// ============================================================
//  SNAPTOGRID — CROSS-STITCH / PIXEL-GRID QUANTIZING
//  `snaptogrid cell [ … ]` snaps every penetration to a lattice. Its
//  defining property is FRAME-INVARIANCE: the grid belongs to the
//  fabric, not the motif, so it's evaluated in fixed hoop space OUTSIDE
//  any enclosing transform. Stamp the same motif at many places with
//  translate and they all snap to ONE shared lattice, registering across
//  the whole piece — that's the difference between a "grid" and mere
//  "rounding". Pure and drawless: same config → same lattice, no seed.
// ============================================================

let cell    = 1.5    // [0.5:0.5:3] lattice pitch, mm — the pixel size
let span    = 4      // [2:6] motifs per side of the stamped grid
let pitch   = 16     // [10:1:20] mm between stamped motifs
let arm     = 5      // [3:1:8] motif arm length, mm

// a bold diamond motif, drawn once in its own local frame with sides
// several cells long, so quantizing steps the edges without collapsing.
// push/pop keeps each stamp from drifting the turtle.
def diamond() [
  push
  seth 45
  repeat 4 [ fd arm rt 90 ]   // a closed square, stood on its corner
  pop
]

color 6

// One global lattice. Each stamped copy lives under its own translate,
// but snaptogrid ignores those local frames and quantizes to the SAME
// hoop-space grid — so every motif's stitches land on shared nodes and
// line up cleanly across the whole piece (the cross-stitch aesthetic).
let off = (span - 1) * pitch / 2
snaptogrid cell [
  for gy = 0 to span - 1 [
    for gx = 0 to span - 1 [
      translate (gx * pitch - off) (gy * pitch - off) [
        diamond()
      ]
      trim
    ]
  ]
]

// The functional companion: snappath quantizes generated data to the
// same fixed lattice. A smooth circle pulled onto the grid becomes a
// blocky, stair-stepped pixel ring instead of a clean curve.
color 7
let circ = []
for i = 0 to 48 [
  append(circ, vfromheading(i * (360 / 48), 30))
]
sewpath(snappath(circ, cell))