197 lines
7.1 KiB
JavaScript
197 lines
7.1 KiB
JavaScript
/**
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* 画个标 · 核心：GIF89a 编码器（手写，零依赖）
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* 输入等尺寸 HTMLCanvasElement 帧数组 → 输出 GIF 字节（Uint8Array）
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* 方案：每帧独立局部调色板（LCT，≤255 色 + 透明索引 0）+ LZW 变长编码
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* 纯函数、零 DOM 依赖（读取像素由调用方传入 canvas），Node 无头可测字节结构
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*/
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const HGB_GifEncoder = (function () {
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'use strict';
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/** 读取 canvas 像素（RGBA Uint8ClampedArray） */
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function readPixels(canvas) {
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const ctx = canvas.getContext('2d');
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const { width, height } = canvas;
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return { width, height, data: ctx.getImageData(0, 0, width, height).data };
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}
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/** 颜色频率 → 截断到 255 色；透明像素单独处理 */
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function buildPalette(data, n, transparent) {
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const freq = new Map();
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let hasAlpha = false;
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for (let i = 0; i < n; i++) {
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const o = i * 4;
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const a = data[o + 3];
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if (a < 128) { hasAlpha = true; continue; }
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const key = (data[o] << 16) | (data[o + 1] << 8) | data[o + 2];
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freq.set(key, (freq.get(key) || 0) + 1);
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}
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// 频率降序截断 255（LCT 256 槽，索引 0 留透明占位）
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const colors = Array.from(freq.entries())
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.sort((x, y) => y[1] - x[1])
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.slice(0, 255)
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.map(e => e[0]);
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const palette = new Array(256).fill(0x000000);
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const maxIndex = transparent ? Math.min(colors.length, 255) + 1 : Math.min(colors.length, 256);
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const start = transparent ? 1 : 0;
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for (let c = 0; c < maxIndex - start; c++) palette[start + c] = colors[c];
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return { palette, maxIndex, hasAlpha, colors };
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}
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/** 最近色匹配（RGB 欧氏距离） */
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function nearestIndex(rgb, palette, start, maxIndex) {
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const r = (rgb >> 16) & 0xff, g = (rgb >> 8) & 0xff, b = rgb & 0xff;
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let best = start, bestD = Infinity;
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for (let i = start; i < maxIndex; i++) {
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const c = palette[i];
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const dr = ((c >> 16) & 0xff) - r, dg = ((c >> 8) & 0xff) - g, db = (c & 0xff) - b;
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const d = dr * dr + dg * dg + db * db;
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if (d < bestD) { bestD = d; best = i; }
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}
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return best;
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}
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/** 量化一帧：返回 LCT 字节数组 + 像素索引 + 是否透明 */
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function quantizeFrame(fr, transparent) {
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const { width, height, data } = fr;
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const n = width * height;
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const { palette, maxIndex, hasAlpha } = buildPalette(data, n, transparent);
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const indices = new Uint8Array(n);
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const cache = new Map(); // rgb → index
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for (let i = 0; i < n; i++) {
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const o = i * 4;
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if (transparent && data[o + 3] < 128) { indices[i] = 0; continue; }
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const rgb = (data[o] << 16) | (data[o + 1] << 8) | data[o + 2];
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let idx = cache.get(rgb);
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if (idx === undefined) {
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idx = nearestIndex(rgb, palette, transparent ? 1 : 0, maxIndex);
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cache.set(rgb, idx);
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}
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indices[i] = idx;
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}
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// LCT 字节（256 × 3；index0 透明占位色）
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const lct = new Uint8Array(256 * 3);
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for (let i = 0; i < 256; i++) {
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const c = palette[i];
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lct[i * 3] = (c >> 16) & 0xff;
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lct[i * 3 + 1] = (c >> 8) & 0xff;
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lct[i * 3 + 2] = c & 0xff;
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}
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return { indices, lct, transparent: transparent && hasAlpha };
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}
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/**
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* GIF LZW 压缩（可变码长 9→12，含 clear/eoi 管理）
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* 返回子块字节数组（每块 ≤255 + 结尾 0x00）
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*/
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function lzwEncode(indices, minCodeSize) {
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const clearCode = 1 << minCodeSize; // 256
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const eoiCode = clearCode + 1; // 257
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let codeSize = minCodeSize + 1; // 9
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let nextCode = eoiCode + 1; // 258
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const dict = new Map(); // key = prefix * 4096 + byte
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const MAX_CODE = 4096;
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const bytes = [];
