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JPEG file format

Published on February 20, 2012

This articles provides information on how a JPEG file is constructed at binary level. It is accompanied with this article: JPEG scan data. The conversion from a raw bitmap to JPEG is implemented in this tool: JPEG

General structure

The JPEG file format uses markers to denote the beginning of a certain block. These markers are 2 bytes long and start with 0xFF followed by the marker for the following block. This consequently means that if we were to write a 0xFF that does not denote the beginning of a block, we must escape it by appending 0x00 right after the 0xFF. This means that no block is following and only the 0xFF is interpreted as being part of the data.

Markers and segments

The following segments are discussed in the order they’d appear in a basic JPEG file. Some are optional segments and some optional segments are left out of the discussion. If you were to decode a JPEG file, you can often skip those optional segments.

Start Of Image

0xFF 0xD8 Marker

APP0

0xFF 0xE0 Marker 0x?? 0x?? Segment length is the length of the segment excluding the marker 0x4A46494600 Reads JFIF 0x01 0x02 Version 1.2 0x00 No density units used (defaults to 72 DPI I believe) 0x00 0x01 Scaling of 100% in the X direction (no scaling) 0x00 0x01 Scaling of 100% in the Y direction (no scaling) 0x00 0x00 Width and height of thumbnail are zero

Quantization table

Usually two tables are provided, one for the luminance and one for the chrominance. See bottom of the page for the default quantization tables. The elements must be provided in the serpentine / zigzag order.

0xFF 0xDB Marker 0x00 0x43 Segment length is the length of the segment excluding the marker (3 + 64) 0x0? Insignificant four bits denote the table ID, significant four the precision of its elements (set to 0 for default 8bit per element) ... 64 values of the quantization table

Start Of Frame

0xFF 0xC0 Marker 0x00 0x11 Segment length is the length of the segment excluding the marker (17 for us) 0x08 Precision of 8bits 0x?? 0x?? Height of image in 2 bytes 0x?? 0x?? Width of image in 2 bytes 0x03 Three color components 0x01 0x11 0x?? Component 1 (luminance), 1x1 sample (no subsampling), quantization table ID 0x02 0x11 0x?? Component 2 (chrominance blue) 0x03 0x11 0x?? Component 3 (chrominance red)

Huffman table

Usually four tables are provided, two for the luminance and two for the chrominance. Each is split into tables for DC and AC values. See bottom of the page for the default Huffman tables and see JPEG scan data for an explanation to use it.

0xFF 0xC4 Marker 0x?? 0x?? Segment length is the length of the segment excluding the marker (3 + 16 + X) 0x?? Insignificant four bits denote the table ID, the 5th bit is true for AC and false for DC ... 16 values for the bit lengths ... X values for the codes

Start of Scan

Multiple scans may exist for interlacing. We use only one.

0xFF 0xDA Marker 0x00 0x0C Segment length is the length of the segment excluding the marker and scan data (12) 0x03 Three color components 0x01 0x?? Component 1 (luminance), huffman table ID 0x02 0x?? Component 2 (chrominance blue) 0x03 0x?? Component 3 (chrominance red) 0x00 0x3F 0x00 Skipping three bytes (no idea what the 0x3F is for…) ... Entropy-encoded scan data, see JPEG scan data

End Of Image

0xFF 0xD9 Marker

Default quantization tables

/* Luminance */
const uint8_t Qy_[] = {
    16,  11,  10,  16,  24,  40,  51,  61,
    12,  12,  14,  19,  26,  58,  60,  55,
    14,  13,  16,  24,  40,  57,  69,  56,
    14,  17,  22,  29,  51,  87,  80,  62,
    18,  22,  37,  56,  68, 109, 103,  77,
    24,  35,  55,  64,  81, 104, 113,  92,
    49,  64,  78,  87, 103, 121, 120, 101,
    72,  92,  95,  98, 112, 100, 103,  99
};

/* Chrominance */
const uint8_t Qc_[] = {
    17, 18, 24, 47, 99, 99, 99, 99,
    18, 21, 26, 66, 99, 99, 99, 99,
    24, 26, 56, 99, 99, 99, 99, 99,
    47, 66, 99, 99, 99, 99, 99, 99,
    99, 99, 99, 99, 99, 99, 99, 99,
    99, 99, 99, 99, 99, 99, 99, 99,
    99, 99, 99, 99, 99, 99, 99, 99,
    99, 99, 99, 99, 99, 99, 99, 99
};

Default Huffman tables

/* Luminance DC */
const uint8_t Hy_dc_bits[] = {
    0, 1, 5, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0
};

const uint8_t Hy_dc_val[] = {
    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11
};

/* Luminance AC */
const uint8_t Hy_ac_bits[] = {
    0, 2, 1, 3, 3, 2, 4, 3, 5, 5, 4, 4, 0, 0, 1, 0x7d
};

const uint8_t Hy_ac_val[] = {
    0x01, 0x02, 0x03, 0x00, 0x04, 0x11, 0x05, 0x12,
    0x21, 0x31, 0x41, 0x06, 0x13, 0x51, 0x61, 0x07,
    0x22, 0x71, 0x14, 0x32, 0x81, 0x91, 0xa1, 0x08,
    0x23, 0x42, 0xb1, 0xc1, 0x15, 0x52, 0xd1, 0xf0,
    0x24, 0x33, 0x62, 0x72, 0x82, 0x09, 0x0a, 0x16,
    0x17, 0x18, 0x19, 0x1a, 0x25, 0x26, 0x27, 0x28,
    0x29, 0x2a, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39,
    0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49,
    0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59,
    0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69,
    0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79,
    0x7a, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89,
    0x8a, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98,
    0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
    0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6,
    0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3, 0xc4, 0xc5,
    0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2, 0xd3, 0xd4,
    0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda, 0xe1, 0xe2,
    0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9, 0xea,
    0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
    0xf9, 0xfa
};

/* Chrominance DC */
const uint8_t Hc_dc_bits[] = {
    0, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0
};

const uint8_t Hc_dc_val[] = {
    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11
};

/* Chrominance AC */
const uint8_t Hc_ac_bits[] = {
    0, 2, 1, 2, 4, 4, 3, 4, 7, 5, 4, 4, 0, 1, 2, 0x77
};

const uint8_t Hc_ac_val[] = {
    0x00, 0x01, 0x02, 0x03, 0x11, 0x04, 0x05, 0x21,
    0x31, 0x06, 0x12, 0x41, 0x51, 0x07, 0x61, 0x71,
    0x13, 0x22, 0x32, 0x81, 0x08, 0x14, 0x42, 0x91,
    0xa1, 0xb1, 0xc1, 0x09, 0x23, 0x33, 0x52, 0xf0,
    0x15, 0x62, 0x72, 0xd1, 0x0a, 0x16, 0x24, 0x34,
    0xe1, 0x25, 0xf1, 0x17, 0x18, 0x19, 0x1a, 0x26,
    0x27, 0x28, 0x29, 0x2a, 0x35, 0x36, 0x37, 0x38,
    0x39, 0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48,
    0x49, 0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58,
    0x59, 0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68,
    0x69, 0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78,
    0x79, 0x7a, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
    0x88, 0x89, 0x8a, 0x92, 0x93, 0x94, 0x95, 0x96,
    0x97, 0x98, 0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5,
    0xa6, 0xa7, 0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4,
    0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3,
    0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2,
    0xd3, 0xd4, 0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda,
    0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9,
    0xea, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
    0xf9, 0xfa
};

More information: http://class.ee.iastate.edu/ee528/Reading%20material/JPEG_File_Format.pdf