jpeg格式 - kkl_fly - 网易博客.htm
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<P style="TEXT-INDENT: 2em">units=0：无单位</P>
<P style="TEXT-INDENT: 2em">units=1：点数/英寸</P>
<P style="TEXT-INDENT: 2em">units=2：点数/厘米</P></TD></TR>
<TR>
<TD vAlign=top width="7%">
<P></P>
<P style="TEXT-INDENT: 2em">14</P></TD>
<TD vAlign=top width="8%">
<P></P>
<P style="TEXT-INDENT: 2em">2 bytes</P></TD>
<TD vAlign=top width="10%"> </TD>
<TD vAlign=top width="36%">
<P></P>
<P style="TEXT-INDENT: 2em"><Xdensity></P></TD>
<TD vAlign=top width="39%">
<P></P>
<P style="TEXT-INDENT: 2em">水平方向像素密度</P></TD></TR>
<TR>
<TD vAlign=top width="7%">
<P></P>
<P style="TEXT-INDENT: 2em">16</P></TD>
<TD vAlign=top width="8%">
<P></P>
<P style="TEXT-INDENT: 2em">2 bytes</P></TD>
<TD vAlign=top width="10%"> </TD>
<TD vAlign=top width="36%">
<P></P>
<P style="TEXT-INDENT: 2em"><Ydensity></P></TD>
<TD vAlign=top width="39%">
<P></P>
<P style="TEXT-INDENT: 2em">垂直方向像素密度</P></TD></TR>
<TR>
<TD vAlign=top width="7%">
<P></P>
<P style="TEXT-INDENT: 2em">18</P></TD>
<TD vAlign=top width="8%">
<P></P>
<P style="TEXT-INDENT: 2em">1 byte</P></TD>
<TD vAlign=top width="10%"> </TD>
<TD vAlign=top width="36%">
<P></P>
<P style="TEXT-INDENT: 2em"><Xthumbnail></P></TD>
<TD vAlign=top width="39%">
<P></P>
<P style="TEXT-INDENT: 2em">缩略图水平像素数目</P></TD></TR>
<TR>
<TD vAlign=top width="7%">
<P></P>
<P style="TEXT-INDENT: 2em">19</P></TD>
<TD vAlign=top width="8%">
<P></P>
<P style="TEXT-INDENT: 2em">1 byte</P></TD>
<TD vAlign=top width="10%"> </TD>
<TD vAlign=top width="36%">
<P></P>
<P style="TEXT-INDENT: 2em"><Ythumbnail></P></TD>
<TD vAlign=top width="39%">
<P></P>
<P style="TEXT-INDENT: 2em">缩略图垂直像素数目</P></TD></TR>
<TR>
<TD vAlign=top width="7%"> </TD>
<TD vAlign=top width="8%">
<P></P>
<P style="TEXT-INDENT: 2em">3n</P></TD>
<TD vAlign=top width="10%"> </TD>
<TD vAlign=top width="36%">
<P></P>
<P style="TEXT-INDENT: 2em">< Thumbnail RGB bitmap></P></TD>
<TD vAlign=top width="39%">
<P></P>
<P style="TEXT-INDENT: 2em">缩略RGB位图(n为缩略图的像素数)</P></TD></TR>
<TR>
<TD vAlign=top width="7%"> </TD>
<TD vAlign=top width="8%"> </TD>
<TD vAlign=top width="10%"> </TD>
<TD vAlign=top width="36%">
<P></P>
<P style="TEXT-INDENT: 2em">Optional JFIF extension APP0 marker
segment(s)</P></TD>
<TD vAlign=top width="39%">
<P></P>
