// 量出「立绘在屏幕上的实际足迹」：正常帧与藏 canvas 参考帧的差值掩码。
// 不依赖对视口/相机数学的任何建模——直接测渲染结果。
// 输出：足迹包围盒、四条边是否被画布切掉、覆盖面积占比、足迹底边距画布底部的距离。
// 用法：node tools/subject.mjs <normal.png> <ref.png> [threshold] [edgeBand]

import { spawn } from "node:child_process";

const [normal, ref, threshold = "8", edgeBand = "2"] = process.argv.slice(2);
if (!normal || !ref) {
  console.error("usage: node tools/subject.mjs <normal.png> <ref.png> [threshold] [edgeBand]");
  process.exit(2);
}
const T = Number(threshold);
const BAND = Number(edgeBand);

function probe(file) {
  return new Promise((resolve) => {
    const p = spawn("ffprobe", ["-v", "error", "-select_streams", "v:0", "-show_entries", "stream=width,height", "-of", "csv=p=0", file]);
    let out = "";
    p.stdout.on("data", (d) => (out += d));
    p.on("close", () => {
      const [w, h] = out.trim().split(",").map(Number);
      resolve({ w, h });
    });
  });
}

const { w: W, h: H } = await probe(normal);

const buf = await new Promise((resolve, reject) => {
  const p = spawn("ffmpeg", ["-v", "error", "-i", normal, "-i", ref, "-lavfi", "blend=all_mode=difference,format=gray", "-f", "rawvideo", "-pix_fmt", "gray", "-"], {
    stdio: ["ignore", "pipe", "pipe"],
  });
  const chunks = [];
  let err = "";
  p.stdout.on("data", (d) => chunks.push(d));
  p.stderr.on("data", (d) => (err += d));
  p.on("close", (code) => (code === 0 ? resolve(Buffer.concat(chunks)) : reject(new Error(err || `exit ${code}`))));
});

if (buf.length < W * H) {
  console.error(`差分流只返回 ${buf.length} 字节，期望 ${W * H}`);
  process.exit(1);
}

let minX = W, maxX = -1, minY = H, maxY = -1, count = 0;
const edge = { left: 0, right: 0, top: 0, bottom: 0 };

for (let y = 0; y < H; y++) {
  for (let x = 0; x < W; x++) {
    if (buf[y * W + x] <= T) continue;
    count++;
    if (x < minX) minX = x;
    if (x > maxX) maxX = x;
    if (y < minY) minY = y;
    if (y > maxY) maxY = y;
    if (x < BAND) edge.left++;
    if (x >= W - BAND) edge.right++;
    if (y < BAND) edge.top++;
    if (y >= H - BAND) edge.bottom++;
  }
}

if (!count) {
  console.log(`${normal}  没有超过阈值 ${T} 的像素——立绘层是空的？`);
  process.exit(1);
}

const pct = (n) => ((n / (W * H)) * 100).toFixed(3) + "%";
console.log(`${normal}  (${W}x${H}, 阈值 ${T})`);
console.log(`  足迹包围盒    x:[${minX}, ${maxX}]  y:[${minY}, ${maxY}]  尺寸 ${maxX - minX + 1}x${maxY - minY + 1}`);
console.log(`  覆盖面积      ${count} 像素 (${pct(count)})`);
console.log(`  底部余量      画布底 ${H} - 足迹底 ${maxY} = ${H - 1 - maxY} px`);
console.log(`  顶部余量      ${minY} px`);
console.log(`  左右余量      左 ${minX} px / 右 ${W - 1 - maxX} px`);
console.log(
  `  贴边情况      ${[
    edge.left ? `左 ${edge.left}` : "左 -",
    edge.right ? `右 ${edge.right}` : "右 -",
    edge.top ? `上 ${edge.top}` : "上 -",
    edge.bottom ? `下 ${edge.bottom}` : "下 -",
  ].join("  ")}   (${BAND}px 带内)`,
);
const cut = edge.left || edge.right || edge.top || edge.bottom;
console.log(`  判定          ${cut ? "被画布边缘切到" : "四边都未触边"}`);