# Scripting and prefabs

Scenes and prefabs are written in [Rhai](https://rhai.rs/book/), a small
scripting language embedded in isokit. Nothing is compiled: save the file and
the Studio redraws.

## A prefab

A prefab is an object type that can be placed in the Studio. It is two
functions in a module that is listed in `isokit.toml` under `prelude`:

```rust
fn meta_kiosk() {                      // label, palette group and property form
    #{ label: "Gate kiosk", category: "Site", icon: "⌂", help: "…",
       props: [
         #{ key: "h", type: "float", def: 2.8, min: 2.2, max: 4.0, step: 0.1, label: "Height (m)" },
         #{ key: "boom", type: "bool", def: true, label: "Boom barrier" },
       ] }
}

fn prefab_kiosk(o) {                   // draw around (0, 0) in the object's own frame
    let s = sprite();                  // sprite ids default to the object id
    s.box(-1.4, -1.1, 1.4, 1.1, 0.0, o.p.h, "brick", #{ top: "roof3" });
}
```

- Property types: `float`, `int`, `bool`, `choice` (with `choices`),
  `material` (a theme material, optionally with `choices`), `text`.
  Write `def:` for the default (`default` is a reserved word in Rhai).
- How the Studio places it: `shape: "line"` (drag along the map: sets `rot`
  and the length prop `w`), `shape: "rect"` (drag a rectangle: sets `w` and
  `d`), default a click. `mount: ["house_wall"]`: the prefab sits on objects of
  those types; the Studio snaps it onto the nearest one and moves, turns,
  copies and deletes it with its host. `storey: true`: placed on the storey the
  view shows (level `1f` puts it at z = 1 × `storey_h`). `directed: true` (with
  `shape: "line"`): the way matters, so the Studio turns the piece to run from
  where the drag starts (rot 0, 90, 180 or 270; conveyors).
- `o` holds `id, type, name, x, y, z, rot, p` (the props with defaults
  filled in), `pts`, `tag`, `info`, `level`.
- The engine sets the object's frame before calling the prefab. All
  coordinates are local: the origin is the object's position, and local x and y
  turn with `rot`. `vis()`, `dv()`, `tag()` and `focus()` use the frame too.
- A prefab with the same name in the project overrides the built-in one.
- If a prefab fails, the engine logs a warning for that object and still draws the rest.

## Drawing API

Numbers can be ints or floats. Classes are CSS classes without the `iso-`
prefix (`"wall"` becomes `iso-wall`). A material sets the face colour, which is
shaded automatically by face direction.

### Scene

| Function | |
|---|---|
| `sprite()` / `sprite(id)` / `sprite(id, cls)` | new sprite (the unit of the depth sort) in the current layer |
| `ground()` | the shared unsorted ground sprite |
| `layer("ground" \| "floor" \| "main")`, `layer(name, #{sort, zfirst, cls, before})` | where new sprites go |
| `tag(x, y, z, text[, prio[, id[, kind]]])` | label; `kind` "signtag" / "warntag" always shows |
| `info(id, name[, detail])`, `info(id, #{name, detail, rows: [[k, v], …]})` | tooltip |
| `info_row(key, value)` | one row more in the tooltip of the object being drawn (a belt's speed) |
| `focus(x, y, z)`, `bounds(x0, y0, x1, y1[, depth])`, `defs(svg)`, `warn(msg)` | framing and extras |
| `level()`, `rot()`, `cfg()`, `cfg(name)`, `objects()` | inputs (`objects()`: every document object, props resolved) |
| `objects_of(types)` | objects of a type or a list of types shown on this level |
| `objects_at(types, x, y)` | … whose footprint (`p.w` × `p.d` around the origin) holds the world point |
| `objects_on(types, x, y, tol)` | … whose centre line (local x, length `p.w`) passes within `p.t / 2 + tol` of the point, with `u, v` = the point in their frame |
| `mount_of(o, hosts[, tol])` | the host o sits on (nearest centre line within `p.t / 2 + tol`, default 0.3 m, same height), with `u, v` = o's position in the host's frame; `()` if none |
| `mounted(h, types, hosts[, tol])` | the objects of `types` whose host is h, each with `u, v` |
| `frame(o)`, `frame(x, y, z, rot)` | draw in another frame from here on (a window draws in its wall's frame) |
| `to_world(x, y)`, `to_local(x, y)`, `local_of(o, x, y)`, `world_of(o, u, v)` | between the current frame (or o's) and world plan coordinates |
| `vis(nx, ny)` | is a face with that outward normal turned to the viewer? |
| `dv(dx, dy, dz)` | screen vector of a world vector (CSS motion) |
| `along_x()` | should a mover run along x in this rotation? |
| `hash(x)`, `fmt(n)`, `round1(x)`, `fmin`, `fmax`, `clamp`, `px()` | helpers |
| `rect(x0, y0, x1, y1)`, `ellipse(cx, cy, a, b, n)`, `rsegs(r)`, `sides(x0, y0, x1, y1)` | geometry |

