Anatomy

Anatomy

This skill makes figures like a small instrument on a bench: a heavy base on feet, rails with tick rulers, a carriage on bushings, knurled knobs, coil springs, a dial gauge whose needle moves, screws as tiny dots. The reader understands the idea because they recognise the machine. Every number the figure shows comes from the same model that moves its parts.

Eight complete examples ship with the skill. Open the closest one before you start. Each framework-free example is built with node examples/<name>/build.mjs, which writes a standalone HTML page next to the script; add --light for the light theme.

• examples/dial-indicator/ is the smallest: a dial gauge on a stand, about 300 lines in all.
• examples/desk-computer/ is framework-free SVG: a desk computer with a keyboard you can type on. The typed line appears on a CRT screen drawn in the plane of the case front, and the picture collapses to a line and a dot when you switch it off. It has about 85 solids and 850 paths.
• examples/arcade-cabinet/ is an upright arcade cabinet with its side panel off, a Hero of about 240 solids and 15 cables. A quarter flies into the slot, rolls down the coin chute at the speed a rolling disc reaches on a 30° slope, trips the coin-switch wire and drops into the cash box; the switch's pulse runs chunk by chunk along its cable to the board, and the CRT warms up, self-tests and runs an attract mode drawn in the plane of the tilted monitor. The stick tilts and closes a microswitch under the panel. It shows how to place a camera so a cutaway's mechanism stays in view, a concave board drawn as a prism along any axis, moving parts rebuilt in the page from the same module the build uses, and live coins and a wire drawn in reserved slots of the settled order. node build.mjs --audit proves the geometry.
• examples/raptor-engine/ is a rocket engine hanging in a test stand. Its round parts come from kit/lathe.mjs and its pipes and harness from kit/tube.mjs. It comes apart into 11 assemblies with staggered springs and per-frame depth re-sorting. Every part records its true shape, and node build.mjs --audit (kit/audit.mjs) proves, at rest, apart and in every replayed frame of the explode, that nothing passes through anything, that every end sits on a real mount, that no bend pinches, folds or lands on another line, that no curved profile's slope wobbles, and that every overlap is drawn in depth order. A WebGL plume, shock, splash and steam render between the back and front SVG. It is the reference for dense pipework, smooth shading and fades.
• examples/ripple-tank/ is a school ripple tank on tall legs over a paper screen, seen from 44° up. Two WebGL layers (lamp light, shadows and caustics on the paper, and the water's reflections and glass) are registered to the isometric planes and driven by a pure dispersion and optics model (model.mjs); a frequency change travels out from the dippers through a history texture.
• examples/test-rig/ is a production figure from a real site (React/Next): a glass button on a spring-mounted carriage, a finger probe on a rail that presses and pulls, and a dial gauge reading the lean. It has 53 solids and about 470 paths.
• examples/spot-plate/ is a second production figure from the same site: six pairs of glass drops on a spot plate, each pair drawn from one distance field, so a pair closer than merge runs together into one piece of glass. A fan of feeler blades sets merge, one blade per gap. It is the reference for shapes that morph into each other and for one control driving many parts.
• examples/turning-dial/ is the optional 3D mode: a combination dial that turns in true 3D with a small robot hand on it. It has about 90 live parts in metal materials, a second turn group, a hinge, a three-hinge finger that lifts and a pod that breaks off as a free body. A WebGL LED, its light pool and sparks follow the moving parts. stress.mjs beside it is a dense hand of about 380 parts in a cluttered workshop, used to measure performance.

The test rig is the quality bar for the drawing itself. The Raptor is the bar for shader work: a WebGL effect has to reach its standard of world-space, no-pop, seamless rendering.

The one idea that matters

The object is the explanation. Don't draw boxes, arrows and labels. Invent a real, well-made physical thing whose working is the concept. Here is how earlier figures mapped:

Two ways this fails, both seen in practice:

• Too abstract. Stacked plates, floating slabs or labelled boxes don't read as anything.
• Too flat and bland. One big slab with a few lines on it has no volume, no parts and nothing to discover.

