Artstr-Studio/docs/PIXEL_ART_FEATURE.md

Pixel Art — editable pixel layers & raster export

Goals

Make Artstr a genuine, Nostr-native pixel-art editor. The user paints on a hard-edged pixel grid; the editable grid lives in the project JSON as compact palette-indexed data; PNG (and later GIF) are produced as exports, never as the source of truth.

The core realisation

Pixel art fits Artstr's JSON/Nostr model better than ordinary raster editing does. A 64×64 sprite is a tiny indexed grid — compact, portable, forkable, replaceable as a kind:30078 event. The rule:

Never store pixel art as a PNG while editing. Store it as an editable palette-indexed grid in the Artstr JSON; rasterise to PNG/GIF on export.

Two faces — the disc-design / slide pattern, generalised

Pixel art has the same dual nature as a disc design or a slide — a standalone publishable thing and an embeddable component — but generalised: because it is a layer type, its host is not one specific template, it is any design.

Standalone design	Embeds into	As a…
Disc design	a disc-label sheet	disc slot
Slide	a slide deck	card
Pixel art	any design	layer

• Standalone — a kind:30078 event, casewrap-pixelart, a "Pixel Art" layout on the Designer tab beside Custom Art / Slide / Disc Design. Forkable, editable-in-place, naddr-shareable, encryptable.
• Embedded — a pixelart layer inside any design (cover, custom art, slide, deck card, stack card). The pixel data embeds inline in the host's event — the host stays self-contained, exactly like an embedded slide or a disc-in-slot.

Product goals

1. A pixelart layer type — a palette-indexed pixel grid, usable in any design, rendered crisp (nearest-neighbour) at any scale.
2. A Pixel Editor — its own canvas-area view with a contextual pixel tool set: paint, fill, pick, palette, grid, zoom.
3. A standalone Pixel Art Designer-tab layout, publishing as casewrap-pixelart; importable into any design as a layer.
4. PNG export at 1× / 2× / 4× / 8×, transparency preserved.
5. (Later) animation frames and GIF / sprite-sheet export.

Non-goals (v1 = Phase A)

• GIF / animation in v1 — frames are designed into the data model from day one, but the timeline UI and the (vendored) GIF encoder are Phase C.
• Whole-design → PNG export. Rasterising an arbitrary layered Artstr artboard is a separate, harder problem (the DOM-to-raster question). This spec only rasterises pixel content. General design→PNG is out of scope here.
• Image import / quantization, selection/move, dither, onion skin, symmetry, tile mode — Phase C/D.
• Embedding binary images — pixel data is always the indexed grid, never an embedded PNG blob.
• External media offload (Blossom) — inline only for v1; a manifest escape hatch is sketched for later.

Decisions (proposed — confirm before building)

1. Layer-type-first. The pixelart layer is the core. The standalone "Pixel Art" mode is a thin wrapper — a project that is one pixel grid.
2. The Pixel Editor is its own canvas-area view (#pixelEditorView), not a toolbar tool — joining #discDesignerView and #deckSorterView. The left tool palette is reused as a contextual host showing pixel tools while it's active.
3. Frames from day one — pixelArt.frames[] always exists; a v1 static sprite is simply a one-frame stack with no timeline UI.
4. Indexed palette + RLE, never raw RGBA. Raster is export-only.
5. Additive schema — a pixelart layer type and a pixelArt payload object. No SCHEMA_VERSION bump.
6. Publishing is gated on a measured event-size budget (derived from the relays' NIP-11 limits), not on grid dimensions alone — see Size limits.

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Why this fits the repo

What's reused

• The layer system — a pixelart layer moves / resizes / rotates / z-orders / locks / clips / sets opacity like any other layer; it shows in the Layers panel; it rides in every payload and preview.
• The "separate canvas-area view" pattern — #previewShell already swaps between #sheetWrap, #discDesignerView, and #deckSorterView. #pixelEditorView is one more.
• The edit round-trip — entering the pixel editor for an embedded layer and writing back on exit is the deck's "edit a slide → Back to deck" pattern (persistEditingDeckSlide / editingIndex).
• The Designer-tab layout system — a pixelart templateMode slots into applyLayout, WORKFLOW_ORDER, updateBodyClasses, normalizeTemplateMode, MODE_LABELS, exactly as slide did.
• Undo/redo — the 50-deep snapshot history already covers state; a pixel layer's data is in state, so a paint stroke is an ordinary history entry.
• Publish / fork / feed / encryption — a pixel-art design is a normal kind:30078 design; the private-publishing envelope wraps it free.
• Import-as-component — pulling a published casewrap-pixelart in as a layer mirrors the disc-design → disc-slot import flow.

