📄 chap02.htm
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"Times New Roman"'>色图来表示了。为什么会这样呢？难道是一种巧合？其实并不是。</span></p>
<p style='line-height:18.0pt'><span
style='font-family:宋体;"Times New Roman"'>在表示颜色的方法中，除了</span><span lang=EN-US>RGB</span><span
style='font-family:宋体;"Times New Roman"'>外，还有一种叫</span><span lang=EN-US>YUV</span><span
style='font-family:宋体;"Times New Roman"'>的表示方法，应用也很多。电视信号中用的就是一种类似于</span><span lang=EN-US>YUV</span><span
style='font-family:宋体;"Times New Roman"'>的颜色表示方法。</span></p>
<p style='line-height:18.0pt'><span
style='font-family:宋体;"Times New Roman"'>在这种表示方法中，</span><span lang=EN-US>Y</span><span
style='font-family:宋体;"Times New Roman"'>分量的物理含义就是亮度，</span><span lang=EN-US>U</span><span
style='font-family:宋体;"Times New Roman"'>和</span><span lang=EN-US>V</span><span style='font-family:
宋体;"Times New Roman"'>分量代表了色差信号</span><span
lang=EN-US>(</span><span style='font-family:宋体;
"Times New Roman"'>你不必了解什么是色差，只要知道有这么一个概念就可以了</span><span
lang=EN-US>)</span><span style='font-family:宋体;
"Times New Roman"'>。使用这种表示方法有很多好处，最主要的有两点：</span></p>
<p style='line-height:18.0pt;
'> <span
lang=EN-US>(1)<span style='font:7.0pt "Times New Roman"'> </span></span>
<span
style='font-family:宋体;"Times New Roman"'>因为</span><span lang=EN-US>Y</span><span style='font-family:
宋体;"Times New Roman"'>代表了亮度，所以</span><span
lang=EN-US>Y</span><span style='font-family:宋体;
"Times New Roman"'>分量包含了灰度图的所有信息，只用</span><span
lang=EN-US>Y</span><span style='font-family:宋体;
"Times New Roman"'>分量就能完全能够表示出一幅灰度图来。当同时考虑</span><span
lang=EN-US>U</span><span style='font-family:宋体;
"Times New Roman"'>，</span><span lang=EN-US>V</span><span
style='font-family:宋体;"Times New Roman"'>分量时，就能够表示出彩色信息来。这样，用同一种表示方法可以很方便的在灰度和彩色图之间切换，而</span><span
lang=EN-US>RGB</span><span style='font-family:宋体;
"Times New Roman"'>表示方法就做不到这一点了。</span></p>
<p style='line-height:18.0pt;
'> <span
lang=EN-US>(2)<span style='font:7.0pt "Times New Roman"'> </span></span>
<span
style='font-family:宋体;"Times New Roman"'>人眼对于亮度信号非常敏感，而对色差信号的敏感程度相对较弱。也就是说，图象的主要信息包含在</span><span
lang=EN-US>Y</span><span style='font-family:宋体;
"Times New Roman"'>分量中。这就提示我们：如果在对</span><span
lang=EN-US>YUV</span><span style='font-family:宋体;
"Times New Roman"'>信号进行量化时，可以“偏心”一点，让</span><span
lang=EN-US>Y</span><span style='font-family:宋体;
"Times New Roman"'>的量化级别多一些</span><span lang=EN-US>(</span><span
style='font-family:宋体;"Times New Roman"'>谁让它重要呢？</span><span lang=EN-US>)</span><span
style='font-family:宋体;"Times New Roman"'>而让</span><span lang=EN-US>UV</span><span style='font-family:
宋体;"Times New Roman"'>的量化级别少一些，就可以实现图象信息的压缩。这一点将在第</span><span
lang=EN-US>9</span><span style='font-family:宋体;
"Times New Roman"'>章介绍图象压缩时仔细研究，这里就不深入讨论了。而</span><span
lang=EN-US>RGB</span><span style='font-family:宋体;
"Times New Roman"'>的表示方法就做不到这一点，因为</span><span
lang=EN-US>RGB</span><span style='font-family:宋体;
"Times New Roman"'>三个分量同等重要，缺了谁也不行。</span><span
lang=EN-US>YUV</span><span style='font-family:宋体;
"Times New Roman"'>和</span><span lang=EN-US>RGB</span><span
style='font-family:宋体;"Times New Roman"'>之间有着如下的对应关系</span></p>
<p align=center style='text-align:center;line-height:18.0pt'><span
lang=EN-US><sub> <img width=352 height=77
src="chap02.files/image014.gif" v:shapes="_x0000_i1071"> </sub> </span></p>
