imagespec

Render images from a declarative YAML/dict spec — shapes, text, charts, QR/barcodes — for e-paper ESL tags and label printers.

imagespec is the rendering core behind hass-ble-esl (BLE e-paper shelf labels) and hass-niimbot (label printers): both Home Assistant integrations depend on it from PyPI and keep only a thin adapter of their own (see docs/integrating.md). The library has no framework dependency, so it can be used and tested standalone. The rendering engine was originally adapted from OpenEPaperLink's Home Assistant Integration (imagegen module, Apache License 2.0) and has since been substantially rewritten and extended — see NOTICE for the full attribution.

Status

Used in production by the two integrations above. Every registered element is pinned by a golden-image test (the README previews below double as the goldens), alongside unit tests for palettes, rotation, dithering, template-string coercion and error handling; CI runs the suite on Python 3.13/3.14 and against the lowest supported dependency versions.

Design

• No framework dependency. The core never imports Home Assistant. Anything host-specific is injected through RenderContext:

  • font_resolver(name) -> path | None — e.g. an integration's hass.config.path("www/fonts") lookup.
  • history_provider(entity_ids, start, end) -> states — for the plot element (HA recorder). Optional.
  • palette — the device's supported colors (see below).

• Registry dispatch. Each element type is a handler registered with @element("type") in imagespec/elements/, replacing the original giant if/elif chain. Adding an element = adding a function.

• RenderState. Threaded through handlers; holds the (reassignable) img and the pos_y flow cursor.

• Device-dependent palette (RenderContext.palette), not unified — panels support different colors. Define it as a list of the colors the device supports (names, HEX, or RGBA tuples):

  RenderContext(palette=["black", "white", "red"])  # names
  RenderContext(palette=["#000000", "#ffffff", "#ff0000"])  # HEX
  RenderContext(palette=[(0, 0, 0), (255, 255, 255)])  # RGBA tuples

  Shorthand names are optional convenience for common panels: "2"/"bw", "3"/"bwr", "4", "7"/"acep". Any requested color in a payload is then quantized to the nearest color in this list — on a 2-color device red becomes black; on 4-color a blue #1e90ff becomes white; on 7-color it stays blue. Elements are drawn in full color and this mapping is applied to the whole image once at the end of render() (dithered or flat, per dither — see Dithering).

• Device-dependent rotation (rotate_mode), not unified — the two behaviours are physically different:

  • "canvas" (fixed-resolution e-ink panels, e.g. ble-esl): the drawing surface rotates; output stays width×height.
  • "image" (variable-size label printers, e.g. niimbot): the drawing rotates; output dimensions swap.

Usage

from imagespec import render, RenderContext

ctx = RenderContext(
    font_resolver=my_font_lookup,  # optional
    history_provider=my_history_lookup,  # optional, only for `plot`
)
image = render(payload, width=296, height=128, rotate=0, background="white", context=ctx)  # -> PIL.Image (RGB)

Run the smoke test (no fonts required):

pip install -e .
python examples/smoke_test.py

Development & testing

pip install -e ".[dev,datamatrix]"   # dev pulls in numpy, so both dither paths are exercised
pytest                 # unit + golden-image tests: every element, palettes, rotation, dither, errors
pytest --update-golden # rewrite the golden PNGs after an intentional rendering change
ruff check . && ruff format --check .   # lint + format
mypy                   # type-check src/ (the package ships py.typed)
python -m build        # build sdist + wheel (bundles fonts/icons)

CI runs on every push/PR (.github/workflows/ci.yml): ruff lint+format, mypy, the test suite on Python 3.13/3.14 (plus a lowest-pinned-dependencies job), and a build that asserts the bundled fonts/icons are present in the wheel.

Releasing: add a version section to CHANGELOG.md (behaviour changes go under Changed with before/after), bump version in pyproject.toml, merge, then tag: pushing a v* tag triggers .github/workflows/release.yml to build and publish to PyPI (trusted publishing).

The test matrix (tests/test_elements.py) asserts it covers every registered element type, so adding a new @element(...) without a sample fails the suite — keeping coverage exhaustive by construction.