Genuinely new work

1. The pixelart layer type + the indexed-grid + RLE data model.
2. A shared renderPixelGridToCanvas() — the app's first real raster/<canvas> layer (everything is DOM/SVG today).
3. The Pixel Editor view + the contextual pixel tool set + palette.
4. A PNG export pipeline — <canvas>.toBlob(); the app has no direct canvas raster export today (JPEG goes via PDF→PDF.js).
5. (Later) the frame timeline and a vendored GIF encoder.

Current-state facts the plan is built on

• Layer types today: image, text, color, shape, qr; layers render as absolutely-positioned DOM nodes from inch coordinates.
• The canvas region swaps views by mode (#sheetWrap / #discDesignerView / #deckSorterView); the tool palette is already rendered contextually (only ▶ Present shows in the deck sorter).
• casewrap-* typed kind:30078 events; /share/<naddr>; the community feed; categories include slide, slide-deck.
• "Export JPEG" round-trips through PDF + PDF.js — there is no direct canvas.toBlob() raster path yet.

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Data model

The pixelart layer

{
  id: 'l_x1',
  type: 'pixelart',
  name: 'Pixel sprite',
  // standard layer box — inches, on the host canvas:
  x, y, w, h, rotate, opacity, z, visible, locked, clip,
  pixelArt: {
    width: 64, height: 64,            // grid size, in cells
    palette: [                        // up to 256 entries, #rrggbbaa
      '#00000000',                    // index 0 — transparent by convention
      '#1a1a2eff',
      '#e94560ff'
    ],
    frames: [
      { id: 'f1', name: 'Frame 1', durationMs: 120, pixels: <RLE> }
    ],
    loop: true, fps: 8                // animation playback (Phase C)
  }
}

• frames always exists. A v1 static sprite has exactly one frame and no timeline UI — but the format never needs migrating when animation lands.

• pixels is run-length-encoded, row-major: an array of [paletteIndex, runLength] pairs.

  pixels: [ [0,128], [1,24], [2,8], [1,24] ]
  // 128 transparent, 24 colour-1, 8 colour-2, 24 colour-1
  

  RLE suits pixel art's long flat runs and stays human-portable JSON, but is worse than raw for noisy art — so each frame stores whichever is smaller: pixels (RLE [[index,count],…]) or pixelsB64 (base64 of a raw Uint8Array of indices, ~1.34 B/cell). Exactly one is present per frame. This bounds the worst case — see Size limits.

• Palette holds #rrggbbaa strings (alpha lets index 0 be fully transparent); capped at 256 so an index is one byte.

• The layer box defaults to the grid's aspect ratio, and its W/H aspect-lock (already a layer feature) defaults on — pixels stay square; the host never stretches them.

Standalone Pixel Art design (templateMode: 'pixelart')

{
  version: SCHEMA_VERSION,
  templateMode: 'pixelart',
  templateType: 'pixelart',
  meta: { … },                        // title / category / language
  pixelArt: { width, height, palette, frames, loop, fps }
}

The standalone project is one pixel grid — pixelArt sits at the top level (mirroring how a slide carries slide, a disc design carries discDesign). Importing it into another design lifts that pixelArt into a fresh pixelart layer; publishing a layer standalone does the reverse. After import the two are independent (a fork).

Nostr

• Tag casewrap-pixelart; d-tag pixelart:<title|id> (random when published privately, per PRIVATE_PUBLISHING_FEATURE.md).
• New pixel-art category (joining slide, slide-deck).
• Forkable, editable-in-place (NIP-33), naddr-shareable, encryptable.

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Size limits & the event budget

A pixel layer must never silently break publishing. Relays cap event size individually (commonly 64 KB; some 128 / 256 KB), and an oversized event is rejected by some relays and accepted by others — so the design seems to publish but is unreliable. The budget covers the whole signed event: every other layer, meta, tags, and ~270 B of id / pubkey / sig.

Why the indexed grid is small

Coordinates are never stored — cells are row-major, so position is free. Hex is stored once in palette; each cell is just a small index. RLE then collapses flat runs. A filled 64×64 sprite stored naively as [x, y, hex] triples is ~90 KB; the same sprite as an indexed RLE grid is ~1–5 KB — a 20–90× difference. Dropping coordinates and de-duplicating the hex is the whole efficiency win.

Rough sizes — encoded pixel data, one frame

RLE size is content-dependent (great for flat art, worse than raw for noise); the encoder picks the smaller of RLE / base64, bounding the worst case at ~1.34 B/cell.

Grid	Cells	Typical (RLE)	Worst case
16×16	256	~0.2 KB	~0.35 KB
32×32	1,024	~0.5–1.5 KB	~1.4 KB
64×64	4,096	~1–5 KB	~5.5 KB
128×128	16,384	~3–20 KB	~22 KB
256×256	65,536	~10–60 KB	~88 KB

Palette is minor (~12 B per colour). Animation multiplies by frame count — that is where the budget is actually spent.

