JPEG头文件结构及组成

2018-11-25

最近在做关于图片显示方面的嵌入式程序.发现国内这方面资料介绍得还是比较少的,所以发表在这里.

Before the image data is ever loaded when a JPEG image is selected for viewing the markers must be read.  In a JPEG image, the very first marker is the SOI, or Start Of Image, marker.  This is the first "hey, I"m a JPEG" declaration by the file.  The JPEG standard, as written by the Joint Picture Expert"s Group, specified the JPEG interchange format.  This format had several shortcomings for which the JFIF (JPEG File Interchange Format) was an attempted remedy.  The JFIF is the format used by almost all JPEG file readers/writers.  It tells the image readers, "Hey, I"m a JPEG that almost anyone can understand."

Most markers will have additional information following them.  When this is the case, the marker and its associated information is referred to as a "header."   In a header the marker is immediately followed by two bytes that indicate the length of the information, in bytes, that the header contains.  The two bytes that indicate the length are always included in that count.

A marker is prefixed by FF (hexadecimal).  The marker/header information that follows does not specify all known markers, just the essential ones for baseline JPEG.

A component is a specific color channel in an image.  For instance, an RGB image contains three components; Red, Green, and Blue.

1998 by James R. Weeks

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)

the very last marker

JPEG文件由八个部分组成，每个部分的标记字节为两个，首字节固定为：0xFF，当然，准许在其前面再填充多个0xFF，以最后一个为准。下面为各部分的名称和第二个标记字节的数值，用ultraedit的16进制搜索功能可找到各部分的起始位置，在嵌入式系统中可用类似的数值匹配法定位。

一、图像开始SOI(Start of Image)标记，数值0xD8

二、APP0标记(Marker)，数值0xE0

1、APP0长度(length)                            2byte

2、标识符(identifier)                     5byte

3、版本号(version)                        2byte

4、X和Y的密度单位(units=0：无单位；units=1：点数/英寸；units=2：点数/厘米)                        1byte

5、X方向像素密度(X density)                      2byte

6、Y方向像素密度(Y density)                       2byte

7、缩略图水平像素数目(thumbnail horizontal pixels)                       1byte

8、缩略图垂直像素数目(thumbnail vertical pixels)                            1byte

9、缩略图RGB位图(thumbnail RGB bitmap)，由前面的数值决定，取值3n，n为缩略图总像素   3n byte

三、APPn标记(Markers)，其中n=1～15，数值对应0xE1～0xEF

1、APPn长度(length)

2、应用细节信息(application specific information)

四、一个或者多个量化表DQT(difine quantization table)，数值0xDB

1、量化表长度(quantization table length)

2、量化表数目(quantization table number)

3、量化表(quantization table)

五、帧图像开始SOF0(Start of Frame)，数值0xC0

1、帧开始长度(start of frame length)

2、精度(precision)，每个颜色分量每个像素的位数(bits per pixel per color component)

3、图像高度(image height)

4、图像宽度(image width)

5、颜色分量数(number of color components)

6、对每个颜色分量(for each component)

包括：ID、垂直方向的样本因子(vertical sample factor)、水平方向的样本因子(horizontal sample factor) 、量化表号(quantization table#)

六、一个或者多个霍夫曼表DHT(Difine Huffman Table)，数值0xC4

1、霍夫曼表的长度(Huffman table length)

2、类型、AC或者DC(Type, AC or DC)

3、索引(Index)

4、位表(bits table)

5、值表(value table)

七、扫描开始SOS(Start of Scan)，数值0xDA

1、扫描开始长度(start of scan length)

2、颜色分量数(number of color components)

3、每个颜色分量

包括：ID、交流系数表号(AC table #)、直流系数表号(DC table #)

4、压缩图像数据(compressed image data)

八、图像结束EOI(End of Image)，数值0xD9

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