Embed text into a PNG file (by draw.io project)

writePngWithText.js

// Source: https://github.com/jgraph/drawio/blob/v14.9.9/src/main/webapp/electron.js#L970

//NOTE: Key length must not be longer than 79 bytes (not checked)
function writePngWithText(origBuff, key, text, compressed, base64encoded)
{
	var isDpi = key == 'dpi';
	var inOffset = 0;
	var outOffset = 0;
	var data = text;
	var dataLen = isDpi? 9 : key.length + data.length + 1; //we add 1 zeros with non-compressed data, for pHYs it's 2 of 4-byte-int + 1 byte
	
	//prepare compressed data to get its size
	if (compressed)
	{
		data = zlib.deflateRawSync(encodeURIComponent(text));
		dataLen = key.length + data.length + 2; //we add 2 zeros with compressed data
	}
	
	var outBuff = Buffer.allocUnsafe(origBuff.length + dataLen + 4); //4 is the header size "zTXt", "tEXt" or "pHYs"
	
	try
	{
		var magic1 = origBuff.readUInt32BE(inOffset);
		inOffset += 4;
		var magic2 = origBuff.readUInt32BE(inOffset);
		inOffset += 4;
		
		if (magic1 != 0x89504e47 && magic2 != 0x0d0a1a0a)
		{
			throw new Error("PNGImageDecoder0");
		}
		
		outBuff.writeUInt32BE(magic1, outOffset);
		outOffset += 4;
		outBuff.writeUInt32BE(magic2, outOffset);
		outOffset += 4;
	}
	catch (e)
	{
		log.error(e.message, {stack: e.stack});
		throw new Error("PNGImageDecoder1");
	}

	try
	{
		while (inOffset < origBuff.length)
		{
			var length = origBuff.readInt32BE(inOffset);
			inOffset += 4;
			var type = origBuff.readInt32BE(inOffset)
			inOffset += 4;

			if (type == PNG_CHUNK_IDAT)
			{
				// Insert zTXt chunk before IDAT chunk
				outBuff.writeInt32BE(dataLen, outOffset);
				outOffset += 4;
				
				var typeSignature = isDpi? 'pHYs' : (compressed ? "zTXt" : "tEXt");
				outBuff.write(typeSignature, outOffset);
				
				outOffset += 4;

				if (isDpi)
				{
					var dpm = Math.round(parseInt(text) / 0.0254) || 3937; //One inch is equal to exactly 0.0254 meters. 3937 is 100dpi

					outBuff.writeInt32BE(dpm, outOffset);
					outBuff.writeInt32BE(dpm, outOffset + 4);
					outBuff.writeInt8(1, outOffset + 8);
					outOffset += 9;

					data = Buffer.allocUnsafe(9);
					data.writeInt32BE(dpm, 0);
					data.writeInt32BE(dpm, 4);
					data.writeInt8(1, 8);
				}
				else
				{
					outBuff.write(key, outOffset);
					outOffset += key.length;
					outBuff.writeInt8(0, outOffset);
					outOffset ++;

					if (compressed)
					{
						outBuff.writeInt8(0, outOffset);
						outOffset ++;
						data.copy(outBuff, outOffset);
					}
					else
					{
						outBuff.write(data, outOffset);	
					}

					outOffset += data.length;				
				}

				var crcVal = 0xffffffff;
				crcVal = crc.crcjam(typeSignature, crcVal);
				crcVal = crc.crcjam(data, crcVal);

				// CRC
				outBuff.writeInt32BE(crcVal ^ 0xffffffff, outOffset);
				outOffset += 4;

				// Writes the IDAT chunk after the zTXt
				outBuff.writeInt32BE(length, outOffset);
				outOffset += 4;
				outBuff.writeInt32BE(type, outOffset);
				outOffset += 4;

				origBuff.copy(outBuff, outOffset, inOffset);

				// Encodes the buffer using base64 if requested
				return base64encoded? outBuff.toString('base64') : outBuff;
			}

			outBuff.writeInt32BE(length, outOffset);
			outOffset += 4;
			outBuff.writeInt32BE(type, outOffset);
			outOffset += 4;

			origBuff.copy(outBuff, outOffset, inOffset, inOffset + length + 4);// +4 to move past the crc
			
			inOffset += length + 4;
			outOffset += length + 4;
		}
	}
	catch (e)
	{
		log.error(e.message, {stack: e.stack});
		throw e;
	}
}