The fix for both is the same: a recognisable machine, built from many small, precise, purposeful parts.

Every creation is highly detailed. This holds for every figure, however small the idea or the request: a quick explainer is still a dense, real machine, never a few slabs. What scales with the size (workflow step 1) is how far you push it: a Figure gets the 4× sweep over its subject and mechanism, a Hero or an Epic over the whole drawing.

• Read the real thing. Build it the way the real object is built: every screw, seam, bevel, bushing, label plate, cable with its connector, scale with its ticks, and the parts that hold the parts.
• Two looks. At 1× it reads as one clear machine. At 4× every region still has something true to find; a bare face or an empty corner is unfinished work, not calm.
• Purposeful, never noise. Detail comes from parts that belong to the mechanism. Each one sits on another part, ends on an edge, and fades rather than stops.
• Check it last. Before you finish, zoom to 4× (over the subject and mechanism for a Figure, the whole drawing for a Hero or an Epic) and add the real parts that are missing wherever a region is plain.

Workflow

Work in this order. Write the concept and the parts list before any geometry. Most of the quality is decided there.

1. Size it before you start. Tell the user which size the request is and roughly how long it will take, before any other work.
   • Figure: one mechanism, 40–80 parts, SVG with at most one WebGL layer. About 30–90 minutes. Build it without asking.
   • Hero: a dense object, several WebGL layers or one 3D subject, 80–150 parts. About 2–4 hours. Say so, then build it unless the user asked for something quick.
   • Epic: a whole vehicle, building or plant with many moving systems, or the 3D mode on more than one subject. 6 hours or more. Do not start it. Give the estimate, propose a Hero version that keeps the core of the idea, and list what the full version would add. Build the Epic only when the user chooses it.
   • Whatever the size, have a working page that opens in the browser within the first hour, and deepen it from there. Run date at the start and at each step, and when a step runs past twice its share of the estimate, tell the user where it stands and what is left.
2. Truth first. Collect the real numbers and behaviour of the thing being explained: sizes, counts, thresholds, curves. The readout, motion and caption must all be true. If you can, write the model as a small pure module, like rig.ts in the test-rig example, and drive everything from it.
3. Invent the object. Pick a machine, tool, instrument or piece of furniture whose mechanism maps one-to-one onto the concept. Say in one sentence what moves and what that motion means. Never copy a reference image: take its finish, not its object.
4. Parts list. Write at least 40 parts (40–80 for a Figure, 80–150 for a Hero, more for an Epic), grouped: base (plate, feet, groove, corner screws, label plate, ruler strip), structure (posts, rails, frame bars, end stops), mechanism (carriage, bushings, springs, plunger, needle), controls (knurled knob, thumb screw, crown) and the subject itself, which is the one lit part. Each part sits on another part. references/craft.md has a vocabulary of parts with recipes.
5. Plan the world. Use world units of about 1 px of the real thing: x and y on the floor, z up. Lay out plans as rounded rectangles {x, y, w, d, r} and circles. Choose the camera: azimuth 45° by default, 35–60° to show the long side, elevation 30° (higher, like the ripple tank's 44°, to look into something open). Then fit with fitProjection(points, width, height, { pad, azimuth }).
6. Build geometry once. Precompute every static path in a geometry module with the kit: slabOf, cylinder, extrude, planOutline, topTicks, knurl, coil, radialTicks and so on. Round parts on any axis (bells, flanges, valves, actuators) come from kit/lathe.mjs, and pipes and cables from kit/tube.mjs. Moving parts are built once at a rest position and moved with translateAlong. Only parts whose shape changes are rebuilt per frame.
7. Paint back to front, bottom to top. Sort repeated parts with depthOf. Split anything that wraps another part into a back half and a front half: the springs around a rod, the bars of a frame around a carriage. A moving part is drawn in the slot where it physically sits. See "Painter's order" in references/craft.md. Once a figure has pipes, cables and round parts, stop tuning keys by eye: record each item's shape as you put it into A.recorder(P, { order, spring }), let A.settle(R, P) measure true depth wherever two items overlap and set the keys, and end the build with A.auditOrExit(R, P). Route pipes before you draw them: real ports at both ends, legs along the structure, crossings on straights, collars gapping the pipe (references/craft.md, section 8). Record the true shape of what you draw: a stand-in shape that silences the audit is a bug, and the audit fails one. A rod that slides out of a gland is one rod from G.rodOut, recorded with A.slide.
8. Frame it. Put the figure in a card: Fig n at top left, a short title at top right, the interaction hint at bottom left and the live readout at bottom right. Legend keys and a true caption go under the card, and words stay out of the drawing apart from at most one tiny engraved label.
9. Make it live. Use calm time-based motion: exponential follow or real springs. An idle tour loops when nobody touches it. The figure responds to pointer and keyboard, stops when offscreen, and goes still under reduced motion. Write attributes on refs and don't re-render per frame. See references/motion.md.
10. Verify like a critic. The owner finds faults by zooming in, so you zoom in first. Read every PNG you make.

