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Rosse: galaxies, drawn by hand. A barred spiral on Paper, a grand-design spiral on Chalkboard, both drawn by the engine.

Rosse is a galaxy-drawing engine. Every mark it puts on the page comes from a library of real hand drawings: dots, strokes, knots, stars and whole galaxies, drawn with a fineliner, scanned and cut out. The engine builds each galaxy in three dimensions, then places, transforms and inks those drawings onto Paper or Chalkboard.

This repository is the rebuild of Rosse v21 (a single HTML page, kept in assets/reference/ as the oracle) on WebGPU and WGSL, with a matching CPU engine for browsers that have no WebGPU. Every picture on this page, the banner's galaxies included, was drawn by this engine, not by v21.

principle in practice
Hand-drawn, never procedural There is no generated mark. Dots, knots, strokes, cores, stars and 156 whole galaxies are scanned pen drawings; the engine only chooses where they go, how they are turned and how heavy the pen is.
On the GPU Sampling, projection, culling, stroke expansion and merger integration run in WGSL compute passes. The CPU builds a small scene description and nothing is read back on the frame path.
A CPU twin for every kernel Without WebGPU, TypeScript twins of the kernels run with the same random numbers, f32 arithmetic and a software rasteriser, and agree with the GPU within tolerance (npm run test:gpu).
Deterministic A counter-based RNG keyed by seed, stream and index. The same seed gives the same galaxy, and orbiting the camera never re-rolls the marks.
Measured against v21 Parity is statistical (same structure, ink, pen weight and mark counts, not the same dots) and is tested against 582 captures of v21 with a calibrated metric.

Beyond v21

v21 stays the oracle: the engine's core draws what v21 draws, and the goldens hold it to that. On top of that core, the page switches on a set of natural features, each an optional flag (off in the core, on in the page, listed per ADR in docs/architecture.md), so a galaxy reads as a body in space and not a flat print. The pictures below are the page's.

the page draws what changes ADR
Depth and perspective marks shrink with distance and as the view zooms out; the disc dims a star behind it; the near and far sides of a galaxy and its debris differ 0084, 0086, 0088
Natural dust and bulges a dust layer in the disc, deprojected Sersic bulges, a boxy-peanut bulge for barred galaxies on request, dust as brush strokes at their own heights 0075, 0076, 0087
A thin disc and 3D line-work a dense sharp plane with a few wanderers; arms, dust lanes, jets, streams and tails as 3D stroke bundles that stay put when you orbit 0081, 0083, 0089
Stellar populations and colour old stars drawn fine and young ones large, clumps, halo clusters and streams of stars; a fifteen-ink colour plate whose colour follows temperature, radius, arms and dust, in pastel chalks on the Chalkboard and pencils on Paper 0090, 0091
The drawn core behind the disc the central bitmap is drawn first, under the disc's stars, dust and strokes 0092
Mergers that keep their galaxies' character a spiral holds its inner disc and crisper arms through the first pass, an elliptical is a soft glow, dust lanes and a bridge thin the stars and travel into the tails, tails carry tidal dwarfs, strokes taper at the tear, discs are thicker 0093

Open ?dustAuto=0&bulgeAuto=0&starsAuto=0&cosmicAuto=0&strokesAuto=0&lineWorld=0&thinAuto=0&popAuto=0&depthAuto=0&occlAuto=0&coreAuto=0&mergeAuto=0 to see the core's drawing alone.


An atlas

Every picture below was drawn by this engine, on SwiftShader (software WebGPU), at a fixed seed. The pictures are plates in the sense of an atlas: numbered, captioned, laid on a grid. Every galaxy, seed, parameter and camera path is in tools/readme-assets/plan.mjs, and one command re-draws all of them (see Regenerating). The colour you see is the engine's own (the colour plates); nothing is added.


Galaxies from the Zoo

Galaxy Zoo 2 asked volunteers a tree of questions about 239,695 SDSS galaxies. For each kind of galaxy in the catalogue's own list (smooth, spiral, barred, edge-on, merger, ring, irregular, many arms, tight arms, loose arms), these are the galaxies with the highest vote fraction for the question that defines it. Rosse draws each one from its votes alone (fromVotes), at the seed its object id gives, with the bars showing the votes it was given.

