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pure JavaScript jpeg quality value recover
// javascript jpeg decoder: https://github.com/eugeneware/jpeg-js/blob/79c4c7ea876d2e7b46981a563fcc4ae8f9e19849/lib/decoder.js
// Determine the JPEG compression quality from the quantization tables: https://github.com/ImageMagick/ImageMagick/blob/59f28acff016f6ae5e8b7e6300bcdf97e94affc9/coders/jpeg.c#L879
// JPEG File Layout and Format: http://vip.sugovica.hu/Sardi/kepnezo/JPEG%20File%20Layout%20and%20Format.htm
// jpeg constants: https://github.com/LuaDist/libjpeg/blob/6c0fcb8ddee365e7abc4d332662b06900612e923/jpeglib.h#L45
function getQuantizationTables(data) {
var quantizationTables = []
var dctZigZag = new Int32Array([
0,
1, 8,
16, 9, 2,
3, 10, 17, 24,
32, 25, 18, 11, 4,
5, 12, 19, 26, 33, 40,
48, 41, 34, 27, 20, 13, 6,
7, 14, 21, 28, 35, 42, 49, 56,
57, 50, 43, 36, 29, 22, 15,
23, 30, 37, 44, 51, 58,
59, 52, 45, 38, 31,
39, 46, 53, 60,
61, 54, 47,
55, 62,
63
]);
var offset = 0, length = data.length;
function readUint16() {
var value = (data[offset] << 8) | data[offset + 1];
offset += 2;
return value;
}
var fileMarker = readUint16();
if (fileMarker != 0xFFD8) { // SOI (Start of Image)
throw new Error("SOI not found");
}
fileMarker = readUint16();
markerLoop:
while (fileMarker != 0xFFD9) { // EOI (End of image)
var i, j, l;
switch(fileMarker) {
case 0xFF00: break;
case 0xFFE0: // APP0 (Application Specific)
case 0xFFE1: // APP1
case 0xFFE2: // APP2
case 0xFFE3: // APP3
case 0xFFE4: // APP4
case 0xFFE5: // APP5
case 0xFFE6: // APP6
case 0xFFE7: // APP7
case 0xFFE8: // APP8
case 0xFFE9: // APP9
case 0xFFEA: // APP10
case 0xFFEB: // APP11
case 0xFFEC: // APP12
case 0xFFED: // APP13
case 0xFFEE: // APP14
case 0xFFEF: // APP15
case 0xFFFE: // COM (Comment)
// ---
case 0xFFC0: // SOF0 (Start of Frame, Baseline DCT)
case 0xFFC1: // SOF1 (Start of Frame, Extended DCT)
case 0xFFC2: // SOF2 (Start of Frame, Progressive DCT)
case 0xFFC4: // DHT (Define Huffman Tables)
case 0xFFDD: // DRI (Define Restart Interval)
offset += readUint16(); // skip
break;
case 0xFFDB: // DQT (Define Quantization Tables)
var quantizationTablesLength = readUint16();
var quantizationTablesEnd = quantizationTablesLength + offset - 2;
while (offset < quantizationTablesEnd) {
var quantizationTableSpec = data[offset++];
var tableData = new Int32Array(64);
if ((quantizationTableSpec >> 4) === 0) { // 8 bit values
for (j = 0; j < 64; j++) {
var z = dctZigZag[j];
tableData[z] = data[offset++];
}
} else if ((quantizationTableSpec >> 4) === 1) { //16 bit
for (j = 0; j < 64; j++) {
var z = dctZigZag[j];
tableData[z] = readUint16();
}
} else
throw new Error("DQT: invalid table spec");
quantizationTables[quantizationTableSpec & 15] = tableData;
}
break;
case 0xFFDA: // SOS (Start of Scan)
break markerLoop;
case 0xFFFF: // Fill bytes
if (data[offset] !== 0xFF) { // Avoid skipping a valid marker.