### Sprite methods (chainable)

| Method | |
|---|---|
| `box(x0, y0, x1, y1, z0, z1, mat[, #{id, top, side, no_top, no_side}])` | axis-aligned box |
| `prism(pts, z0, z1, mat[, opts])` | convex counter-clockwise footprint; z0/z1 one value or one per vertex |
| `cyl(cx, cy, r, z0, z1, mat[, opts])`, `frust(cx, cy, r0, r1, z0, z1, mat[, sides[, opts]])` | round things |
| `slope(x0, x1, y0, y1, za, zb, th, mat)` | ramp / stair flight |
| `face(pts3, mat[, #{id, two_sided}])` | any planar polygon, shaded and culled by its normal (roof slopes, gables) |
| `beam(a, b, n, w, mat)` | square bar `w` wide between 3D points a and b, `n` a direction across it (robot arms, trusses) |
| `poly(polys3, cls[, id])` | 3D polygons as one path (solar modules) |
| `flat(poly or polys, z, cls[, id])` | flat polygon(s); later ones cut holes |
| `open(a, b, [[u0, u1, z0, z1], …], cls[, id])` | windows / doors on the face a→b (outward normal on the right) |
| `lines(segs, z, cls)`, `disc(cx, cy, z, r, cls[, id])` | markings |
| `circle(x, y, z, r_px, cls[, dx, dy])`, `stem(x, y, z0, z1, w_px, cls)` | screen-space details (crowns, poles) |
| `person(x, y, z[, cls])`, `sit(x, y, z, cls)`, `head(x, y, z, r)`, `led(x, y, z[, cls])` | figures |
| `puffs(x, y, z[, kind[, n, dur]])`, `fan(x, y, z, r)` | animated steam / smoke / fans |
| `swim(x, y, z, [#{ rx, ry, dur, delay, cw, len, cls }, ...])` | fish going round ellipses (radii rx, ry along the object's axes) about (x, y) on the plane at z, one round per `dur` s, clockwise when `cw`; `cls` koi / koiw / koig (the pond) |
| `seq([[x, y, \|q\| …], …])` | items drawn back to front by their anchor |
| `group(cls[, css], \|q\| …)`, `begin(cls[, css])` … `end()` | `<g>` around parts (animation) |
| `ext(x, y, z)`, `ext_box(…)` | widen the sort box |
| `raw([x, y, z,] svg)` | raw SVG (`{uid}` becomes the view's id) |

Library helpers (`stdlib/*.rhai`): `s.tree_at(x, y, r)`, `s.car(x0, y0, x1, y1, cls)`,
`s.drive(dx, dy, dur, delay)` … `s.end()`, `cut_walls(...)`, `win_list(...)`,
`floor_of(level)`, `building_windows`, `gable_roof`, `flat_roof_kit`; for house
parts (`stdlib/house.rhai`, `stdlib/furniture.rhai`): `storey_of(z)`,
`house_view(o)` (0 closed, 1 on the storey cut open, 2 above it: leave out),
`wall_top(w)`, `opening_at(o)`, `s.slab(a, b, th, z0, z1, mat)`,
`s.chair_at(x, y, bx, by, mat)`, `s.office_chair(x, y, mat)`.

## Joints of walls and fences

`line_joint(o, e, same, stops)` (util.rhai) tells a line piece (centre line
along local x, length `p.w`) how it meets others at its end `e` (-1 or 1):
`#{ ext, post }`, how far it reaches past the end and whether it draws its end
post. `same` are the types it joins with, `stops` the types it ends against;
half thicknesses come from `line_ht(o)` (`p.t / 2`, for fences the design's).
Alone a piece reaches half its thickness past the end; on a straight run the
lower piece runs through and draws the shared post; at a corner the thicker
piece (equal: the one along world x) runs through and the other stops at its
face; at a T the stem stops at the face without an end post; against a `stops`
piece it stops at the face and keeps its post. Pieces never overlap, which the
depth sort needs. `fence` (designs `style`: mesh, wood, bars, half stone,
cobble, brick) and the site `wall` use it; `house_wall` has its own version
(`wall_ext`, house.rhai).

