands most cells on a few near-identical poses.
--effects-asset
OFF
Opt INTO content-aware routing on a DENSE grid (>= 4x4 with >= 16 cells). Use ONLY for sparse-but-cross-boundary content: fire breath, plasma trails, projectile auras. Keep dense character grids on strict-pitch slicing because content-aware routing can drop cells via centroid drift (ADR-207 RC-1).
Per-mode gate selection:
Mode
Entry
Gates
spritesheet
sprite_pipeline.py run_pipeline
verify_no_magenta, verify_grid_alignment, verify_anchor_consistency, verify_frames_have_content, verify_frames_distinct, verify_pixel_preservation (when {name}_sheet_raw.png is present), verify_raw_vs_final_cell_parity (ADR-207 Rule 3 — same precondition as verify_pixel_preservation)
portrait
portrait_pipeline.py run_pipeline (mode=portrait)
verify_no_magenta (single-image mode; per-cell gates do not apply)
verifier_verdict (ADR-207 Rule 2) is the contracted consumer-facing field. Manifest writers, orchestrators, and downstream classifiers MUST derive their own status fields from this string; the toolkit's write_manifest_record writer asserts at write time that verifier_verdict == "PASS" implies an empty verifier_failures list (and vice versa).
Exit codes:
0: passed: true (or --no-verify for portrait / portrait-loop modes).
2: at least one gate failed (distinct from generic pipeline error rc=1 so CI can branch on "verifier said no" vs "the pipeline blew up").
3: --no-verify was passed in spritesheet mode (ADR-207 Rule 4 — VERIFIER_SKIPPED_EXIT_CODE). Orchestrators that explicitly want to skip spritesheet verification must explicitly accept exit code 3.
Logging (ADR-202)
Both pipelines emit diagnostic logs via stdlib logging on stderr. Verifier JSON and other structured machine-readable output stays on stdout. Logger name pattern: sprite-pipeline.<module> (e.g., sprite-pipeline.sprite_pipeline, sprite-pipeline.sprite_bg). Default level: INFO.
Flag
Default
Effect
(none)
—
INFO level. Phase boundaries, backend selection, asset paths, per-phase status.
--quiet / -q
—
WARNING level. Suppresses INFO chatter; only fallback warnings + errors reach stderr. Mutually exclusive with --verbose.
--verbose / -v
—
DEBUG level. Adds detailed parameter dumps and intermediate counters. Mutually exclusive with --quiet.
Error: "BackendUnavailableError: No image-generation backend available"
Cause: Neither codex CLI nor GEMINI_API_KEY/GOOGLE_API_KEY is available.
Solution: Install Codex CLI (npm install -g @openai/codex or per-OS) and run codex auth, OR set GEMINI_API_KEY (or GOOGLE_API_KEY) in your environment to use the Nano Banana fallback. Both paths use keys the user already holds; pick whichever is available. Never set OPENAI_API_KEY for this skill — direct OpenAI calls are not an authorized path.
Error: "Aspect ratio X outside allowed range [1:1.5, 1:2.5]"
Cause: Character rendered too wide (crouched pose) or too tall (full-extension jump pose).
Solution: Re-generate with a neutral standing prompt. If the character must have an atypical pose, add --force-dimensions (logs loudly, should not become routine).
Cause: Connected-components merged neighboring frames, or small fragments were filtered.
Solution: Increase cell-size (gives more separation gap), re-generate, or use --chroma-threshold to tighten the bg mask. See references/frame-detection.md.
Error: "Codex CLI grid-template input not supported"
Cause: Codex imagegen may not consume a magenta grid canvas as a structural input.
Solution: Skill auto-falls-back to per-frame generation + Pillow compositing. Slower but deterministic.
Error: "rembg not installed"
Cause: --bg-mode rembg requested without the opt-in dependency.
Solution: pip install rembg onnxruntime (~200MB ONNX model). Or re-run with default magenta chroma key.
Error: "road-to-aew directory not found at ~/road-to-aew"
Cause: --target road-to-aew resolves to a path that does not exist.
Solution: Pass --target-dir <explicit-path> or clone the repo to ~/road-to-aew. The deploy step refuses to guess.
Failure Patterns
Failure mode: Codex regeneration as a post-processing fix
What it looks like: A verifier flags a blank cell, clipped fire, missing silhouette, or any other final-asset defect. The reflex is to re-run codex exec to redraw the raw with a different prompt or seed. STOP.
Why wrong: Codex generation is treated as ground truth. The raw PNG in <your-output-dir>/raw/<slug>.png is what Codex painted, and it is almost always correct. If the final-sheet shows blank cells, clipped effects, or missing silhouettes, the bug is in one of the post-processing steps:
slice_grid_cells derived the wrong pitch (raw_size / grid math) and cut the cell at the wrong place
slice_with_content_awareness claimed a component to the wrong cell (centroid mapping bug)
The despill chain (chroma_pass2_edge_flood, kill_pink_fringe, neutralize_interior_magenta_spill) ate the silhouette
The mass-centroid anchor pinned the wrong body part to the ground line
The LANCZOS resize between magenta padding and content created pink fringe
The user's framing: "the codex generation has never failed, it is working perfectly, the rest has failed."