The limits

• Dimensions — 256×256 hard max; 128×128 recommended ceiling; 32×32 default for a new grid. Palette ≤ 256 entries (1-byte indices).
• Event byte gate — the real guard (dimensions alone can't predict size). Before publishing, the app measures the would-be event size against a budget:
  • The budget is the minimum limitation.max_event_size advertised via NIP-11 across the user's relay set, falling back to 64 KB when unknown.
  • Soft warning at ~50% of budget — suggests a smaller grid, fewer colours, or fewer frames.
  • Hard block at ~90% — publishing is refused with a clear message, instead of letting the event be silently dropped.
  • So a clean 256×256 publishes fine; a pathologically noisy one is caught before it half-publishes.
• Live readout — the Pixel Editor always shows current data size against the budget (e.g. "Sprite data: 6.2 KB / 48 KB").
• Oversized animations route to the external-manifest / Blossom escape hatch (Phase C); inline storage covers everything that fits.

In short: single static sprites up to ~128×128 are always safe inline; 256×256 works unless the art is pathological; multi-frame animation is the real pressure point and gets the offload path.

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Rendering

One shared helper, used by the editor, every preview renderer, the feed cards, and export:

renderPixelGridToCanvas(pixelArt, frameIndex = 0) -> HTMLCanvasElement

• Decodes the frame's RLE into the palette, draws each cell as a 1×1 fill onto an offscreen <canvas> sized exactly width × height.
• A pixelart layer renders as that <canvas> placed in the layer's div, width:100%; height:100%; image-rendering: pixelated — the CSS equivalent of imageSmoothingEnabled = false, so any up-scaling stays crisp.
• The decoded canvas is memoised keyed by the frame's pixel data, so the per-render layer rebuild stays cheap.
• renderCustomArtPreviewDOM (feed cards / publish preview) uses the same helper — pixel layers preview correctly everywhere.

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The Pixel Editor

#pixelEditorView — a new canvas-area view, shown when:

• a standalone Pixel Art design is open, or
• the user picks "Edit pixels" on a selected pixelart layer (or double-clicks it) inside any other design.

It contains a zoomed, paintable pixel-grid <canvas> with its own zoom/pan, a palette panel, and the contextual tool palette (the left palette slot, populated with pixel tools instead of the vector tools — exactly as the deck sorter shows only ▶ Present).

Entering / leaving follows the deck round-trip: for an embedded layer, "Edit pixels" enters; "Done" writes the grid back to the layer and returns to the host artboard. For a standalone design the editor is the project.

v1 tools (Phase A core)

Pencil      paint cells with the current palette index
Eraser      paint index 0 (transparent)
Fill        flood-fill contiguous same-index region
Eyedropper  set current index from a cell

Plus: a palette panel (swatches; add / edit / remove a colour; pick the current colour), a grid overlay toggle, mirror X / Y drawing, and the editor's own zoom (fit / 1× / + / −). Drawing supports click-and-drag; a whole drag-stroke is one undo entry (history is pushed at stroke start, not per cell).

Phase B tools

Line, Rectangle, Ellipse; whole-grid Flip H / V and Rotate 90°; resize the grid (change width/height, anchored).

Phase C / D tools

Selection + move; dither brush; onion-skin (needs frames); symmetry; tile preview; image-import → pixelate/quantize.

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Raster export

PNG first — a clean, self-contained <canvas> pipeline, independent of the app's PDF/JPEG route:

exportPixelArtPng(pixelArt, { scale, transparentBg, includeGrid }):
  native = renderPixelGridToCanvas(pixelArt, frame)      // width × height
  out    = canvas(width*scale, height*scale)
  ctx.imageSmoothingEnabled = false
  ctx.drawImage(native, 0, 0, out.width, out.height)
  return out.toBlob('image/png')

• Scale: 1× / 2× / 4× / 8× (nearest-neighbour always).
• Transparent background on/off; include grid lines on/off.
• image/png is the one format toBlob() is guaranteed to support; WebP is an easy later add.

Animation export (Phase C): GIF — Canvas can't toBlob('image/gif'), so this needs a vendored JS/WASM encoder (e.g. gif.js), fed each frame's rendered ImageData + durationMs. Also sprite-sheet PNG (all frames tiled) + a metadata.json, which needs no new dependency.

