RecipesCam/src/utils/horizon.ts

import { Skia, FilterMode, MipmapMode, type SkImage } from '@shopify/react-native-skia';

// FRAME-tab AUTO straighten: the angle, in degrees, of the horizon in the photo.
// Null when the picture has no line worth trusting (a portrait, a flat wall), so
// AUTO leaves the fine angle alone instead of inventing one.
//
// Deskew by projection profile, not a gradient-orientation histogram: for every
// candidate angle the edge map is sheared and projected onto the rows, and the
// angle whose projection is the sharpest single line wins. The histogram looks
// cheaper, but a real edge is a staircase one pixel tall — a 7 degree line only
// steps every 8 px — so its gradients point at 0 and 45 degrees instead of at
// the line, and every small tilt reads as zero. Shearing cancels a staircase
// instead of averaging it away.
//
// ponytail: 1 degree coarse pass, 0.5 degree refine around the winner. The
// STRAIGHTEN slider is a 0.5 degree grid, so finer is invisible. Add a third
// pass if a residual tilt ever shows up.
export function detectTilt(image: SkImage | null, maxAngle = 45): number | null {
  if (!image) return null;
  let iw = image.width();
  let ih = image.height();
  if (!(iw > 8 && ih > 8)) return null;

  // Scan a thumbnail, never the 12 MP original: the profile only needs the
  // gross shape of the lines, and this runs on the JS thread. Halve instead of
  // resampling in one go — a one-shot 3x bilinear step samples 4 taps spread
  // over 3 source pixels and leaves exactly the staircase the search undoes.
  let cur = image;
  while (Math.max(iw, ih) > 256) {
    const w = Math.max(8, Math.round(iw / 2));
    const h = Math.max(8, Math.round(ih / 2));
    const surface = Skia.Surface.Make(w, h);
    if (!surface) break;
    surface
      .getCanvas()
      .drawImageRectOptions(
        cur,
        Skia.XYWHRect(0, 0, iw, ih),
        Skia.XYWHRect(0, 0, w, h),
        FilterMode.Linear,
        MipmapMode.Linear
      );
    const next = surface.makeImageSnapshot();
    if (!next) break;
    cur = next;
    iw = w;
    ih = h;
  }

  const raw = cur.readPixels();
  if (!raw) return null;
  // Bytes for the 8-bit raster surfaces this app builds; the float branch is
  // there because readPixels() reports whatever the image holds.
  const unit = raw instanceof Float32Array ? 255 : 1;
  const w = iw;
  const h = ih;

  const gray = new Float32Array(w * h);
  for (let i = 0; i < w * h; i++) {
    gray[i] = (0.299 * raw[i * 4] + 0.587 * raw[i * 4 + 1] + 0.114 * raw[i * 4 + 2]) * unit;
  }

  // The edge map, kept as a list: the angle search touches every edge once per
  // candidate, so skipping the flat pixels is most of the saving.
  const px = new Float64Array(w * h);
  const py = new Float64Array(w * h);
  const pw = new Float64Array(w * h);
  let n = 0;
  for (let y = 1; y < h - 1; y++) {
    for (let x = 1; x < w - 1; x++) {
      const i = y * w + x;
      const dx = gray[i + 1] - gray[i - 1];
      const dy = gray[i + w] - gray[i - w];
      const mag = Math.sqrt(dx * dx + dy * dy);
      if (mag < 20) continue; // flat patch: no line edge here
      px[n] = x - w / 2;
      py[n] = y;
      pw[n] = mag;
      n++;
    }
  }
  if (n < 32) return null; // too few edges to call anything a horizon

  // Score an angle by how sharply the sheared edge map projects onto the rows:
  // a line sheared to its own angle lands in one bin, any other angle smears it
  // over |delta| * w / 57 bins wide and the squared-sum collapses.
  const limit = Math.ceil((w / 2) * Math.tan((maxAngle * Math.PI) / 180)) + 2;
  const size = h + 2 * limit;
  const acc = new Float64Array(size);
  const score = (deg: number) => {
    acc.fill(0);
    const t = Math.tan((deg * Math.PI) / 180);
    for (let k = 0; k < n; k++) acc[Math.round(py[k] - px[k] * t) + limit] += pw[k];
    let ss = 0;
    let tot = 0;
    for (let i = 0; i < size; i++) {
      ss += acc[i] * acc[i];
      tot += acc[i];
    }
    return tot > 0 ? ss / (tot * tot) : 0;
  };

  const coarse: number[] = [];
  let best = 0;
  let bestScore = -1;
  for (let deg = -maxAngle; deg <= maxAngle; deg++) {
    const s = score(deg);
    coarse.push(s);
    if (s > bestScore) {
      bestScore = s;
      best = deg;
    }
  }
  // A picture with no line scores about the same at every angle; only a real
  // one stands clear of the middle of the field.
  coarse.sort((a, b) => a - b);
  if (bestScore < 3 * coarse[coarse.length >> 1]) return null;

  let angle = best;
  for (let deg = best - 1; deg <= best + 1; deg += 0.5) {
    const s = score(deg);
    if (s > bestScore) {
      bestScore = s;
      angle = deg;
    }
  }
  return angle;
}