index.js

/* adapted from https://github.com/hughsk/png-chunks-extract (MIT) and https://github.com/SheetJS/js-crc32 (Apache 2.0) (C) 2014-present SheetJS -- http://sheetjs.com */
window.rewrite_pHYs_chunk = function (data, ppmx, ppmy) {
	
	/* crc32.js (C) 2014-present SheetJS -- http://sheetjs.com */
/* vim: set ts=2: */
/*exported CRC32 */
var CRC32;
(function (factory) {
	/*jshint ignore:start */
	if(typeof DO_NOT_EXPORT_CRC === 'undefined') {
		if('object' === typeof exports) {
			factory(exports);
		} else if ('function' === typeof define && define.amd) {
			define(function () {
				var module = {};
				factory(module);
				return module;
			});
		} else {
			factory(CRC32 = {});
		}
	} else {
		factory(CRC32 = {});
	}
	/*jshint ignore:end */
}(function(CRC32) {
CRC32.version = '1.1.1';
/* see perf/crc32table.js */
/*global Int32Array */
function signed_crc_table() {
	var c = 0, table = new Array(256);

	for(var n =0; n != 256; ++n){
		c = n;
		c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
		c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
		c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
		c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
		c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
		c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
		c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
		c = ((c&1) ? (-306674912 ^ (c >>> 1)) : (c >>> 1));
		table[n] = c;
	}

	return typeof Int32Array !== 'undefined' ? new Int32Array(table) : table;
}

var T = signed_crc_table();
function crc32_bstr(bstr, seed) {
	var C = seed ^ -1, L = bstr.length - 1;
	for(var i = 0; i < L;) {
		C = (C>>>8) ^ T[(C^bstr.charCodeAt(i++))&0xFF];
		C = (C>>>8) ^ T[(C^bstr.charCodeAt(i++))&0xFF];
	}
	if(i === L) C = (C>>>8) ^ T[(C ^ bstr.charCodeAt(i))&0xFF];
	return C ^ -1;
}

function crc32_buf(buf, seed) {
	if(buf.length > 10000) return crc32_buf_8(buf, seed);
	var C = seed ^ -1, L = buf.length - 3;
	for(var i = 0; i < L;) {
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
	}
	while(i < L+3) C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
	return C ^ -1;
}

function crc32_buf_8(buf, seed) {
	var C = seed ^ -1, L = buf.length - 7;
	for(var i = 0; i < L;) {
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
		C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
	}
	while(i < L+7) C = (C>>>8) ^ T[(C^buf[i++])&0xFF];
	return C ^ -1;
}

function crc32_str(str, seed) {
	var C = seed ^ -1;
	for(var i = 0, L=str.length, c, d; i < L;) {
		c = str.charCodeAt(i++);
		if(c < 0x80) {
			C = (C>>>8) ^ T[(C ^ c)&0xFF];
		} else if(c < 0x800) {
			C = (C>>>8) ^ T[(C ^ (192|((c>>6)&31)))&0xFF];
			C = (C>>>8) ^ T[(C ^ (128|(c&63)))&0xFF];
		} else if(c >= 0xD800 && c < 0xE000) {
			c = (c&1023)+64; d = str.charCodeAt(i++)&1023;
			C = (C>>>8) ^ T[(C ^ (240|((c>>8)&7)))&0xFF];
			C = (C>>>8) ^ T[(C ^ (128|((c>>2)&63)))&0xFF];
			C = (C>>>8) ^ T[(C ^ (128|((d>>6)&15)|((c&3)<<4)))&0xFF];
			C = (C>>>8) ^ T[(C ^ (128|(d&63)))&0xFF];
		} else {
			C = (C>>>8) ^ T[(C ^ (224|((c>>12)&15)))&0xFF];
			C = (C>>>8) ^ T[(C ^ (128|((c>>6)&63)))&0xFF];
			C = (C>>>8) ^ T[(C ^ (128|(c&63)))&0xFF];
		}
	}
	return C ^ -1;
}
CRC32.table = T;
CRC32.bstr = crc32_bstr;
CRC32.buf = crc32_buf;
CRC32.str = crc32_str;
}));

// Used for fast-ish conversion between uint8s and uint32s/int32s.
// Also required in order to remain agnostic for both Node Buffers and
// Uint8Arrays.
	var uint8 = new Uint8Array(4);
	var int32 = new Int32Array(uint8.buffer);
	var uint32 = new Uint32Array(uint8.buffer);
	
	var pHYsFound = false;

	if (data[0] !== 0x89 || data[1] !== 0x50 || data[2] !== 0x4E || data[3] !== 0x47 || data[4] !== 0x0D || data[5] !== 0x0A || data[6] !== 0x1A || data[7] !== 0x0A) {
		throw new Error('Invalid .png file header: possibly caused by DOS-Unix line ending conversion?');
	}

  var ended = false
  var idx = 8

  while (idx < data.length) {
    // Read the length of the current chunk,
    // which is stored as a Uint32.
    uint8[3] = data[idx++]
    uint8[2] = data[idx++]
    uint8[1] = data[idx++]
    uint8[0] = data[idx++]