## Production lines

`producer`, `conveyor` and `robot_arm` (stdlib/industry.rhai) work together
through the document, like house parts:

- A producer makes one of its `product` goods every 60 / `rate` seconds (its
  ram goes down once per item) and puts them out on its local +x side.
- A conveyor carries goods along its local +x at `speed` m/s. When its start
  lies on a producer's output side, it carries that producer's goods at its
  rate; when its start is at another conveyor's end, it carries on with what
  that one carries (followed back up to 12 belts). The gap between goods is
  speed / rate, so a faster belt spreads them out. A stopped producer or belt
  stops the goods after it. Alone a conveyor carries its own `item` every
  `gap` metres.
- Belts meeting at a corner do not overlap: the belt a feeding belt runs into
  runs through the corner and the feeding one stops at its side; at a T the
  belt stops at the side. `rise` lifts the end (an incline).
- The goods ride a group that moves one gap per loop (`iso-ride`), so their
  drawing order never changes; the first fades in, the last fades out.
  Tooltips show speed, rate and where the goods come from (`info_row`).
- A robot arm picks goods off the end of the nearest conveyor within `reach`
  (+0.4 m) and puts them on the start of another conveyor within reach, or on
  a pallet a quarter turn on (`pick` / `put`: auto or a side of the arm). It
  makes one move per item coming in (`cycle` 0) or every `cycle` seconds, and
  waits while the line before it stands. The belt it puts on carries its goods
  and starts its loop when they land, so arm and belts stay in step.
- The arm is a flipbook: 16 poses (two-link inverse kinematics, links drawn
  with `beam`), each shown for a sixteenth of the cycle by a stepped CSS
  animation (`iso-flip`); a rotation cannot turn a 3D arm in this projection.
  Poses pointing away from the viewer are drawn before the base, the others
  after it.

## House parts

`house_wall`, `window`, `door`, `floor`, `roof`, `stairs` and the furniture
prefabs work together through the document, not through a parent object:

- A window or door at a point on a wall's centre line (within the wall's half
  thickness + 0.3 m, same height) sits on that wall (`mount_of`). The wall
  leaves a gap for it (`mounted`), the window draws the opening in the wall's
  frame with the wall's height, thickness and material. On no wall it stands
  alone as a short wall piece. Its rotation relative to the wall picks a
  door's swing side (0, 90: towards the wall's +y side) and hinge (0, 180:
  at the low end).
- Wall ends adapt to the walls they meet, so no two walls overlap (the depth
  sort needs them apart): along a line two walls just meet, at a T the stem
  stops at the other wall's face, at a corner the wall along world x runs
  through and the other stops at it.
- A part's storey is `floor((z + 0.5) / storey_h)` (`storey_h`: project
  parameter, default 3 m). On level `gf` (`1f`, …) the parts on that storey
  are cut open: a wall stays full height only when a floor lies on the
  viewer's side of it and none behind it (it is a back wall), all others are
  cut to 1 m. Roofs on that storey become an outline on the floor, parts on
  storeys above are left out, the storeys below stay closed.
- Ground-level floors and rugs go to the `floor` layer; upper floors are slabs
  whose top is at the object's z. Furniture starts 0.02 m above its z.

## Rules that keep the depth sort right

These are lessons from the CNX/Mooban views, and the engine relies on them:

- **One sprite per thing that can stand in front of another.** Split long walls,
  fences and solar rows into pieces (≤ 30 m). One big sprite overlapping many
  others creates sort cycles (`isokit check` reports them).
- **Things under a cover belong in the cover's sprite**: draw cars under a carport
  before its posts and roof (see `prefab_carport`).
- **Floors of cut-open storeys go in the `floor` layer** (ground floor) or in
  the same sprite as the storey below. Start walls and furniture standing on
  them a little above the floor (+0.02 m).
- **Movers** (`drive`) run along the axis the sort compares second
  (`along_x()`). Along the other axis they stay still.
- Street furniture stays off drive areas, and pipe runs end on walls or supports.

## Themes

`theme.toml` stacks on the built-in palette:

```toml
[palette.dark]
brick = "#8a4f3e"
[palette.light]
brick = "#b8664c"
[materials]
brick = "brick"          # class iso-brick, colour var(--iso-brick)
[rules]
".iso-svg .iso-sign2" = "fill: #ffcc00; stroke: none"
```

`isokit check` warns about any class in the drawing that has no style.