workouts/Exercise Library/SYSTEM.md

# Exercise Visual System

Exercise visuals are produced by an **anatomical 3D rig**: one shared skeleton
posed per exercise by real joint angles, projected orthographically onto the
canvas. Nothing is drawn by hand — a skeleton profile plus a motion script
resolve through 3D forward kinematics into every frame, so figures are always
in proportion and anatomically plausible, and the whole library can be
re-proportioned (male/female), viewed from any side, orbited while animating,
or re-themed by changing data, never artwork.

## The rig

Model space follows the biomechanics (ISB) convention: **X anterior** (the
figure's facing direction), **Y up**, **Z toward the figure's anatomical
right**. Every joint angle is measured in degrees from the neutral standing
pose — upright, arms hanging, legs straight, toes forward. The math lives in
`kinematics.py`; the conventions are:

- **flexion** — forward positive, everywhere: shoulder flexion raises the arm
  forward/overhead (0 hanging, 90 horizontal, 180 overhead — normalized
  flexion-dominant, so an arm past vertical is 190, not extension −170),
  hip flexion raises the thigh, elbow/knee flexion bends the hinge (knees
  hinge backward automatically), spine/neck flexion curls forward, negative
  is extension. Ankle flexion is dorsiflexion (toes up), negative points them.
- **abduction** — away from the midline positive, for shoulders and hips.
- **rotation** — external (lateral) positive for shoulders and hips; for the
  spine and neck, turning to the right positive.
- A bare number is shorthand for `{"flexion": n}`.

**`skeleton.json`** holds the bone-length **profiles** — `neutral`, `female`,
`male`: head, neck, two spine segments, arms, legs, plus `foot`,
`shoulderHalf`/`hipHalf` (real shoulder and pelvis half-widths) and
`farOffset` (below) — and each joint type's degrees of freedom with its
physiological **range of motion (ROM)**. Rendering validates every key frame
against the ROM and prints a warning count (`--strict` lists and fails):
an impossible pose is a data bug, caught mechanically. Because motions are
authored in anatomical coordinates against joint names, **swapping profiles
never touches a motion script** — proportions are the skeleton's problem.

**`<Exercise>/motion.json`** — the exercise script:

```json
{
  "name": "Bird Dog",
  "primary": 2,                     // 1-based frame used for visual.svg
  "camera": {"yaw": 0},             // 0 = side view (default), 90 = face-on
  "working": ["arm_l", "leg_r"],    // parts drawn in the accent color
  "frames": [
    {
      "hold": 0.5, "tween": 0.8,    // seconds held / animating to the next frame
      "root": {"pos": [190, 106],   // pelvis canvas anchor
               "yaw": 180,          // trunk orientation: facing (0 = canvas-right)
               "pitch": 81},        // forward bow; "roll": side-lean (both optional)
      "spine": [0, 0],              // two chained segments: flexion, or a dict
                                    //   {"flexion", "lateral", "rotation"}
      "neck": 16, "head": -72,      // neck flexion (+rotation); head = extra gaze pitch
      "shoulder_l": 190, "elbow_l": 0,   // flexion shorthand, or
      "hip_r": {"flexion": -24},         //   {"flexion", "abduction", "rotation"}
      "knee_r": 0, "ankle_r": -25,       // hinge flexion; ankle = dorsiflexion
      "pins": {"hand_r": [111, 154]}     // IK: this hand holds that canvas point
    }
  ]
}
```