Fig. 01: ten galaxies from Galaxy Zoo 2, one for each kind, drawn by Rosse from the volunteers' votes

Ties are common (a fraction of 1.00 is easy to reach), so the choice is: the highest fraction, then the most volunteers, then the lowest object id. The rule is written out in GZ2_RULES in the plan, and the choices (with their votes and parameters) are recorded in gz2-picks.json.

Fig. 01.2: the spiral with the highest vote fraction, drawn by Rosse, with its votes Fig. 01.3: the barred galaxy with the highest vote fraction
Fig. 01.6: the ring galaxy with the highest vote fraction Fig. 01.5: the merger with the highest vote fraction
Fig. 01.4: the edge-on galaxy with the highest vote fraction Fig. 01.8: the galaxy with the most arms by vote fraction
The other four: smooth, irregular, tight arms, loose arms
Fig. 01.1: the smooth galaxy with the highest vote fraction Fig. 01.7: the irregular galaxy with the highest vote fraction
Fig. 01.9: the tightest-armed spiral by vote fraction Fig. 01.10: the loosest-armed spiral by vote fraction

Photograph and drawing

The catalogue has votes but no pictures. The asset pack does have the SDSS photographs of 42 real galaxies, so here they are side by side with Rosse's drawing of each, drawn from the same votes. (Photograph: 160 px, enlarged.) Where a cutout of a catalogue galaxy is in docs/img/gz2/cutouts/, the plates above show it too; see Regenerating.

Fig. 02: a barred spiral, SDSS photograph and Rosse's drawing Fig. 02: a ringed disc, SDSS photograph and Rosse's drawing
Fig. 02: an edge-on disc with a dust lane, photograph and drawing Fig. 02: a two-armed spiral, photograph and drawing
Ten more pairs
a round elliptical, photograph and drawing a cigar-shaped elliptical, photograph and drawing
an edge-on disc, photograph and drawing a spiral with a ring, photograph and drawing
a three-armed barred spiral, photograph and drawing a four-armed barred spiral, photograph and drawing
a spiral, photograph and drawing a six-armed spiral, photograph and drawing
an irregular galaxy, photograph and drawing two galaxies merging, photograph and drawing

Every preset

All 45 presets of src/core/presets.ts, as the page draws them at seed 7. The sheet is made from the preset list itself, so a new preset appears in it the next time the pictures are regenerated.

Fig. 03: a contact sheet of all 45 presets: spirals, ellipticals, mergers, stars, artefacts, lenses, the deep field and more


In motion

Each film is a camera path through the engine's own camera (azimuth, inclination, roll, zoom), eased, timed, and looped where it can be. The marks are fixed in three dimensions, so nothing is re-rolled as the camera moves.

Fig. 04: a spiral galaxy on the Chalkboard, turning once with the camera pushing in and out
04  ORBIT, SPIRAL
one turn; inclination 40 to 52 degrees; a slow push-in		 Fig. 05: an edge-on galaxy with a dust lane, the camera opening the disc and closing it again
05  ORBIT, EDGE-ON
the disc opens from 84 to 62 degrees and closes
Fig. 08: a barred spiral changing shape as its pitch, bar and arm width are swept
08  A GALAXY MORPHING
one seed; pitch, bar and arm width swept		 Fig. 07: the same spiral printed in one ink, then in slipped colour plates, then in population colours
07  THE PLATES BREATHING
one ink, slipped plates, population colours
Fig. 06: a line sweeping across a spiral, Chalkboard on its left and Paper on its right
06  PAPER AND CHALKBOARD
one set of marks, two surfaces; only the last, cheap pass changes

Up close

The pen keeps its weight at any zoom, so a mark is always a mark. At the deepest zoom the nucleus resolves into the scanned core drawings it is made of, and a star into its drawn heart.

Fig. 09: one tightly wound spiral at three zooms, ending in individual hand-drawn core marks

Fig. 10: a bright star at three zooms, from its spikes to its drawn heart

Fig. 11: the camera pushing in on the nucleus of a spiral to nine times, then back
11  PUSHING IN
x1.5 to x9 on the nucleus and back		 Fig. 12: the camera pushing in on a bright star from the whole star to its heart, then back
12  PUSHING IN ON A STAR
spikes, rings, then the drawn heart

By layers

A galaxy is a pile of kinds of mark, laid in a fixed order. Here is one galaxy, with each layer added in turn: dots, knots, strokes, drawings (pen lines and whole drawings), drawn stars and cores, and the sky.

