456. The emoji face is Noto, drawn from its paint graphs

Date: 2026-09-23

Status

Accepted. Supersedes the choice of face in ADR-0384 and the choice of format in ADR-0393; the artifact boundary of the first and the itemization of the second stand.

Context

goldberry-emoji shipped OpenMoji’s COLRv0 build: flat-coloured layers, drawn a layer at a time. The request was to switch to Noto Color Emoji — the face Android and ChromeOS draw, whose pictures are the ones most readers expect — and to keep it in colour.

Google publishes Noto Color Emoji two ways, and the choice between them is the one ADR-0393 made for OpenMoji, with different numbers:

The bitmap build needs a PNG decoder in the font pipeline and blurs above the one size it was drawn at, on a toolkit whose claim is that it is crisp at every scale (docs/ARCHITECTURE.md §6.2 already rejected it once). The COLRv1 build is half the size and is outlines — but it is not a list of flat layers. Nothing the toolkit had could draw it: ColorLayers reads version 0 records and Noto ships none, so every emoji would have come out as its bare base outline.

What version 1 is, measured over the face (3,993 colour glyphs, 72,825 shared layers):

• PaintGlyph — fill inside an outline — 66,000 times;
• its fill is a solid 58,600 times, a linear gradient 3,000, a radial gradient 4,900, and a gradient under a transform of its own 4,200;
• transforms wrap glyphs 12,400 times — affine, translate, scale about a centre;
• PaintComposite 567 times, every one of them a flag: the stripes, masked SRC_IN, with a shading ramp laid over them SOFT_LIGHT so the flag waves.

Decision

Read the graph in Java, as a sealed tree

text.font.sfnt gains ColorPaints, the COLR version 1 reader, and ColorPaint, the tree it produces — a sealed interface of records, so the painter is an exhaustive switch and a node added later stops it compiling rather than falling through a default that draws nothing.

What is folded at read time, so the painter has fewer cases:

• Eleven transform formats become one Transform matrix — translate, scale, rotate, skew, each with and without a centre.
• Variable formats read as their defaults. Every PaintVar… is its static twin plus a variation index, and the toolkit draws the default instance.
• Palette indices become colours, keeping 0xFFFF — “the text’s colour” — as a flag.

The index — base glyphs, layer offsets, clip boxes, palette — is read when the face is. A glyph’s graph is parsed on first use and kept, and layers are cached by index, because the same eyes recur across a dozen faces. Parsing all 3,993 graphs up front would be work and memory spent on emoji nobody sent; ColourEmojiTest.everyGraphReads parses them all anyway — the whole test class runs in well under a second — to prove none is malformed.

A malformed graph — an unknown format, an index past a list, a cycle deeper than 64 — answers null, and the pen draws the glyph as its own outline. That is ColorLayers’ rule and it stays.

Outlines are read too, because a glyph is now a clip

PaintGlyph fills inside an outline with a paint that may be a gradient, and the rasterizer’s glyph call fills with a colour and nothing else. So the painter needs the outline as a path, and GlyphOutlines reads glyf — simple glyphs with their implied on-curve midpoints, and composites placed by offset and matrix. It sends to an OutlineSink rather than returning a Path, because Path is in paint and paint reads sfnt. It is all-or-nothing: a glyph whose data runs off the table sends nothing, not half a shape.

GlyphFace reads outlines only for a face with paint graphs, and caches each glyph’s Path. A face of letters never builds one.

A clip is a fill, and a transform under a glyph is the gradient’s

ColourGlyphPainter concatenates one matrix — size / unitsPerEm, y flipped, at the glyph’s origin — and draws the graph in design units.

The rasterizer has no path clip, and the shipped face does not need one: filling an outline with a brush is the same picture as clipping the brush to the outline. So PaintGlyph over a solid or a gradient is one fillPath. A transform between the glyph and its gradient — 4,200 of them — must move the ramp and not the outline, so it becomes the gradient’s own matrix, which bl_gradient_init_as already took and nothing had passed.

The one case that is a real clip — a glyph whose fill is itself a picture — is drawn as the composite it means: the picture offscreen, kept SRC_IN to the outline.

Composites are two small layers

PaintComposite blends two finished pictures, so both are rendered into Layers sized to the glyph’s clip box on the device, snapped outwards to whole pixels, and the source is blitted onto the backdrop with the font’s operator. Noto’s flags nest one composite inside another; the inner one is drawn onto the outer one’s layer, so the offscreen area travels with the surface being drawn on rather than being fixed to the frame.

The native layer grows constants, not symbols

Everything needed was already exported: bl_gradient_init_as builds radial and conic gradients from a different values struct, and bl_context_set_comp_op takes any operator. What was missing is what the layout verifier guards:

• BLRadialGradientValues and BLConicGradientValues rows, every field named — Blend2D’s radial puts the focal circle, where the first stop sits, second, and a COLRv1 radial puts it first. Swapped, both render.
• Every BLCompOp a font may name — twenty-two constants, not the two the face uses, because a font is data the toolkit does not choose. The four HSL modes have no Blend2D operator and draw as source-over.

BlendGradient gains radial and conic and an extend mode and a BlendMatrix; BlendContext gains compOp. No export list changed.

The asset is a file, not an archive

Noto’s release has no assets, and its source zip is the whole repository — hundreds of megabytes of PNGs for one 5 MB font. Asset gains a Packaging: FILE pins the font itself, by the tag in its URL and by its checksum, which is the promise an archive’s checksum made.

The licence, and why the artifact stays

Noto Color Emoji is SIL OFL 1.1 — the licence Inter and JetBrains Mono are under. It asks for the licence text to travel with the font and for a modified font not to be called Noto; it does not ask for credit on screen, which is what put OpenMoji in an artifact of its own. The artifact stays for ADR-0384’s other reason: five megabytes is a lot to inherit for an application that never draws an emoji. NotoColorEmojiFont.CREDIT remains for an About box that wants it.

Consequences

Emoji are Noto’s, shaded and sharp at any scale, in every Paragraph drawn through a Fonts book, exactly as ADR-0393 routed them.

A colour glyph costs more than it did. A Noto glyph is a dozen filled outlines where an OpenMoji glyph was fourteen flat layers — comparable — but a gradient fill builds a native gradient per fill, and a flag allocates two small offscreen layers. None of it has been benchmarked yet; a wall of flags is the case to measure first if it ever shows. Caching a rasterized glyph per size is the optimisation, and it costs the crispness under a transform that this decision is for, so it is not taken now.

Written down rather than discovered:

• A sweep gradient’s stops outside one turn are clamped into it, and a sweep that repeats or reflects is drawn padded. Noto uses no sweeps.
• A radial gradient whose stops run past a circle of zero radius is clamped at zero.
• PaintColrGlyph draws the glyph it names without that glyph’s own clip box, and stops after sixteen references.
• A composite glyph placed by matching points is placed at its origin. No colour face uses the form.
• CFF outlines are not read; a COLRv1 face with a CFF table draws its glyphs as outlines.

The version 0 reader stays, for any face that is still version 0, and the pen asks the graph first where a face carries both.

An emoji is as tall as Noto makes it. Noto ascends 950 and descends 250 in 1024 units, so at 14 px an emoji reaches 13.0 above the baseline against Inter’s 13.6 — it sits inside the line box now rather than three-quarters of a pixel over it, which retires one consequence ADR-0393 recorded.