SlabUploader/frontend/src/lib/pipeline/crop.ts

import { ASPECT_H, ASPECT_W, FILL_TARGET, OUTPUT_PX_MIN } from "./constants.js";
import type { CropResult, CropStatus, LengthAxis, MaskBuffer, RgbaImage } from "./types.js";

/**
 * Find the bounding box of the slab in the mask.
 */
export function findSlabBounds(mask: MaskBuffer, width: number, height: number): {
  left: number;
  top: number;
  right: number;
  bottom: number;
  widthPx: number;
  heightPx: number;
} {
  let left = width;
  let top = height;
  let right = 0;
  let bottom = 0;
  for (let y = 0; y < height; y++) {
    for (let x = 0; x < width; x++) {
      if (mask[y * width + x]! >= 128) {
        if (x < left) left = x;
        if (x > right) right = x;
        if (y < top) top = y;
        if (y > bottom) bottom = y;
      }
    }
  }
  return {
    left,
    top,
    right,
    bottom,
    widthPx: right - left + 1,
    heightPx: bottom - top + 1,
  };
}

/**
 * Compute target 3:4 frame dimensions so the slab fills FILL_TARGET (80%)
 * at the extremes. Returns the frame size in pixels.
 */
export function computeCropTarget(slabWidthPx: number, slabHeightPx: number): {
  width: number;
  height: number;
} {
  const targetAspect = ASPECT_W / ASPECT_H;
  const slabAspect = slabWidthPx / slabHeightPx;

  if (slabAspect >= targetAspect) {
    // Width is limiting; height gets extra margin
    const frameHeight = slabHeightPx / FILL_TARGET;
    return { width: Math.round(frameHeight * targetAspect), height: Math.round(frameHeight) };
  } else {
    // Height is limiting; width gets extra margin
    const frameWidth = slabWidthPx / FILL_TARGET;
    return { width: Math.round(frameWidth), height: Math.round(frameWidth / targetAspect) };
  }
}

/**
 * Crop the source to a 3:4 portrait frame, center the slab, with 80% fill
 * at extremes, transparent background. No upscaling.
 *
 * @param image  Original source RGBA image.
 * @param mask   Binary mask (255 = slab, 0 = background).
 * @param axis   Confirmed length axis for slab bounds.
 * @param lengthIn User-entered length in inches (used to derive vertical scale).
 * @returns CropResult with the cropped image and metadata.
 */
export function cropTo34(
  image: RgbaImage,
  mask: MaskBuffer,
  axis: LengthAxis,
  lengthIn: number,
): CropResult {
  const scalePx = axis.lengthPx / lengthIn;
  const slabBounds = findSlabBounds(mask, image.width, image.height);

  const target = computeCropTarget(slabBounds.widthPx, slabBounds.heightPx);
  const { width: frameW, height: frameH } = target;

  // Check minimum output size before any scaling
  const shorterSide = Math.min(frameW, frameH);
  const meetsMinPx = shorterSide >= OUTPUT_PX_MIN;

  // Compute the scale factor: we want the slab to fill FILL_TARGET of the frame
  // at the extremes. The slab is scaled down by FILL_TARGET, then the frame
  // adds extra margin.
  const slabW = slabBounds.widthPx;
  const slabH = slabBounds.heightPx;
  const slabAspect = slabW / slabH;
  const targetAspect = ASPECT_W / ASPECT_H;

  // Compute how much to scale the slab to fit 80% of the frame
  let scaleDown: number;
  if (slabAspect >= targetAspect) {
    // Width limiting: frameH = slabH / 0.8, frameW = frameH * 0.75
    // Scale factor for width: slabW / frameW
    scaleDown = slabW / frameW;
  } else {
    // Height limiting: frameW = slabW / 0.8, frameH = frameW / 0.75
    // Scale factor for height: slabH / frameH
    scaleDown = slabH / frameH;
  }

  // Scaled slab dimensions within the frame
  const scaledSlabW = Math.round(slabW * FILL_TARGET);
  const scaledSlabH = Math.round(slabH * FILL_TARGET);

  // Center position within the frame
  const offsetX = Math.round((frameW - scaledSlabW) / 2);
  const offsetY = Math.round((frameH - scaledSlabH) / 2);

  // Create the output image
  const outData = new Uint8ClampedArray(frameW * frameH * 4);

  // For each output pixel, compute which source pixel to sample
  for (let oy = 0; oy < frameH; oy++) {
    for (let ox = 0; ox < frameW; ox++) {
      const oi = (oy * frameW + ox) * 4;

      // Check if this output pixel is within the slab region
      if (ox < offsetX || ox >= offsetX + scaledSlabW ||
          oy < offsetY || oy >= offsetY + scaledSlabH) {
        // Transparent margin
        outData[oi] = 0;
        outData[oi + 1] = 0;
        outData[oi + 2] = 0;
        outData[oi + 3] = 0;
        continue;
      }

      // Map to slab-local coordinates
      const slabX = ox - offsetX;
      const slabY = oy - offsetY;

      // Nearest-neighbor sample from source
      const srcX = Math.round((slabX / scaledSlabW) * slabW) + slabBounds.left;
      const srcY = Math.round((slabY / scaledSlabH) * slabH) + slabBounds.top;

      // Clamp to source bounds
      const sx = Math.min(Math.max(0, srcX), image.width - 1);
      const sy = Math.min(Math.max(0, srcY), image.height - 1);
      const si = (sy * image.width + sx) * 4;

      outData[oi] = image.data[si]!;
      outData[oi + 1] = image.data[si + 1]!;
      outData[oi + 2] = image.data[si + 2]!;

      // Alpha from the mask at the source location
      const maskVal = mask[sy * image.width + sx]!;
      outData[oi + 3] = maskVal;
    }
  }

  return {
    image: { width: frameW, height: frameH, data: outData },
    width: frameW,
    height: frameH,
    meetsMinPx,
  };
}

/**
 * Check if a source image is too small to produce an OUTPUT_PX_MIN result.
 * Returns CropStatus.
 */
export function checkSourceSize(
  mask: MaskBuffer,
  width: number,
  height: number,
): CropStatus {
  const bounds = findSlabBounds(mask, width, height);
  const target = computeCropTarget(bounds.widthPx, bounds.heightPx);
  const shorterSide = Math.min(target.width, target.height);
  return shorterSide >= OUTPUT_PX_MIN ? "ok" : "undersized";
}