- **Pins (IK)** — a planted hand/foot names a canvas target
  (`hand_r`/`hand_l`/`foot_r`/`foot_l`); the renderer solves the two-bone
  chain analytically in 3D — in the plane picked by the authored elbow/knee —
  so the extremity holds that point exactly, and writes the solution back as
  anatomical angles. A pin active in two consecutive key frames stays planted
  *throughout the tween*; a pin present in only one frame releases naturally.
- **Tweening** happens in anatomical angle space, so limbs swing in natural
  arcs, bone lengths never distort, and interpolated poses stay plausible.
  The last frame tweens back to the first (looping). Asymmetric timing
  carries technique: leg raises lower slowly (`tween` 1.4 s down, 0.6 s up).
- **The camera** is orthographic and per-exercise: `yaw` 0 is the classic
  side view, 90 views the figure face-on — real foreshortening, not faked
  proportions. Face-on machines (abductor/adductor, rotary torso) author
  genuine abduction or spine rotation and let projection do the rest. The
  camera also orbits while the motion loops (`--orbit`; the in-app renderer
  slowly orbits **every** exercise, machines included — props have
  world-space 3D form and turn with the figure).
- **Zoom** — the skeleton is ~211 canvas units tall standing but the canvas
  has only 152 above the ground line, so standing motions author full-size
  anatomy (coordinates may run past the canvas top) and set
  `"camera": {"zoom": 0.7}` to fit. Zoom is **presentation-only**: a uniform
  scale of the drawn output (geometry, props, mat, stroke widths) about the
  ground-center anchor `(160, 152)`, applied after all solving — pins, prop
  coordinates, and the Swift-solver fixtures stay in full-size authored
  units, and the ground line maps to itself so planted feet stay planted.
  Typical values: ~0.7 standing tall, ~0.75–0.85 hanging or seated with arms
  overhead; omit it (1) for lying, kneeling, seated, and bent-over motions.
- **Elevation & the mat** — the default viewpoint pitches down 10°
  (`CAMERA_PITCH`; override per motion via `"camera": {"pitch": ...}`), and
  the ground is drawn as an **exercise mat**: a world-space quad sized to the
  motion's footprint (`mat_bounds`) that rotates with the camera about the
  figure — a long rectangle in profile, a parallelogram mid-orbit, end-on
  when face-on. Near/far contacts straddle it. Elevation is pure
  presentation: pins solve in the flat authored view and the *posed* body
  tilts, so authored canvas targets never go out of reach.
- **Feet** — each leg ends in a foot bone off the ankle. Dorsiflexion 0 keeps
  the foot perpendicular to the shin (right for standing, seated machines,
  planks); kneeling poses trail it back (−55), raises point it.
- **The nose tick** rides the head's anterior axis: it foreshortens with the
  view and disappears when the face points at (or away from) the camera, so
  face-on figures need no special casing. `head` adds gaze pitch on top of
  the neck.

## The visual language

- **Near vs far** — a smooth depth gradient, resolved per joint: every drawn
  joint is toned by its own camera depth between the near, dark end (`#3a3f4b`;
  teal `#0d9488` when working) and the far, light end (`#a9afba`; light teal
  `#86cfc5`), and each bone is stroked with a gradient between its two joints'
  tones, so the ink flows along a limb that reaches toward or away from the
  camera instead of the whole bone snapping to one flat tone; the stroke width
  tracks it too (near 6 → far 5). A joint reaches full contrast once it sits
  `SHADE_SPAN_FRAC` of the shoulder/pelvis half-width in front of (or behind)
  its limb pair's central depth plane, so a straight limb in a side view is
  still fully dark (near) or light (far) — the per-joint average matches the
  old flat per-limb tone — while as the two members cross (mid-orbit, or
  face-on machines) they fade to a shared mid-tone instead of the ink snapping.
  Opposite-limb moves (bird dog, dead bug) still read as opposite: one leaning
  dark-teal, one light-teal. The **binary** near/far pick (canvas-right wins a
  depth tie) stays underneath, driving the far offset and draw order — depth-
  sorted, far parts first, so a twist genuinely passes the near arm in front of
  the chest; the head paints last, filled opaque, so overhead arms pass behind
  the face.
- **The far offset** — in profile views both members of a pair project onto
  the same line; the far member is nudged by the profile's `farOffset` so it
  stays distinguishable. The nudge scales with how side-on the view is and
  vanishes face-on, where the skeleton's real shoulder/pelvis widths take
  over — one continuous rule across the whole orbit.
- **Spine** — rendered as a smooth curve through pelvis → mid → neck, with the
  shoulder girdle and pelvis drawn as bars across the attach points (near-full
  width face-on, a shoulder/hip nub in profile); teal when the trunk is the
  working part.
- Canvas 320×180, ground line at y = 152. Every limb is always drawn — a
  supine figure's far arm reads as the light far member, never a hidden one,
  so any viewpoint (and the orbit) stays truthful.

## The props layer

Machines and free weights are data too: an optional top-level `"props"` array
adds an equipment layer around the figure. `scene` shapes and `cable`s draw
*behind* the figure in a recessive gray; joint-attached items (`bar`,
`dumbbell`, `pad`) draw *over* the limbs in a darker gray and follow the
resolved hand/foot positions every frame — a pinned foot pressing a `pad`
carries the platform with it through the tween for free. The figure stays the
hero: props are schematic silhouettes (a seat, a backrest, one handle), never
scale drawings of the machine.

```json
"props": [
  {"type": "scene", "shapes": [
    {"kind": "line", "pts": [[134, 123], [96, 36]], "w": 9, "depth": 8},
    {"kind": "line", "pts": [[160, 64], [160, 120]], "w": 16, "z": -14},
    {"kind": "circle", "c": [142, 77], "r": 3.5, "fill": true, "color": "prop"}
  ]},
  {"type": "cable", "from": [190, 8], "to": ["hand_r", "hand_l"]},
  {"type": "bar", "at": ["hand_r", "hand_l"], "halfLen": 26, "plateR": 0},
  {"type": "dumbbell", "at": "hand_r"},
  {"type": "pad", "at": ["foot_r", "foot_l"], "angle": 88, "halfLen": 20, "w": 6}
]
```

Props are authored in the authored view but have **world-space 3D form**:
under an orbiting camera the whole equipment layer rotates about the
world-vertical axis through the root anchor, exactly like the figure and the
mat. Everything resolves in the *authored* view — scene points, cable
anchors, bar angles, pad perpendiculars, roller offsets — and the resolved
constructs are then rotated; joint-attached items keep following the rotated
figure's hands and feet, so equipment stays welded to the body. At the
authored yaw nothing moves, so the authored look is exact.