<P style="TEXT-INDENT: 2em">任选的JFIF扩展APP0标记段</P></TD></TR>
<TR>
<TD vAlign=top width="7%"> </TD>
<TD vAlign=top width="8%">
<P></P>
<P style="TEXT-INDENT: 2em">……</P></TD>
<TD vAlign=top width="10%"> </TD>
<TD vAlign=top width="36%">
<P></P>
<P style="TEXT-INDENT: 2em">……</P></TD>
<TD vAlign=top width="39%"> </TD></TR>
<TR>
<TD vAlign=top width="7%"> </TD>
<TD vAlign=top width="8%">
<P></P>
<P style="TEXT-INDENT: 2em">2 byte</P></TD>
<TD vAlign=top width="10%">
<P></P>
<P style="TEXT-INDENT: 2em">0xFFD9</P></TD>
<TD vAlign=top width="36%">
<P></P>
<P style="TEXT-INDENT: 2em">(EOI) end-of-file</P></TD>
<TD vAlign=top width="39%">
<P></P>
<P style="TEXT-INDENT: 2em">图像文件结束标记</P></TD></TR></TBODY></TABLE></P></DIR>
<P></P>
<P style="TEXT-INDENT: 2em"> </P>
<P style="TEXT-INDENT: 2em"> JPEG文件使用的颜色空间是CCIR
601推荐标准进行的彩色空间(参看第7章)。在这个彩色空间中，每个分量、每个像素的电平规定为255级，用8位代码表示。从RGB转换成YCbCr空间时，使用下面的精确的转换关系：</P>
<P style="TEXT-INDENT: 2em"> Y = 256 * E'y</P>
<P style="TEXT-INDENT: 2em"> Cb = 256 * [E'Cb] + 128</P>
<P style="TEXT-INDENT: 2em"> Cr = 256 * [E'Cr] + 128</P>
<P style="TEXT-INDENT: 2em"> 其中亮度电平E'y和色差电平E'Cb和E'Cb分别是CCIR
601定义的参数。由于E'y的范围是0～1，E'Cb和E'Cb的范围是-0.5～+0.5，因此Y,
Cb和Cr的最大值必须要箝到255。于是RGB和YCbCr之间的转换关系需要按照下面的方法计算。</P>
<P style="TEXT-INDENT: 2em"> (1) 从RGB转换成YCbCr</P>
<P style="TEXT-INDENT: 2em">
YCbCr(256级)分量可直接从用8位表示的RGB分量计算得到：</P>
<P style="TEXT-INDENT: 2em">
Y = 0.299R + 0.587G + 0.114B</P>
<P style="TEXT-INDENT: 2em"> Cb =
-0.1687R - 0.3313G + 0.5B + 128</P>
<P style="TEXT-INDENT: 2em"> Cr = 0.5R - 0.4187G - 0.0813B + 128</P>
<P style="TEXT-INDENT: 2em"> 需要注意的是不是所有图像文件格式都按照R0，G0，B0，……
Rn，Gn，Bn的次序存储样本数据，因此在RGB文件转换成JFIF文件时需要首先验证RGB的次序。</P>
<P style="TEXT-INDENT: 2em"> (2) 从YCbCr转换成RGB </P>
<P style="TEXT-INDENT: 2em"> RGB分量可直接从YCbCr(256级)分量计算得到：</P>
<P style="TEXT-INDENT: 2em"> R = Y + 1.402(Cr-128)</P>
<P style="TEXT-INDENT: 2em"> G = Y - 0.34414(Cb-128) - 0.71414(Cr-128)</P>
<P style="TEXT-INDENT: 2em"> B = Y + 1.772(Cb-128)</P>
<P style="TEXT-INDENT: 2em"><A></A></P>
<P style="TEXT-INDENT: 2em">
在JFIF文件格式中，图像样本的存放顺序是从左到右和从上到下。这就是说JFIF文件中的第一个图像样本是图像左上角的样本。 </P>
<OL>
<LI>SOI 0xD8 图像开始
<LI>APP0 0xE0 JFIF应用数据块
<LI>APPn 0xE1 - 0xEF 其他的应用数据块(n, 1～15)
<LI>DQT 0xDB 量化表
<LI>SOF0 0xC0 帧开始
<LI>DHT 0xC4 霍夫曼(Huffman)表
<LI>SOS 0xDA 扫描线开始
<LI>EOI 0xD9 图像结束 </LI></OL>
<P></P>
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