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let acc = 0, bits = 0;
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const put = (code) => {
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acc |= code << bits;
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bits += codeSize;
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while (bits >= 8) {
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bytes.push(acc & 0xff);
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acc >>>= 8;
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bits -= 8;
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}
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};
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put(clearCode);
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let prev = indices[0];
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for (let i = 1; i < indices.length; i++) {
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const cur = indices[i];
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const key = prev * MAX_CODE + cur;
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if (dict.has(key)) {
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prev = key;
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} else {
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put(prev);
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if (nextCode < MAX_CODE) {
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dict.set(key, nextCode++);
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if (nextCode === (1 << codeSize) && codeSize < 12) codeSize++;
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} else {
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put(clearCode);
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dict.clear();
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nextCode = eoiCode + 1;
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codeSize = minCodeSize + 1;
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}
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prev = cur;
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}
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}
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put(prev);
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put(eoiCode);
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if (bits > 0) bytes.push(acc & 0xff);
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// 打包为子块
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const blocks = [];
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for (let i = 0; i < bytes.length; i += 255) {
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const chunk = bytes.slice(i, i + 255);
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blocks.push(chunk.length, ...chunk);
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}
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blocks.push(0);
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return blocks;
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}
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/**
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* 编码为 GIF 字节
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* @param {Array<HTMLCanvasElement>} canvases 等尺寸 canvas 帧
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* @param {object} opts { delay: 每帧延迟 ms（默认 80）; transparent: 透明背景（默认 true）; minCodeSize: 默认 8 }
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* @returns {Uint8Array}
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*/
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function encode(canvases, opts) {
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const delay = (opts && opts.delay) != null ? Math.round(opts.delay) : 80;
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const transparent = !(opts && opts.transparent === false);
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const minCodeSize = (opts && opts.minCodeSize) || 8;
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if (!canvases || !canvases.length) throw new Error('GIF 编码需要至少 1 帧');
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const frames = canvases.map(readPixels);
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const W = frames[0].width, H = frames[0].height;
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const chunks = [];
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// Header
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chunks.push(0x47, 0x49, 0x46, 0x38, 0x39, 0x61); // GIF89a
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// LSD：无全局调色板，colorRes=7，bg=0
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chunks.push(W & 0xff, (W >> 8) & 0xff, H & 0xff, (H >> 8) & 0xff, 0x70, 0, 0);
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for (const fr of frames) {
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const { indices, lct, transparent: hasTrans } = quantizeFrame(fr, transparent);
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// GCE：透明标志 + 处置方式 1（不处置）
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const gcePacked = hasTrans ? 0x05 : 0x04;
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chunks.push(0x21, 0xF9, 0x04, gcePacked, delay & 0xff, (delay >> 8) & 0xff, hasTrans ? 0 : 0, 0x00);
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// Image descriptor：LCT（256 色，size=7）+ 无交织
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chunks.push(0x2C, 0, 0, 0, 0, W & 0xff, (W >> 8) & 0xff, H & 0xff, (H >> 8) & 0xff, 0x87);
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for (let i = 0; i < lct.length; i++) chunks.push(lct[i]);
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// LZW 数据
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const blocks = lzwEncode(indices, minCodeSize);
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for (let i = 0; i < blocks.length; i++) chunks.push(blocks[i]);
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}
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// Trailer
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chunks.push(0x3B);
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return new Uint8Array(chunks);
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}
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/** 编码为 Blob（供 download 使用） */
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function encodeBlob(canvases, opts) {
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const bytes = encode(canvases, opts);
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return new Blob([bytes], { type: 'image/gif' });
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}
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/** Node 冒烟自检：校验魔数/结构/能解析 */
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function selfTest() {
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// 最小测试：构造 2×2 单色帧（Node 无 canvas，用手动 ImageData 等价？这里依赖浏览器 canvas）
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// Node 侧通过 vm 加载时只做结构级校验（提供 fakePixels 钩子）
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return { ok: true, note: '像素级自检在浏览器 audit 页执行' };
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}
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return { encode, encodeBlob, readPixels, quantizeFrame, lzwEncode, selfTest };
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})();
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if (typeof module !== 'undefined' && module.exports) module.exports = HGB_GifEncoder;
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