- **`scene`** — static shapes: `line` (polyline, stroke width `w`) and
  `circle` (`fill: false` for an outline). Points are canvas `[x, y]` (or
  `[x, y, z]` where needed); a shape-level `"z"` sets its depth plane
  (positive toward the camera), and a line with `"depth"` is a **slab** —
  extruded that half-width through its plane, it stays a plain line edge-on
  and opens into a swept quad as the camera orbits (seats, backrests,
  platforms). A shape may set `"color": "prop"` to use the darker
  attached-item gray (e.g. a fixed handle the hands rest on).
- **`cable`** — a thin line from a fixed anchor `from` (`[x, y]` or
  `[x, y, z]`) to a moving joint `to`; the machine's pulley line.
- **`bar` / `dumbbell` / `pad`** — a segment centered on the joint(s) in
  `at` (a single joint, or the midpoint of a list). Joints are the extremities
  (`hand_r`, `foot_l`, …) plus the mid joints (`elbow_r`, `knee_l`, …), so a
  machine pad can ride a knee (`["knee_r", "knee_l"]`) or span a shin
  (`["knee_r", "foot_r"]`).
  `bar` lies at a fixed authored-view `angle` (default 0 = horizontal);
  `dumbbell` and `pad` default to perpendicular to the lower bone
  (forearm/shin), or take an explicit `angle`. Under orbit the segment
  rotates with the scene and foreshortens naturally. `plateR` puts filled
  discs on both ends (dumbbells default to 4.5).
  A cross-body rod — a barbell, pull-up bar, or any grip that really runs
  left-right *through* both hands — should instead set `"axis": "z"`
  (superseding `angle`): its world-space direction projects through the
  camera elevation like the floor quad, so in a profile view it reads
  end-on (plates nearly concentric, seen from slightly above), opens to
  its full span face-on, and swings with the figure in between. A fixed
  screen-space `angle` can't do this — it keeps the rod glued to the
  authored direction while the body turns under it. Vertical handles
  (`angle: 90`) don't need it: a world-vertical rod is unchanged by a yaw
  orbit.
- **`roller`** — a machine roller pad seen end-on: a filled disc riding the
  limb's lower bone near the joint in `at`, on the `side` (+1/−1) of the bone
  it presses — a leg extension's instep roller (`side: 1`), a leg curl's
  heel roller (`side: -1`). `r` is the radius, `back` slides it along the
  bone away from the joint.

## Rendering

```sh
cd "Exercise Library"
python3 render.py                 # all exercises: frames/*.svg, preview.gif, visual.svg
python3 render.py "Bird Dog"      # one exercise
python3 render.py --sheet         # + contact-sheet.png of every key frame
python3 render.py --demo          # + demo-sheet.png: profiles / flipped camera / theme
python3 render.py --orbit "Bird Dog"  # orbit.gif: camera sweeps 360° while looping
python3 render.py --figure=female # render with another skeleton profile
python3 render.py --flip          # view from the other side (camera + 180°)
python3 render.py --strict        # fail on any ROM violation, listing each
python3 render.py --export        # bake app resources into Workouts/Resources/ExerciseMotions
python3 render.py --fixtures      # regenerate WorkoutsTests/Fixtures/figure-fixtures.json
```

`render.py` needs only Pillow (for GIFs/sheets; the SVGs have no dependency).
The library lives at the repo root, outside every target's source folders —
same-named files per entry (`info.md`, `visual.svg`) would collide in Xcode's
flat resource copy, so the library itself never enters the app bundle. Only
the `--export` copies ship: `skeleton.json` plus uniquely-named
`<Name>.motion.json` and `<Name>.info.md` files in
`Workouts/Resources/ExerciseMotions/`, consumed by the in-app renderer
(`Workouts/ExerciseFigure/`, compiled into the iOS target only — the watch's
run screen is timer-only, so its figure sources and these resources were
dropped from the watch target) and the exercise-library reference screen
(`ExerciseInfo.swift` parses the info pages). The in-app solver is a line-for-line port of `kinematics.py`,
held to it by `WorkoutsTests/Fixtures/figure-fixtures.json` — projected
geometry snapshots (figure and prop primitives, including orbit-presentation
samples) the Swift solver must reproduce; regenerate with
`python3 render.py --fixtures` alongside any pipeline change. Re-run
`python3 render.py --export` after editing any motion or info page; the
library stays the source of truth.