png-chunk-itxt/src/index.ts

Create or parse a PNG iTXt chunk that stores unicode text data in PNG images

import zlip from "zlib";
import { Parser } from "binary-parser";

export interface ITxtChunk {
  keyword: string;
  compressionFlag: boolean;
  compressionMethod: number;
  languageTag: string;
  translatedKeyword: string;
  text: string;
}

// http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.iTXt
const iTxtChunkParser = new Parser()
  .string("keyword", { zeroTerminated: true })
  .uint8("compressionFlag")
  .uint8("compressionMethod")
  .string("languageTag", { zeroTerminated: true })
  .string("translatedKeyword", { zeroTerminated: true })
  .buffer("text", { readUntil: "eof" });

export function decodeSync(data: Uint8Array): ITxtChunk {
  const vars = iTxtChunkParser.parse(data);

  const uncompressedTextBuffer = Boolean(vars.compressionFlag)
    ? zlip.inflateSync(vars.text)
    : vars.text;

  return {
    keyword: vars.keyword,
    compressionFlag: Boolean(vars.compressionFlag),
    compressionMethod: vars.compressionMethod,
    languageTag: vars.languageTag,
    translatedKeyword: vars.translatedKeyword,
    text: new TextDecoder().decode(uncompressedTextBuffer)
  };
}

export function encodeSync(chunk: ITxtChunk): Uint8Array {
  const encoder = new TextEncoder();
  const encodedText = encoder.encode(chunk.text);

  const compressedTextBuffer = chunk.compressionFlag
    ? zlip.deflateSync(encodedText)
    : encodedText;

  return Buffer.concat([
    encoder.encode(chunk.keyword),
    Buffer.from([0]),
    Buffer.from([Number(chunk.compressionFlag)]),
    Buffer.from([Number(chunk.compressionMethod)]),
    encoder.encode(chunk.languageTag),
    Buffer.from([0]),
    encoder.encode(chunk.translatedKeyword),
    Buffer.from([0]),
    compressedTextBuffer
  ]);
}