Fig. 13: one spiral in six stages, from dots alone to every layer and the sky

Fig. 14: the same spiral drawn in layer by layer, each spreading from the centre outwards, while the camera drifts
14  DRAWN IN
each layer spreads from the centre; the camera drifts

Lenses

A lens is a galaxy (or a cluster) in front of a source: the source's drawing is bent into rings, arcs and images. The source can be a galaxy, a quasar or one of the owner's hand drawings (Fig. 36.6).

Fig. 15: six lens presets: an Einstein ring, a double ring, a quasar cross, a quad, a cluster with arcs and a giant arc

Fig. 16: the same lens with the source at three offsets: a full ring, an arc and two images

Fig. 17: an Einstein ring breaking into an arc and closing again as the source moves, with the camera pushing in
17  EINSTEIN RING
the source drifts: ring, arc, two images, ring		 Fig. 18: the four images of a lensed quasar circling the cross as the source circles
18  EINSTEIN CROSS
a quasar: four images follow the source round
Fig. 19: a double Einstein ring: two rings that part and close again as one source drifts, with the camera pushing in
19  DOUBLE EINSTEIN RING
two sources behind one lens

Mergers

Mergers are simulated: a core track integrated on the CPU and test stars moved by the GPU, with the tides stretching the discs into tails. The merger's own timeline (mTime) runs from the approach to the settled remnant; each film below moves the camera with it. On the page each galaxy keeps its character through the first pass: a spiral its arms and dust lanes, an elliptical its smooth glow, with dust lanes and tidal dwarfs carried into the tails (ADR 0093).

Fig. 20: nine merger presets, from the Mice to sketches torn apart

Fig. 21: the Mice at six moments, from approach to the settled remnant with long tails

Fig. 22: two spirals approaching, passing and throwing out tails, filmed by a camera that dollies in and then draws back
22  THE MICE
a dolly in on the approach, a turn at the close pass, a draw back as the tails form		 Fig. 23: a merger with long tidal tails on the Chalkboard, the camera orbiting
23  LONG TAILS
a slow orbit while the discs swing past
Fig. 24: two discs at right angles colliding, the camera turning with them
24  POLAR COLLISION
discs at right angles		 Fig. 25: whole hand drawings stretched and torn by the tides
25  SKETCHES, TORN APART
whole drawings, torn by the tides

Shells are what a minor merger leaves behind: faint arcs round a smooth galaxy, simulated and then drawn.

Fig. 26: a shell galaxy at three times: a sharp edge, spreading shells and wide faint shells

Fig. 27: the shells of a shell galaxy forming and spreading
27  SHELLS FORMING
shellTime 12 to 90, with a slow turn

The sky

Behind and in front of every galaxy there is sky: a deep field of small drawn galaxies, foreground stars with their spikes, rings and glare, and the faults of a real exposure (a satellite trail, a ghost reflection, cosmic rays). All of it is drawn from the same library of pen marks.

Fig. 28: the deep field: hundreds of small drawn galaxies round a central elliptical

Fig. 29: a slow glide over the deep field on the Chalkboard, turning the sky
29  THE DEEP FIELD
a long glide; the sky turns with the camera

Fig. 30: six stars: the preset, a plain heart, long spikes, rings, glare and a faint one

Fig. 31: a satellite trail, a ghost reflection, cosmic rays and other seeds of each

Fig. 32: layered presets: a spiral beside a bright star, a barred spiral with a satellite trail, an edge-on with a star on top, a ringed galaxy with a ghost, a lensed merger

Fig. 33: a spiral with no foreground, with foreground stars, and with stars and small companion galaxies


Kinds of galaxy

Fig. 34: flocculent, tightly wound, loose, hand-drawn, dusty and armless discs

Fig. 35: a ringed galaxy, a shell galaxy, a radio jet, stellar streams, a cigar-shaped and a round elliptical

Fig. 36: slipped colour plates, stellar populations, a wobbling hand, both on Paper and on Chalkboard, and a sketch lensed


