Add JPEG XL converter example

examples/convert-to-jxl.mjs

@@ -0,0 +1,74 @@
+/**
+ * Convert an image to JPEG XL.
+ *
+ * sharp can write JPEG XL only when its libvips was built with libjxl
+ * (probed below via sharp.format); the prebuilt binaries currently do
+ * not include it. Fallbacks, in order: cjxl (the libjxl CLI, the only
+ * backend that can transcode JPEG losslessly, bit-exact), ffmpeg with
+ * libjxl, ImageMagick. Success requires a JPEG XL codestream (FF 0A)
+ * or container signature in the output.
+ *
+ *   node convert-to-jxl.mjs [--lossless] [--distance 1.0] [--effort 7] in.png [out.jxl]
+ */
+import { spawnSync } from "node:child_process";
+import { readFile } from "node:fs/promises";
+import sharp from "../lib/index.mjs";
+
+const CODESTREAM = Buffer.from([0xff, 0x0a]);
+const CONTAINER = Buffer.from([0, 0, 0, 0x0c, 0x4a, 0x58, 0x4c, 0x20, 0x0d, 0x0a, 0x87, 0x0a]);
+
+export function isJpegXl(buf) {
+  return buf.subarray(0, 2).equals(CODESTREAM) || buf.subarray(0, 12).equals(CONTAINER);
+}
+
+async function viaSharp(input, { lossless, distance, effort }) {
+  if (!sharp.format?.jxl?.output?.buffer) {
+    throw new Error("this sharp/libvips build cannot write JPEG XL");
+  }
+  const quality = lossless ? 100 : Math.max(10, Math.min(100, Math.round(100 - distance * 10)));
+  return sharp(input).jxl({ quality, lossless, effort }).toBuffer();
+}
+
+function viaTool(args) {
+  const r = spawnSync(args[0], args.slice(1), { stdio: "pipe" });
+  return r.status === 0;
+}
+
+export async function convert(input, output, { lossless = false, distance = 1.0, effort = 7 } = {}) {
+  try {
+    const buf = await viaSharp(input, { lossless, distance, effort });
+    if (!isJpegXl(buf)) throw new Error("sharp produced non-JPEG-XL output");
+    const { writeFile } = await import("node:fs/promises");
+    await writeFile(output, buf);
+    return output;
+  } catch { /* fall through to external libjxl backends */ }
+
+  const tools = [
+    ["cjxl", input, output, `--effort=${effort}`, lossless ? "--lossless=1" : `--distance=${distance}`],
+    ["ffmpeg", "-y", "-i", input, "-c:v", "libjxl", "-effort", String(effort),
+     ...(lossless ? ["-lossless", "1"] : ["-distance", String(distance)]), output],
+    ["magick", input, output],
+  ];
+  for (const args of tools) {
+    if (viaTool(args) && isJpegXl(await readFile(output))) return output;
+  }
+  throw new Error("no working JPEG XL backend (build sharp/libvips with libjxl, or install cjxl, ffmpeg --enable-libjxl, or ImageMagick with jxl delegate)");
+}
+
+if (import.meta.url === `file://${process.argv[1]}`) {
+  const args = process.argv.slice(2);
+  const opts = { lossless: false, distance: 1.0, effort: 7 };
+  const pos = [];
+  for (let i = 0; i < args.length; i++) {
+    if (args[i] === "--lossless") opts.lossless = true;
+    else if (args[i] === "--distance") opts.distance = Number(args[++i]);
+    else if (args[i] === "--effort") opts.effort = Number(args[++i]);
+    else pos.push(args[i]);
+  }
+  if (!pos.length || pos.length > 2) {
+    console.error("usage: convert-to-jxl.mjs [--lossless] [--distance 1.0] [--effort 7] in [out.jxl]");
+    process.exit(2);
+  }
+  const out = pos[1] ?? pos[0].replace(/\.[^.]+$/, ".jxl");
+  console.log(`${pos[0]} -> ${await convert(pos[0], out, opts)}`);
+}