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Phased delivery

Phase A — Pixel layer, editor core, Nostr

• The pixelart layer type + pixelArt data model (RLE, frames-of-one).
• renderPixelGridToCanvas + crisp layer rendering everywhere (editor artboard, preview-DOM renderer, feed cards).
• The #pixelEditorView with the core tools (pencil / eraser / fill / eyedropper), palette panel, grid toggle, mirror, editor zoom.
• "Add Layer → Pixel Art"; the per-layer "Edit pixels" round-trip.
• The standalone "Pixel Art" Designer-tab layout (templateMode: 'pixelart').
• Save / load / publish / fork as casewrap-pixelart; feed card; pixel-art category; pixelart in MODE_LABELS / mode filters.

Ship gate: paint a 32×32 sprite, see it render crisp as a layer in a Custom Art design and as a standalone design; save, reload, publish — it round-trips and shows in the feed.

Phase B — Raster export + editor extras

• PNG export at 1×/2×/4×/8×, transparency + optional grid.
• Line / rectangle / ellipse tools; flip H/V; rotate 90°; grid resize.
• Import a published casewrap-pixelart design as a layer (the disc-design-style remix flow).

Ship gate: export a sprite as a transparent 8× PNG; import a published pixel design into another design as a layer.

Phase C — Animation

• frames[] timeline UI: add / duplicate / delete / reorder frames, per-frame duration, loop/fps; onion-skin; playback preview.
• GIF export (vendored encoder); sprite-sheet PNG + metadata export.

Ship gate: build a 4-frame loop, preview it, export a working GIF and a sprite sheet.

Phase D — Conversion & advanced tools

• Image import → nearest-neighbour downscale / palette quantization → editable grid.
• Selection + move; dither brush; symmetry mode; tile preview.

Later / backlog

• WebP / APNG / animated-WebP export.
• Blossom offload + an external manifest form for very large art.
• Animated pixel layers driving Stack actions (a sprite that plays on onClick).

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Risks & tradeoffs

Risk	Mitigation
First real raster/<canvas> layer in a DOM-vector app	One shared renderPixelGridToCanvas; image-rendering: pixelated; the layer is still a normal DOM node hosting a <canvas>.
No direct canvas raster export exists today	PNG export of pixel content is self-contained (toBlob), independent of the PDF/JPEG route; whole-design PNG is explicitly out of scope.
GIF needs a new vendored dependency	Deferred to Phase C; isolated to export; sprite-sheet export (no dependency) ships alongside as the lighter option.
Per-pixel undo would swamp the 50-deep history	A whole drag-stroke / fill / transform is one pushHistory entry, taken at gesture start.
Large grids / many frames inflate the event → silent relay drops	Encoded size is gated against a NIP-11-derived budget (min max_event_size across the user's relays, 64 KB fallback): soft warning ~50%, hard block ~90%; the encoder stores the smaller of RLE / base64; oversized animations offload via Blossom (Phase C). See Size limits.
Pixels stretched / blurred by a non-matching layer box	Layer box defaults to the grid aspect; W/H aspect-lock defaults on; image-rendering: pixelated everywhere including print.
A pixel layer inside a host's PDF/print export	Verify the print path rasterises the <canvas> crisply; fall back to a high-scale PNG draw if needed.
Palette index overflow	Palette capped at 256 entries (one byte per index).

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Open questions

1. templateMode: 'pixelart' on the Designer tab — confirmed?
2. v1 core tool set — pencil / eraser / fill / eyedropper + palette / grid / mirror / zoom — right cut?
3. New-grid size options — 32×32 default is set; which other presets to offer (16 / 32 / 64 / 128 / custom)?
4. Editor zoom — reuse the bottom #canvasToolbar slot, or a dedicated pixel-editor zoom control?
5. GIF encoder — OK to vendor one (gif.js or similar) in Phase C?
6. Budget thresholds — soft 50% / hard 90% — the right percentages, and is 64 KB the right NIP-11 fallback?

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Acceptance summary

Phase A

• A pixelart layer renders crisp (nearest-neighbour) at any scale, in the editor, previews, and feed cards.
• The Pixel Editor view paints with pencil / eraser / fill / eyedropper; palette add/edit/remove; grid + mirror; editor zoom.
• "Add Layer → Pixel Art" works in any design; "Edit pixels" enters the editor and "Done" writes back.
• A standalone Pixel Art design saves / loads / publishes / forks as casewrap-pixelart and shows in the feed under pixel-art.
• A drag-stroke is a single undo step.
• Publishing is gated on the event-size budget — a live size readout in the editor, a soft warning, then a hard block — so a pixel design can never be silently dropped by relays.

Phase B

• PNG export at 1×/2×/4×/8× with transparency and optional grid.
• Line / rect / ellipse, flip, rotate, grid resize.
• A published pixel design imports into any design as a layer.

Phase C

• A multi-frame sprite plays back; onion-skin works.
• GIF export and sprite-sheet export both produce correct files.

Phase D

• An imported PNG/JPEG converts to an editable pixel grid.
• Selection/move and the advanced brushes work.