Regenerating the pictures

nice -n 15 npm run readme:assets                 # everything: banner, gallery, anatomy, gz2, gifs
nice -n 15 npm run readme:assets -- gz2          # one stage (banner, renders, gallery, anatomy, gz2, gifs)
nice -n 15 npm run readme:assets -- gifs --only=merger-mice
npm run readme:assets -- fetch-gz2               # the SDSS cutouts of the Galaxy Zoo 2 picks (needs the network)
npm run readme:check

Each stage re-draws only its own outputs, deterministically; the plan, the camera paths and the catalogue rule are in tools/readme-assets/ and docs/readme-refresh.md. fetch-gz2 downloads the SDSS cutout of each Galaxy Zoo 2 galaxy above once, into the repository; after that the plates show photograph and drawing together and regenerating needs no network.


Run it

You need Node 22 or later and a browser with WebGPU for the GPU engine (any other browser draws on the CPU engine, and says so under the plate). To validate shaders you also need naga (cargo install naga-cli).

npm ci
npm run dev        # Vite dev server: the page (plate, controls, presets, timeline, exports)

The page has the plate on Paper or the Chalkboard, v21's controls as a recipe of cards, preset cards drawn by this engine, the merger's timeline, the 42 real galaxies with their photographs, the Galaxy Zoo 2 catalogue, and PNG, SVG (for pen plotters, one layer per pen) and GIF (the merger's timeline, a quasar's flare) export. Drag to orbit and tilt, shift-drag or Q/E to roll, wheel to zoom. A link holds the drawing: ?preset= (or ?from=real:<n>, ?from=gz2:<DR7 object id>), ?seed=, ?az=, ?incl=, ?pa=, ?zoom=, ?surface=paper|chalk, any parameter by its name, and ?backend=cpu|webgpu; for example /?preset=Barred%20spiral&seed=11&backend=cpu.

command what it does
npm test unit tests (vitest)
npm run test:gpu browser tests on WebGPU (SwiftShader): RNG vectors, CPU = GPU kernels and raster, the page's UI and extras smoke tests
npm run golden the golden check against v21 (see below; slow)
npm run lint, typecheck, validate:wgsl, build the other CI checks
npm run readme:assets re-draws every picture on this page, by stage (tools/readme-assets; needs ffmpeg and ImageMagick)
npm run readme:check fails if a README image is missing or a picture is over its size budget
npm run screenshots screenshots of the plate on both surfaces and both engines

The tools that drive a browser use Playwright's Chromium (npx playwright install chromium) with software rendering, so no GPU is needed.


How it is built

parameters ──► schema: which tier is dirty?
                 │
  model dirty ───┼─► CPU  scene description (a few kB): variation, curves,
                 │        parts, merger core track
                 │   GPU  stipple samples · merger test stars · shells
  view dirty ────┼─► GPU  project + cull → scan → instances
                 │        curves → ribbons · vectors → capsules
  present ───────┴─► GPU  ink layers in v21's order → ink target
                          → composite: Paper | Chalkboard + plates
tier re-runs when cost of an orbit frame
model a parameter that changes the galaxy none
view the camera or zoom moves the view passes only
present the surface or plates change one composite pass
  • Kernels with CPU twins. Each compute kernel in src/shaders/compute/ (stipple, scan, project, ribbons, vector-expand, merger, merger-sprites, tide, shells, and more) has a TypeScript twin in src/fallback/kernels/ and a parity test.
  • Two ink pipelines. Instanced quads for bitmap drawings, and analytic capsule ribbons for pen lines, into an rgba16float ink target; the composite blends it onto Paper or Chalkboard.
  • Determinism. A counter-based RNG (pcg4d), so each sample owns its randomness and no result depends on thread scheduling.
  • Deliberate differences from v21: orbiting keeps the marks; integer-lattice noise replaces v21's sin-hash noise; drawings have separate mip chains. Listed in docs/architecture.md.

Status

Each milestone is one reviewable pull request. The engine draws single galaxies, mergers, stars, artefacts and the sky, and lensing; the page and the extras are built on it. M0 to M12 are all built; M7, M9 and M12 are in the integration pull request (#15) and reach main with it.