Golden images (tests/test_golden.py) pin the actual pixels: each element preview in examples/elements/ is re-rendered from examples/generate_element_previews.py and compared exactly, and tests/golden/ holds targeted scenes (every dither method, per-element dither phase, rotation modes, less-common handler options). A rendering change that is intended is committed by running pytest --update-golden and checking in the PNGs; an unintended one fails CI with the pixel count and a diff image. Text in the goldens is laid out with Pillow's BASIC engine (RenderContext(layout_engine=...)) so the same Pillow release renders them identically on every OS; set IMAGESPEC_GOLDEN_TOLERANCE=0.05 to allow 5 % of pixels to differ under a different Pillow/FreeType or python-barcode release.

Robustness built in:

• Each handler error is wrapped with element context — you get error rendering element #3 (type 'text'): ..., not a raw PIL traceback.
• render() validates rotate/rotate_mode/size and rejects non-dict elements; unknown element types are warned-and-skipped. Unknown keys are ignored unless you pass strict=True (or call imagespec.validate() yourself).
• dlimg only allows http(s)/data: URLs by default; local paths require RenderContext(allow_local_images=True). Network failures become RenderError. Downloads are streamed and abort past max_image_bytes (20 MB default); image_cache_ttl=<seconds> reuses a fetched image across renders (off by default so camera snapshots are never served stale), and image_fetcher lets the host supply its own url -> bytes.
• Clear errors for missing required args, invalid barcode symbology, malformed polygon points, and a diagram too small for its bars.
• Template-friendly input: numeric keys (x, size, progress, ...) accept strings ("42", "3.5") and boolean flags (visible, show_percentage, ...) accept "False"/"off"/"0", as Home Assistant templates produce them. A non-numeric string fails with 'x' must be a number, got 'oops' naming the element.

Elements

Tip

Copy-paste examples for every element: docs/elements.md

Preview	Element	Module	Notes
	line	shapes	+ dashed lines
	rectangle	shapes
	rectangle_pattern	shapes
	circle	shapes
	ellipse	shapes
	arc	shapes
	polygon	shapes
	gauge	shapes
	text	text	+ rotation, background box
	text_box	text
	multiline	text
	new_multiline	text	fit-to-width/height autosize
	text_fit	text	fit text into a fixed box: shrink font / ellipsis / wrap
	table	text
	qrcode	codes	eclevel, pixel width/height sizing
	barcode	codes
	datamatrix	codes	optional dep pyStrich (imagespec[datamatrix])
	icon	media	Material Design Icons (default) + Font Awesome Free (fa:/fas:/far:/fab:); needs bundled icons/ assets
	dlimg	media	+ fit modes (stretch/fit/fill/contain)
	diagram	charts	bar chart
	plot	charts	needs history_provider; + area_fill, xlegend
	progress_bar	charts	+ rounded corners
	pie	charts	pie / donut (inner_radius)
	sparkline	charts	compact axis-less line from inline values
	rich_text	text	inline spans: icon + text + color on one line
	group	layout	container: child elements at an offset, clipped, optionally rotated
	stack / row / column	layout	auto-layout: packs children along an axis (gap/padding/justify/align), flexbox-style, with optional Tailwind-like class shorthand
	legend	widgets	color-swatch ↔ label rows (vertical/horizontal) for pie/plot
	star_rating	widgets	full/half/empty stars for rating labels
	battery	widgets	vector battery gauge with proportional fill

render(..., dither=True|"atkinson"|…) dithers the whole output (15 algorithms

• none; see docs/dithering.md) and any element can carry its own dither bool/method to override it just for itself.

Payload specification

Payloads are specified as a list (sequence) of dictionary elements, which can be easily authored in YAML or JSON. Each element requires a type string and varying geometric/styling attributes.

Tip

Element-by-element YAML examples: docs/elements.md

Generating payloads with an LLM? See docs/authoring.md — a self-contained authoring guide you can paste straight into an AI's context. It covers the output contract, the layout decision model (stack/row/column vs group vs absolute coordinates), the Tailwind-like class shorthand, common pitfalls (e.g. the YAML "one key per line" trap), device canvas sizes, and full worked examples — every example verified by actually rendering it.