Do instead: Open the raw and the final side-by-side. Confirm the raw has the content (it almost always does). Trace which post-processing step lost it. Specifically for boundary clipping (asset 27 dragon flame, asset 30 plasma trail): set has_effects: True and/or content_aware_extraction: True in the spec to use slice_with_content_awareness, which expands cell windows to recover content extending past conceptual cell boundaries.
Caveat (ADR-207 RC-1, dense-grid downgrade): on dense grids (cols * rows >= 16 AND both dims >= 4 — i.e. 4x4 and denser), the slicer dispatch in sprite_pipeline.py and cmd_extract_frames silently downgrades --content-aware-extraction (and the spec's content_aware_extraction / has_effects slicer leg) to strict-pitch with a WARNING log because content-aware routing on Codex's fractional-pitch raws drops cells via centroid drift. To opt INTO content-aware on a dense grid for genuine sparse effects assets, also pass --effects-asset (orchestrator-side) or set effects_asset: True (spec-side). Character grids with arms touching cell edges should stay on the strict slicer and rely on verify_raw_vs_final_cell_parity to flag genuine slicing bugs.
See references/error-catalog.md for the full diagnostic procedure.
Failure mode: Calling a third-party paid API the user did not authorize
What it looks like: Adding a try/except that falls back from Codex CLI → api.openai.com direct / remove.bg / any service whose billing relationship the user did not establish.
Why wrong: The user-owned-key clause (PHILOSOPHY.md, ADR-198) authorizes fallbacks gated on environment variables the user explicitly set — Codex's auth and GEMINI_API_KEY/GOOGLE_API_KEY for Nano Banana qualify because the user holds the keys. A fallback that hits a service the toolkit pays for, or that silently charges a card the user never connected, does not. That's the failure mode the Local-First principle exists to prevent.
Do instead: Stop at the two authorized backends (Codex CLI, then Nano Banana via user-set Gemini key). When neither is available, raise BackendUnavailableError with both fix paths in the message. Adding a third backend requires a new env-var-gated path AND an ADR amendment — never a silent runtime fallback.
Failure mode: Naive grid-math frame cropping
What it looks like:frame = sheet.crop((col*CELL, row*CELL, (col+1)*CELL, (row+1)*CELL)) for every cell.
Why wrong: Generated frames drift within their cells (the model does not respect grid boundaries precisely). Naive cropping captures neighbor-cell pixels and truncates the current sprite.
Do instead: Use connected-components clustering (flood-fill from non-magenta pixels, bound each cluster by its actual pixel extent). See references/frame-detection.md.
Failure mode: Trusting the backend's alpha channel
What it looks like: Prompting "transparent background" and consuming the output PNG directly.
Why wrong: Both Codex CLI and Nano Banana produce backgrounds that are nominally transparent but often have magenta/white/gray fringing, full-opacity backgrounds, or inconsistent alpha. Downstream consumers get dirty assets.
Do instead: Always post-process: prompt for a known chroma color (magenta #FF00FF), then run local bg removal. Two-pass flood fill handles feathering. references/bg-removal-local.md has the algorithm.
Failure mode: Interactive curation as the default
What it looks like: Every pipeline run opens a contact-sheet viewer and waits for user confirmation.
Why wrong: Blocks automation (batch generation of 50 wrestlers), adds cognitive load, and prevents reproducibility. The user is not always at the keyboard.
Do instead: Deterministic auto-curation is the default (rank by edge-touches → scale variance → seed). Expose --curate for cases where the automated pick is visibly wrong. Reproducibility matters more than picking the single prettiest variant.
Reference files
references/style-presets.md — full catalog of 9 era/hardware presets, prompt fragments, --style custom slot.
Demo orchestrators (e.g. <your-output-dir>/generate.py) should be idempotent: running them again on a partial output skips assets whose assets/<slug>/final.png (or final-sheet.png) AND meta.json already exist. This saves ~30-60s per asset of Codex CLI time when iterating. The pattern:
def_is_done(asset_dir: Path) -> bool:
ifnot (asset_dir / "meta.json").exists():
returnFalseif (asset_dir / "final.png").exists() or (asset_dir / "final-sheet.png").exists():
returnTruereturnFalsedefrun_one(spec, force=False):
ifnot force and _is_done(asset_dir):
return existing_meta # log "skipping" + return
...
Provide --force to override (re-run all) and --force-slug <prefix> to re-run a specific asset. Document the behavior in the orchestrator's docstring so future maintainers don't accidentally regenerate everything.
Related skills
phaser-gamedev — downstream consumer of spritesheet output (atlas JSON with frameDurations and animationTimings).
threejs-builder — may consume portrait output for 3D card framing.
game-asset-generator — sibling umbrella for 3D models / textures / matrix-driven pixel art. Its references/pixel-art-sprites.md redirects AI-driven sprite work here.
game-pipeline — parent lifecycle orchestrator; slot this skill under its ASSETS phase when building a new game.
motion-pipeline — CPU-only motion data processing; can provide BVH-derived reference frames for video-to-sprite extraction.