# milestone state
M0 Plan: reference notes, architecture, ADRs, v21 captures merged
M1 Paper and one bitmap mark; RNG, atlases, ink and composite merged
M2 Stipple disc from the model; the golden comparison harness merged
M3 Camera, orbit and zoom; the cache tiers merged
M4 Stroke ribbons and arms; dust lanes merged
M5 Vector marks: bars, rings, whole drawings, jets, streams merged
M6 The single-galaxy preset set; plates; Chalkboard merged
M7 Stars and artefacts; the sky built (pull request #15)
M8 Mergers and shell galaxies merged
M9 Lensing built (pull request #15)
M10 Performance pass: pooling, kept batches, the CPU engine in a worker, the profiling and L1 harnesses merged; real-hardware numbers open
M11 The page and its UI merged
M12 The extras: Galaxy Zoo 2 catalogue, real galaxies, SVG and GIF export built (pull request #15), wired into the page

Beyond the milestones, the pull request that follows them (#17) adds the natural features above, each behind a flag with its own ADR, and tests them on both engines.

Still open: everything that needs a real GPU (the budget table of docs/architecture.md, the L1 report on a second adapter, and recalibrating the golden thresholds, Q14); measurements on a phone; the licence (Q1) and the SDSS acknowledgement wording the page must show; and the owner's sign-off of the proposed ADRs (0035, 0036, 0061 among them).

Not yet proven: all parity numbers are measured on SwiftShader, a software adapter. Details per milestone are in docs/roadmap.md and docs/milestones/.


Testing

  • Unit tests (vitest) cover the RNG vectors, schema and tiers, the scene description, camera maths, the WGSL resolver, struct layouts and the CPU kernels.
  • GPU tests (npm run test:gpu) run the WGSL kernels and the CPU twins on the same input and compare them.
  • Golden images. tests/golden/reference/ holds 582 captures of v21 (ink, plate and parameters). Because v21 places dots from a sequential random stream and this engine from a parallel one, the dots land elsewhere by design, so plain pixel comparison would fail. The metric compares what must match: total ink, structure (SSIM of blurred density maps), moments and extent, pen weight (stroke widths from a distance transform) and mark counts, with thresholds calibrated from "same galaxy, other dots" re-draws and negative controls that must fail. The engine is also bit-exact against its own goldens on one adapter. See tests/golden/README.md and ADR 0013 and 0015.

Documentation

read for
docs/architecture.md the WebGPU design, tiers, the hard parts and the ADR index
docs/roadmap.md milestones M0 to M12
docs/reference-notes.md how v21 works, stage by stage
docs/open-questions.md decisions waiting for the owner
assets/README.md the asset pack: drawings, objects, fonts, data, and the reference page and source
assets/reference/design-language/ the design language this page borrows (Swiss grid, hairlines, cream paper)
CONTRIBUTING.md, CHANGELOG.md how to contribute; what changed

Credits and licences

  • The drawings are the owner's own pen drawings (dots, strokes, knots, stars, whole galaxies), scanned and cut out.
  • Galaxy Zoo 2. The classifications behind the "Galaxies from the Zoo" plates and the real-galaxy pairs are from Galaxy Zoo 2 (Willett et al. 2013, MNRAS 435, 2835; Hart et al. 2016, MNRAS 461, 3663), the work of the volunteers of galaxyzoo.org, licensed CC BY 4.0. Changes: each vote fraction is quantised to one byte and the catalogue is re-packed; the drawings are Rosse's, not Galaxy Zoo's or SDSS's.
  • SDSS. The photographs of the 42 real galaxies (and any cutout fetched for the Galaxy Zoo 2 plates) are from the Sloan Digital Sky Survey. Funding for SDSS and its acknowledgement, as the survey publishes it: sdss.org/collaboration/citing-sdss. The exact wording is the owner's to confirm and paste here (docs/readme-refresh.md).
  • Fonts. The banner is set in Heros (the files are TeX Gyre Heros, a Helvetica-like face) and IBM Plex Mono (SIL OFL 1.1, text in assets/fonts/ibm-plex-mono/). The handwriting fonts (Threshold Grain, Mark, Patina, Signs) are made from the owner's handwriting. Each keeps its own terms; they are listed in assets/fonts/fonts.json.
  • The project's licence is undecided. Until the owner chooses one (docs/open-questions.md, Q1), all rights are reserved; see LICENSE. Third-party material in assets/ remains under its own terms.

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