• Capture desktop, phone (390 wide), hover and several moments of motion with scripts/capture.mjs; use scripts/drive.mjs for sequences, the apart pose and the --no-webgl fallback.
• Run the audit to zero. With pipes, cables or round parts, node build.mjs --audit must end audit passed, every failing line at 0: clearance at rest, apart and in mid-flight, terminals, loose ends, folds, self-overlap, crowding (against slender solids too), stand-in shapes, support, wobble, saddles and depth order. Fix an interlock in the geometry, never with a key bias.
• Run the line check to zero. The audit never sees line markup. node scripts/lines.mjs page.html --out shots/lines.png (and on the -light page, and in every resting pose with drive.mjs's ["lines"] action) must print line ends: 0: every seam, rule, rib, tick and pointer ends on a stroke, a dot or an outline. Read the ringed PNG.
• 4× close-ups of every junction in both themes (build --light too), assembled and apart: every flange, tee, elbow, clamp, valve, bellows, band, saddle, ball joint and crossing, every curved body (no tone staircase), every place a texture ends (on an edge or fading), flat box faces with their seams, every textured hose at a chunk joint, every sliding rod at rest and at full stroke, and every window with text or shader content in it.
• Shaders: a contact sheet slowed with ["slow", 0.05] across every transition; nothing pops, no colour switch, edges read 0. In light, also log ink per frame through each transition (it must follow the light monotonically) and crop where the ink meets strokes at 8–16× (references/verify.md, "Light-theme shaders").
• Put your PNG next to the examples and ask: is it instantly a real object, rich in small parts with volume, with every line ending on something? Is it highly detailed at 1× and still rewarding at 4×, with no bare face or empty corner? Iterate until yes. Checklist: references/verify.md.

Setup

The kit is in kit/:

Pick the path that fits the project:

• Standalone HTML, a static SVG, a blog post or any non-React site. Write a Node build script like examples/dial-indicator/build.mjs. It imports kit/iso-kit.mjs, assembles strings with solidSvg, lineSvg, dotsSvg, wireSvg and faceTextSvg, wraps them with figureSvg and plateHtml, and writes the page with pageHtml({ script: kitScript() + live }), or kitScript() + glScript() + live when it has a shader. For a lone .svg file use figureSvg({ standalone: "dark" | "light" }), which embeds the styles.
• React or Next.js. Copy kit/ into the project (for example components/iso/). Keep geometry in plain .ts modules, build the static art in a server component with the draw.tsx components, and pass it as element props to one "use client" live component that moves things. references/react.md explains the split and a trap with large pages.