    // Chunk includes name/type for CRC check (see below).
    var length = uint32[0] + 4
    var chunk = new Uint8Array(length)
    chunk[0] = data[idx++]
    chunk[1] = data[idx++]
    chunk[2] = data[idx++]
    chunk[3] = data[idx++]

    // Get the name in ASCII for identification.
    var name = (
      String.fromCharCode(chunk[0]) +
      String.fromCharCode(chunk[1]) +
      String.fromCharCode(chunk[2]) +
      String.fromCharCode(chunk[3])
    );
	
	console.log("chunk found " + name + ", length = " + (length-4));


	var chunkDataStart = idx;

    // Read the contents of the chunk out of the main buffer.
    for (var i = 4; i < length; i++) {
      chunk[i] = data[idx++];
    }

	var crcStart = idx;
    // Read out the CRC value for comparison.
    // It's stored as an Int32.
    uint8[3] = data[idx++];
    uint8[2] = data[idx++];
    uint8[1] = data[idx++];
    uint8[0] = data[idx++];

    var crcActual = int32[0];
    var crcExpect = CRC32.buf(chunk);
    if (crcExpect !== crcActual) {
      throw new Error(
        'CRC values for ' + name + ' header do not match, PNG file is likely corrupted'
      )
    } else {
	console.log("CRCs match! " + crcExpect + " " + crcActual + " " + chunk.length);
	}
	
	if (name == "IDAT") {
		
		chunkDataStart = chunkDataStart - 8;
		
		var len = data.length;
		
		// create new array with pHYs chunk inserted
		// 4+4+13
		var data2 = new Uint8Array(len + 21);
		
		// copy before IEND
		for (var i = 0 ; i < chunkDataStart ; i++) {
			data2[i] = data[i];
		}
		// copy IEND to end
		for (var i = chunkDataStart ; i < len ; i++) {
			data2[i+21] = data[i];
		}
		
		var phys = new Uint8Array(13);
		var i = 0;
		
		// length of pHYs chunk
		int32[0] = 9;
		data2[chunkDataStart++] = uint8[3];
		data2[chunkDataStart++] = uint8[2];
		data2[chunkDataStart++] = uint8[1];
		data2[chunkDataStart++] = uint8[0];
		
		// pHYs (chunk name)
		phys[i++] = data2[chunkDataStart++] = 'p'.charCodeAt(0);
		phys[i++] = data2[chunkDataStart++] = 'H'.charCodeAt(0);
		phys[i++] = data2[chunkDataStart++] = 'Y'.charCodeAt(0);
		phys[i++] = data2[chunkDataStart++] = 's'.charCodeAt(0);
		
		
		// x
		uint32[0] = ppmx;
		phys[i++] = data2[chunkDataStart++] = uint8[3];
		phys[i++] = data2[chunkDataStart++] = uint8[2];
		phys[i++] = data2[chunkDataStart++] = uint8[1];
		phys[i++] = data2[chunkDataStart++] = uint8[0];
		// y 
		uint32[0] = ppmy;
		phys[i++] = data2[chunkDataStart++] = uint8[3];
		phys[i++] = data2[chunkDataStart++] = uint8[2];
		phys[i++] = data2[chunkDataStart++] = uint8[1];
		phys[i++] = data2[chunkDataStart++] = uint8[0];
		// unit = meters
		phys[i++] = data2[chunkDataStart++] = 1;
		
		var physCRC = CRC32.buf(phys);
		int32[0] = physCRC;
		
		data2[chunkDataStart++] = uint8[3];
		data2[chunkDataStart++] = uint8[2];
		data2[chunkDataStart++] = uint8[1];
		data2[chunkDataStart++] = uint8[0];

		return data2;
		
		
	}
	
	if (name == "pHYs") {
		console.log("pHYs chunk found, rewriting!!!!!!!!!!!!!");
		
		var phys = new Uint8Array(13);
		var i = 0;
		
		
		// pHYs (chunk name)
		phys[i++] = 'p'.charCodeAt(0);
		phys[i++] = 'H'.charCodeAt(0);
		phys[i++] = 'Y'.charCodeAt(0);
		phys[i++] = 's'.charCodeAt(0);
		// x
		uint32[0] = ppmx;
		phys[i++] = data[chunkDataStart++] = uint8[3];
		phys[i++] = data[chunkDataStart++] = uint8[2];
		phys[i++] = data[chunkDataStart++] = uint8[1];
		phys[i++] = data[chunkDataStart++] = uint8[0];
		// y 
		uint32[0] = ppmy;
		phys[i++] = data[chunkDataStart++] = uint8[3];
		phys[i++] = data[chunkDataStart++] = uint8[2];
		phys[i++] = data[chunkDataStart++] = uint8[1];
		phys[i++] = data[chunkDataStart++] = uint8[0];
		// unit = meters
		phys[i++] = data[chunkDataStart++] = 1;
		
		var physCRC = CRC32.buf(phys);
		int32[0] = physCRC;
		
		data[crcStart++] = uint8[3];
		data[crcStart++] = uint8[2];
		data[crcStart++] = uint8[1];
		data[crcStart++] = uint8[0];
		
		return data;
	}

  }

  throw new Error('.png file ended prematurely: no IEND or pHYs header was found');

}