300x250
Start of Image (SOI) marker -- two bytes (FFD8)
JFIF marker (FFE0)
length -- two bytes
identifier -- five bytes: 4A, 46, 49, 46, 00 (the ASCII code equivalent of a zero terminated "JFIF" string)
version -- two bytes: often 01, 02
the most significant byte is used for major revisions
the least significant byte for minor revisions
units -- one byte: Units for the X and Y densities
0 => no units, X and Y specify the pixel aspect ratio
1 => X and Y are dots per inch
2 => X and Y are dots per cm
Xdensity -- two bytes
Ydensity -- two bytes
Xthumbnail -- one byte: 0 = no thumbnail
Ythumbnail -- one byte: 0 = no thumbnail
(RGB)n -- 3n bytes: packed (24-bit) RGB values for the thumbnail pixels, n = Xthumbnail * Ythumbnail
Define Quantization table marker (FFDB)
the first two bytes, the length, after the marker indicate the number of bytes, including the two length bytes, that this header contains
until the length is exhausted (loads two quantization tables for baseline JPEG)
the precision and the quantization table index -- one byte: precision is specified by the higher four bits and index is specified by the lower four bits
precision in this case is either 0 or 1 and indicates the precision of the quantized values; 8-bit (baseline) for 0 and up to 16-bit for 1
the quantization values -- 64 bytes
the quantization tables are stored in zigzag format
Define Huffman table marker (FFC4)
the first two bytes, the length, after the marker indicate the number of bytes, including the two length bytes, that this header contains
until length is exhausted (usually four Huffman tables)
index -- one byte: if >15 (i.e. 0x10 or more) then an AC table, otherwise a DC table
bits -- 16 bytes
Huffman values -- # of bytes = the sum of the previous 16 bytes
Start of frame marker (FFC0)
the first two bytes, the length, after the marker indicate the number of bytes, including the two length bytes, that this header contains
P -- one byte: sample precision in bits (usually 8, for baseline JPEG)
Y -- two bytes
X -- two bytes
Nf -- one byte: the number of components in the image
3 for color baseline JPEG images
1 for grayscale baseline JPEG images
Nf times:
Component ID -- one byte
H and V sampling factors -- one byte: H is first four bits and V is second four bits
Quantization table number-- one byte
The H and V sampling factors dictate the final size of the component they are associated with. For instance, the color space defaults to YCbCr and the H and V sampling factors for each component, Y, Cb, and Cr, default to 2, 1, and 1, respectively (2 for both H and V of the Y component, etc.) in the Jpeg-6a library by the Independent Jpeg Group. While this does mean that the Y component will be twice the size of the other two components--giving it a higher resolution, the lower resolution components are quartered in size during compression in order to achieve this difference. Thus, the Cb and Cr components must be quadrupled in size during decompression.
Start of Scan marker (FFDA)
the first two bytes, the length, after the marker indicate the number of bytes, including the two length bytes, that this header contains
Number of components, n -- one byte: the number of components in this scan
n times:
Component ID -- one byte
DC and AC table numbers -- one byte: DC # is first four bits and AC # is last four bits
Ss -- one byte
Se -- one byte
Ah and Al -- one byte
Comment marker (FFFE)
the first two bytes, the length, after the marker indicate the number of bytes, including the two length bytes, that this header contains
whatever the user wants
End of Image (EOI) marker (FFD9
JFIF marker (FFE0)
length -- two bytes
identifier -- five bytes: 4A, 46, 49, 46, 00 (the ASCII code equivalent of a zero terminated "JFIF" string)
version -- two bytes: often 01, 02
the most significant byte is used for major revisions
the least significant byte for minor revisions
units -- one byte: Units for the X and Y densities
0 => no units, X and Y specify the pixel aspect ratio
1 => X and Y are dots per inch
2 => X and Y are dots per cm
Xdensity -- two bytes
Ydensity -- two bytes
Xthumbnail -- one byte: 0 = no thumbnail
Ythumbnail -- one byte: 0 = no thumbnail
(RGB)n -- 3n bytes: packed (24-bit) RGB values for the thumbnail pixels, n = Xthumbnail * Ythumbnail
Define Quantization table marker (FFDB)
the first two bytes, the length, after the marker indicate the number of bytes, including the two length bytes, that this header contains
until the length is exhausted (loads two quantization tables for baseline JPEG)
the precision and the quantization table index -- one byte: precision is specified by the higher four bits and index is specified by the lower four bits
precision in this case is either 0 or 1 and indicates the precision of the quantized values; 8-bit (baseline) for 0 and up to 16-bit for 1
the quantization values -- 64 bytes
the quantization tables are stored in zigzag format
Define Huffman table marker (FFC4)
the first two bytes, the length, after the marker indicate the number of bytes, including the two length bytes, that this header contains
until length is exhausted (usually four Huffman tables)
index -- one byte: if >15 (i.e. 0x10 or more) then an AC table, otherwise a DC table
bits -- 16 bytes
Huffman values -- # of bytes = the sum of the previous 16 bytes
Start of frame marker (FFC0)
the first two bytes, the length, after the marker indicate the number of bytes, including the two length bytes, that this header contains
P -- one byte: sample precision in bits (usually 8, for baseline JPEG)
Y -- two bytes
X -- two bytes
Nf -- one byte: the number of components in the image
3 for color baseline JPEG images
1 for grayscale baseline JPEG images
Nf times:
Component ID -- one byte
H and V sampling factors -- one byte: H is first four bits and V is second four bits
Quantization table number-- one byte
The H and V sampling factors dictate the final size of the component they are associated with. For instance, the color space defaults to YCbCr and the H and V sampling factors for each component, Y, Cb, and Cr, default to 2, 1, and 1, respectively (2 for both H and V of the Y component, etc.) in the Jpeg-6a library by the Independent Jpeg Group. While this does mean that the Y component will be twice the size of the other two components--giving it a higher resolution, the lower resolution components are quartered in size during compression in order to achieve this difference. Thus, the Cb and Cr components must be quadrupled in size during decompression.
Start of Scan marker (FFDA)
the first two bytes, the length, after the marker indicate the number of bytes, including the two length bytes, that this header contains
Number of components, n -- one byte: the number of components in this scan
n times:
Component ID -- one byte
DC and AC table numbers -- one byte: DC # is first four bits and AC # is last four bits
Ss -- one byte
Se -- one byte
Ah and Al -- one byte
Comment marker (FFFE)
the first two bytes, the length, after the marker indicate the number of bytes, including the two length bytes, that this header contains
whatever the user wants
End of Image (EOI) marker (FFD9
300x250