Element reference & JSON Schema

Every element's keys, types and defaults are declared once, next to its handler (@element("circle", fields=[num("x", required=True), ...])), and everything else is generated from those declarations:

• docs/reference.md — the full key reference
• schema/elements.json — JSON Schema (draft 2020-12) for a whole payload; use it in an editor or validator to catch typos, missing required keys and bad enum values before rendering
• template-string coercion ("42" → 42, "False" → False) — driven by the declared key types
• imagespec.validate(payload) — the same checks at runtime, without a JSON Schema library: returns [Issue(path, message), ...] such as [2].fill: unknown key for 'circle', tolerating template strings and null exactly as render() does. render(..., strict=True) runs it first and raises RenderError listing every issue.

python scripts/export_schema.py regenerates the two files; CI fails if they drift from the declarations, and a guard test fails if a handler reads a key it did not declare. The web payload editor in eigger.github.io keeps its supported subset in schema/editor_types.json.

Common attributes

• Colors: names (black, white, red, green, blue, orange, yellow, any CSS name) or HEX (#FF0000, #f00); quantized to the device palette at the end of render().
• Coordinates: pixels from the top-left corner (0, 0).
• Numbers and booleans also accept the string forms Home Assistant templates produce.
• visible, dither, class / layout are accepted by every element (see the reference).

Dithering

imagespec can dither full-color content onto limited palettes (2-color B/W, 3-color BWR, 7-color ACeP, …). Besides flat nearest-color mapping (none), it ships 15 e-ink-oriented dither algorithms (Floyd–Steinberg, Atkinson, Jarvis, Stucki, Burkes, Sierra family, Stevenson–Arce, Bayer 2/4/8/16, clustered-dot 4/8).

• dither=False / "none" (default) → flat nearest color
• dither=True / "floyd" → Floyd–Steinberg (backward compatible)
• dither="atkinson" / "bayer8" / … → any name in DITHER_METHODS

Per-element overrides accept the same bool or method string. Full gallery, API notes, and per-algorithm tiles: docs/dithering.md.

python examples/generate_dither_methods.py   # all-method grids + tiles

Without dithering, colors snap to the nearest palette entry and gradients band badly. Dithering trades spatial resolution for perceived depth.

1. Gradient & 3D Shading

Dithering creates a natural halftone pattern that simulates smooth shading and eliminates color banding.

2. Font Rendering (Anti-aliasing vs. Dithering)

Important

Guidelines for Text: Avoid dithering on text layers. Dithering anti-aliased font edges creates tiny dot noise, which severely degrades readability on low-resolution e-ink screens. For sharp text, use direct quantization or disable anti-aliasing (fontmode = "1"). The built-in text element enforces fontmode = "1" for this reason.

3. Charts & Solid Fills

Dithering is useful when you have solid color regions (like pie slices or bar diagrams) in colors outside your device palette (e.g. orange on a black/white screen). Dithering simulates these colors with dot patterns to help distinguish segments, though it introduces some edge noise.

How palette mapping works

Every element is drawn in full color, and the whole image is mapped to context.palette once at the end of render(). The dither flag only picks how that single mapping happens:

• dither=True / "floyd" → Floyd–Steinberg (or pass another method name)
• dither=False / "none" (default) → flat nearest color

Either way the output is strictly on-palette. Because mapping is deferred, in-palette colors (e.g. black text on white) stay crisp under dithering — error diffusion spreads no error when a pixel already equals a palette color — while the text guidance above still applies to off-palette text you choose to dither.

ctx = RenderContext(palette="bw")
img = render(payload, 296, 128, dither=True, context=ctx)
img = render(payload, 296, 128, dither="atkinson", context=ctx)
# orange/green/blue pie slices -> different dot patterns, not one black blob

Per-element dither override

Any element may carry its own dither: true/false/method name to override the global flag just for itself — so you can dither only the parts that benefit (photos, charts) and keep the rest flat (labels, QR codes), in a single render:

- type: dlimg          # this photo -> halftone
  url: "https://…/photo.png"
  xsize: 100
  ysize: 100
  dither: atkinson
- type: pie            # this chart -> ordered screen
  x: 60
  y: 60
  radius: 40
  values: "Gas,30,orange;Water,25,blue;Elec,45,red"
  dither: bayer8
- type: text           # left flat regardless of the global flag
  x: 10
  y: 110
  value: "Energy mix"

An element with an explicit dither is rendered in isolation and mapped to the palette immediately (then composited in payload order), so its choice survives the final whole-image pass. Elements without the key follow the global dither argument. (QR/barcode and black text are pure palette colors, so they stay crisp under the global flag anyway — set dither: false only for off-palette content you want kept solid.)

Device samples

Both labels below mix crisp content (text, QR, barcode) with charts authored in off-palette colors and marked dither: true — the charts become halftones so their segments stay distinguishable, while everything else stays sharp.

Electronic shelf label — 3-color (black / white / red):

Label printer — 2-color (black / white):

Regenerate them with:

python examples/generate_dither_labels.py

Older Floyd-vs-none comparison boards:

python examples/compare_dither.py

Fonts & assets

Bundled in the package (offline baseline, ~12 MB total):

• icons/materialdesignicons-webfont.ttf + _meta.json — icon's default set (mdi: prefix, or no prefix).
• icons/fontawesome-free-{solid,regular,brands}.otf + trimmed metadata — icon's second set (fa:/fas:/far:/fab: prefix).
• fonts/NotoSansKR-Regular.ttf — the only bundled font, and the default for every payload (RenderContext.default_font).

Anything else is resolved at runtime, in order: font_resolver (host) → bundled font of the same basename → bundled default. Helpers in imagespec.resolvers:

• directory_resolver(dir) — look up fonts in a host directory (e.g. www/fonts).
• caching_resolver(cache_dir, sources) — download on first use, cache to disk, reuse offline (internet needed only once per font).
• google_fonts_resolver(cache_dir, families=None) — a caching_resolver preset over verified-license Google Fonts (ofl/ directory → all SIL OFL 1.1), covering scripts the bundled Noto Sans KR doesn't: Japanese, Simplified/ Traditional Chinese, Arabic, Thai, plus a broader Latin/Cyrillic/Greek family. See GOOGLE_FONTS_SOURCES for the exact list.
• chain_resolvers(a, b, ...) — try several in order.

The package bundles only this baseline: decorative or other-script fonts are better downloaded-and-cached (google_fonts_resolver/caching_resolver) or served by the host (e.g. Home Assistant's www/fonts) through font_resolver. Only fonts with a verifiable license are bundled — see Licensing & attribution below.

Licensing & attribution

imagespec is MIT AND Apache-2.0 (see the license field in pyproject.toml) — not pure MIT — because it's a combined work:

Component	License	Source
imagespec's own code/modifications	MIT	LICENSE
Rendering engine origin (registry dispatch, element handlers)	Apache License 2.0	OpenEPaperLink Home Assistant Integration — see NOTICE
icons/materialdesignicons-webfont.ttf (+ metadata)	Apache License 2.0	Pictogrammers / Templarian MaterialDesign-Webfont
icons/fontawesome-free-*.otf (+ metadata)	SIL OFL 1.1 (fonts) / CC BY 4.0 (icons)	Font Awesome Free
fonts/NotoSansKR-Regular.ttf	SIL Open Font License 1.1	Google Noto Fonts

The engine was originally adapted from OpenEPaperLink's code and has since been substantially rewritten and extended (palette/color model, device-aware rotation, dithering, new elements, ...); NOTICE documents this per the Apache License's redistribution terms. Full license texts ship in the package: LICENSE-APACHE-2.0 (covers the engine origin and the MDI font), icons/LICENSE (a co-located copy for the icons directory), icons/LICENSE-FONTAWESOME (Font Awesome Free — also notes brand-icon trademark restrictions), and fonts/OFL.txt.

Integrating into a host

Build a RenderContext (palette, font lookup, history provider), call render(), translate RenderError into the host's error type — about 60 lines. docs/integrating.md walks through it with a Home Assistant adapter.