offset--;
}
break;
default:
if (data[offset - 3] == 0xFF &&
data[offset - 2] >= 0xC0 && data[offset - 2] <= 0xFE) {
// could be incorrect encoding -- last 0xFF byte of the previous
// block was eaten by the encoder
offset -= 3;
break;
}
throw new Error("unknown JPEG marker " + fileMarker.toString(16));
}
fileMarker = readUint16();
}
return quantizationTables;
}
const DCTSIZE2 = 64
const NUM_QUANT_TBLS = 4
const hash_q01 = [ 1020, 1015, 932, 848, 780, 735, 702, 679, 660, 645, 632, 623, 613, 607, 600, 594, 589, 585, 581, 571, 555, 542, 529, 514, 494, 474, 457, 439, 424, 410, 397, 386, 373, 364, 351, 341, 334, 324, 317, 309, 299, 294, 287, 279, 274, 267, 262, 257, 251, 247, 243, 237, 232, 227, 222, 217, 213, 207, 202, 198, 192, 188, 183, 177, 173, 168, 163, 157, 153, 148, 143, 139, 132, 128, 125, 119, 115, 108, 104, 99, 94, 90, 84, 79, 74, 70, 64, 59, 55, 49, 45, 40, 34, 30, 25, 20, 15, 11, 6, 4, 0 ];
const sums_q01 = [ 32640, 32635, 32266, 31495, 30665, 29804, 29146, 28599, 28104, 27670, 27225, 26725, 26210, 25716, 25240, 24789, 24373, 23946, 23572, 22846, 21801, 20842, 19949, 19121, 18386, 17651, 16998, 16349, 15800, 15247, 14783, 14321, 13859, 13535, 13081, 12702, 12423, 12056, 11779, 11513, 11135, 10955, 10676, 10392, 10208, 9928, 9747, 9564, 9369, 9193, 9017, 8822, 8639, 8458, 8270, 8084, 7896, 7710, 7527, 7347, 7156, 6977, 6788, 6607, 6422, 6236, 6054, 5867, 5684, 5495, 5305, 5128, 4945, 4751, 4638, 4442, 4248, 4065, 3888, 3698, 3509, 3326, 3139, 2957, 2775, 2586, 2405, 2216, 2037, 1846, 1666, 1483, 1297, 1109, 927, 735, 554, 375, 201, 128, 0 ];
const hash_q0 = [ 510, 505, 422, 380, 355, 338, 326, 318, 311, 305, 300, 297, 293, 291, 288, 286, 284, 283, 281, 280, 279, 278, 277, 273, 262, 251, 243, 233, 225, 218, 211, 205, 198, 193, 186, 181, 177, 172, 168, 164, 158, 156, 152, 148, 145, 142, 139, 136, 133, 131, 129, 126, 123, 120, 118, 115, 113, 110, 107, 105, 102, 100, 97, 94, 92, 89, 87, 83, 81, 79, 76, 74, 70, 68, 66, 63, 61, 57, 55, 52, 50, 48, 44, 42, 39, 37, 34, 31, 29, 26, 24, 21, 18, 16, 13, 11, 8, 6, 3, 2, 0 ];
const sums_q0 = [ 16320, 16315, 15946, 15277, 14655, 14073, 13623, 13230, 12859, 12560, 12240, 11861, 11456, 11081, 10714, 10360, 10027, 9679, 9368, 9056, 8680, 8331, 7995, 7668, 7376, 7084, 6823, 6562, 6345, 6125, 5939, 5756, 5571, 5421, 5240, 5086, 4976, 4829, 4719, 4616, 4463, 4393, 4280, 4166, 4092, 3980, 3909, 3835, 3755, 3688, 3621, 3541, 3467, 3396, 3323, 3247, 3170, 3096, 3021, 2952, 2874, 2804, 2727, 2657, 2583, 2509, 2437, 2362, 2290, 2211, 2136, 2068, 1996, 1915, 1858, 1773, 1692, 1620, 1552, 1477, 1398, 1326, 1251, 1179, 1109, 1031, 961, 884, 814, 736, 667, 592, 518, 441, 369, 292, 221, 151, 86, 64, 0 ]
function getQualityFromQuantizationTables(quantizationTables) {
var i, j;
var sum = 0;
for ( i=0; i < NUM_QUANT_TBLS; i++ ) {
if ( quantizationTables[i] )
for ( j=0; j < DCTSIZE2; j++ )
sum += quantizationTables[i][j];
}
if ( quantizationTables[0] && quantizationTables[1] ) {
var qvalue = quantizationTables[0][2] + quantizationTables[0][53] + quantizationTables[1][0] + quantizationTables[1][DCTSIZE2-1];
for ( i=0; i < 100; i++ ) {
if ( (qvalue < hash_q01[i]) && (sum < sums_q01[i]) )
continue;
if ( ((qvalue <= hash_q01[i]) && (sum <= sums_q01[i])) || (i >= 50) )
return i+1;
break;
}
} else
if ( quantizationTables[0] ) {
var qvalue = quantizationTables[0][2] + quantizationTables[0][53];
for ( i=0; i < 100; i++ ) {
if ((qvalue < hash_q0[i]) && (sum < sums_q0[i]))
continue;
if (((qvalue <= hash_q0[i]) && (sum <= sums_q0[i])) || (i >= 50))
return i+1;
break;
}
}
return undefined;
}
module.exports = function getJpegQuality(jpegData) {
// the Buffer class implements the Uint8Array API.
return getQualityFromQuantizationTables(getQuantizationTables(jpegData));
}
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