动态缩略图生成

js/apng-encoder.js

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+'use strict';
+
+const zlib = require('zlib');
+
+const PNG_SIGNATURE = Buffer.from([137, 80, 78, 71, 13, 10, 26, 10]);
+const crcTable = new Uint32Array(256);
+for (let n = 0; n < 256; n++) {
+	let value = n;
+	for (let k = 0; k < 8; k++) value = (value & 1) ? (0xedb88320 ^ (value >>> 1)) : (value >>> 1);
+	crcTable[n] = value >>> 0;
+}
+
+function crc32(buffer) {
+	let value = 0xffffffff;
+	for (const byte of buffer) value = crcTable[(value ^ byte) & 0xff] ^ (value >>> 8);
+	return (value ^ 0xffffffff) >>> 0;
+}
+
+function chunk(type, data) {
+	const name = Buffer.from(type, 'ascii');
+	const body = Buffer.from(data || []);
+	const out = Buffer.allocUnsafe(12 + body.length);
+	out.writeUInt32BE(body.length, 0);
+	name.copy(out, 4);
+	body.copy(out, 8);
+	out.writeUInt32BE(crc32(Buffer.concat([name, body])), 8 + body.length);
+	return out;
+}
+
+function paeth(left, above, upperLeft) {
+	const estimate = left + above - upperLeft;
+	const leftDistance = Math.abs(estimate - left);
+	const aboveDistance = Math.abs(estimate - above);
+	const cornerDistance = Math.abs(estimate - upperLeft);
+	return leftDistance <= aboveDistance && leftDistance <= cornerDistance ? left
+		: aboveDistance <= cornerDistance ? above : upperLeft;
+}
+
+function filterFrame(rgba, width, height) {
+	const stride = width * 4;
+	const result = Buffer.allocUnsafe((stride + 1) * height);
+	const candidates = Array.from({ length: 5 }, () => Buffer.allocUnsafe(stride));
+	for (let y = 0; y < height; y++) {
+		const rowStart = y * stride;
+		let bestType = 0;
+		let bestScore = Infinity;
+		for (let type = 0; type < 5; type++) {
+			const candidate = candidates[type];
+			let score = 0;
+			for (let x = 0; x < stride; x++) {
+				const raw = rgba[rowStart + x];
+				const left = x >= 4 ? rgba[rowStart + x - 4] : 0;
+				const above = y > 0 ? rgba[rowStart - stride + x] : 0;
+				const upperLeft = y > 0 && x >= 4 ? rgba[rowStart - stride + x - 4] : 0;
+				let predicted = 0;
+				if (type === 1) predicted = left;
+				else if (type === 2) predicted = above;
+				else if (type === 3) predicted = Math.floor((left + above) / 2);
+				else if (type === 4) predicted = paeth(left, above, upperLeft);
+				const filtered = (raw - predicted + 256) & 0xff;
+				candidate[x] = filtered;
+				score += Math.abs(filtered < 128 ? filtered : filtered - 256);
+			}
+			if (score < bestScore) { bestScore = score; bestType = type; }
+		}
+		const outputStart = y * (stride + 1);
+		result[outputStart] = bestType;
+		candidates[bestType].copy(result, outputStart + 1);
+	}
+	return result;
+}
+
+function encode(options) {
+	const width = Number(options.width);
+	const height = Number(options.height);
+	const frames = options.frames || [];
+	if (!Number.isInteger(width) || width <= 0 || !Number.isInteger(height) || height <= 0) throw new TypeError('APNG 尺寸无效');
+	if (!frames.length) throw new TypeError('APNG 至少需要一帧');
+	const ihdr = Buffer.alloc(13);
+	ihdr.writeUInt32BE(width, 0); ihdr.writeUInt32BE(height, 4);
+	ihdr[8] = 8; ihdr[9] = 6; // 8-bit RGBA
+	const animationControl = Buffer.alloc(8);
+	animationControl.writeUInt32BE(frames.length, 0);
+	animationControl.writeUInt32BE(Math.max(0, Number(options.plays) || 0), 4);
+	const chunks = [PNG_SIGNATURE, chunk('IHDR', ihdr), chunk('acTL', animationControl)];
+	let sequence = 0;
+	for (let index = 0; index < frames.length; index++) {
+		const frame = frames[index];
+		const rgba = Buffer.from(frame.rgba.buffer, frame.rgba.byteOffset, frame.rgba.byteLength);
+		if (rgba.length !== width * height * 4) throw new TypeError('APNG 帧像素数量无效');
+		const control = Buffer.alloc(26);
+		control.writeUInt32BE(sequence++, 0);
+		control.writeUInt32BE(width, 4); control.writeUInt32BE(height, 8);
+		control.writeUInt32BE(0, 12); control.writeUInt32BE(0, 16);
+		control.writeUInt16BE(Math.max(1, Math.min(65535, Number(frame.delayNumerator) || 1)), 20);
+		control.writeUInt16BE(Math.max(1, Math.min(65535, Number(frame.delayDenominator) || 1000)), 22);
+		control[24] = 0; // keep the previous frame
+		control[25] = 0; // replace the full canvas
+		chunks.push(chunk('fcTL', control));
+		const compressed = zlib.deflateSync(filterFrame(rgba, width, height), {
+			level: Math.max(0, Math.min(9, Number(options.compressionLevel) || 9)),
+		});
+		if (index === 0) chunks.push(chunk('IDAT', compressed));
+		else {
+			const frameData = Buffer.allocUnsafe(4 + compressed.length);
+			frameData.writeUInt32BE(sequence++, 0);
+			compressed.copy(frameData, 4);
+			chunks.push(chunk('fdAT', frameData));
+		}
+	}
+	chunks.push(chunk('IEND'));
+	return Buffer.concat(chunks);
+}
+
+module.exports = { encode };