<p align=right style='text-align:right;line-height:18.0pt'><span
lang=EN-US>(2.3)</span></p>
<p align=center style='text-align:center;line-height:18.0pt'><span
lang=EN-US><sub> <img width=324 height=75
src="chap02.files/image016.gif" v:shapes="_x0000_i1072"> </sub> </span></p>
<p align=right style='text-align:right;line-height:18.0pt'><span
lang=EN-US>(2.4)</span></p>
<p style='line-height:18.0pt'><span
style='font-family:宋体;"Times New Roman"'>当</span><span lang=EN-US>RGB</span><span style='font-family:
宋体;"Times New Roman"'>三个分量的大小一样时，假设都是</span><span
lang=EN-US>a</span><span style='font-family:宋体;
"Times New Roman"'>，代入公式</span><span lang=EN-US>(2.3)</span><span
style='font-family:宋体;"Times New Roman"'>，得到</span><span lang=EN-US>Y=a</span><span style='font-family:
宋体;"Times New Roman"'>，</span><span
lang=EN-US>U=0</span><span style='font-family:宋体;
"Times New Roman"'>，</span><span lang=EN-US>V=0 </span><span
style='font-family:宋体;"Times New Roman"'>。你现在该明白我前面所说不是巧合的原因了吧。</span></p>
<p style='line-height:18.0pt'><span
style='font-family:宋体;"Times New Roman"'>使用灰度图有一个好处，那就是方便。首先</span><span lang=EN-US>RGB</span><span
style='font-family:宋体;"Times New Roman"'>的值都一样；其次，图象数据即调色板索引值，也就是实际的</span><span lang=EN-US>RGB</span><span
style='font-family:宋体;"Times New Roman"'>值，也就是亮度值；另外，因为是</span><span lang=EN-US>256</span><span
style='font-family:宋体;"Times New Roman"'>色调色板，所以图象数据中一个字节代表一个象素，很整齐。如果是</span><span lang=EN-US>2</span><span
style='font-family:宋体;"Times New Roman"'>色图或</span><span lang=EN-US>16</span><span style='font-family:
宋体;"Times New Roman"'>色图，还要拼凑字节，很麻烦。如果是彩色的</span><span
lang=EN-US>256</span><span style='font-family:宋体;
"Times New Roman"'>色图，由于图象处理后有可能会产生不属于这</span><span
lang=EN-US>256</span><span style='font-family:宋体;
"Times New Roman"'>种颜色的新颜色，就更麻烦了；这一点，今后你就会有深刻体会的。所以，做图象处理时，一般采用灰度图。为了将重点放在算法本身上，</span><b><span
style='font-family:黑体'>今后给出的程序如不做特殊说明，都是针对<span lang=EN-US>256级灰度图的。</span></span></b><span
style='font-family:宋体;"Times New Roman"'>其它颜色的情况，你可以自己想一想，把算法补全。</span></p>
<p style='line-height:18.0pt'><span
style='font-family:宋体;"Times New Roman"'>如果想得到一幅灰度图，可以使用</span><span lang=EN-US>Sea</span><span
style='font-family:宋体;"Times New Roman"'>或者</span><span lang=EN-US>PhotoShop</span><span
style='font-family:宋体;"Times New Roman"'>等软件提供的颜色转换功能将彩色图转换成灰度图。</span></p>
<p style='line-height:18.0pt'><span
style='font-family:宋体;"Times New Roman"'>好了，言归正传，下面给出</span><span lang=EN-US>Translation</span><span
style='font-family:宋体;"Times New Roman"'>的源代码。算法的思想是先将所有区域填成白色，然后找平移后显示区域的左上角点</span><span
lang=EN-US>(x<sub>0</sub>,y<sub>0</sub>) </span><span style='font-family:宋体;
"Times New Roman"'>和右下角点</span><span
lang=EN-US>(x<sub>1</sub>,y<sub>1</sub>) </span><span style='font-family:宋体;
"Times New Roman"'>，分几种情况进行处理。</span></p>
<p style='line-height:18.0pt'><span
style='font-family:宋体;"Times New Roman"'>先看</span><span lang=EN-US>x</span><span style='font-family:
宋体;"Times New Roman"'>方向</span><span
lang=EN-US>(width</span><span style='font-family:宋体;
"Times New Roman"'>指图象的宽度</span><span lang=EN-US>)</span></p>
<p style='line-height:
18.0pt;'> <span
lang=EN-US>(1)<span style='font:7.0pt "Times New Roman"'> </span></span>
<span
lang=EN-US>t<sub>x</sub></span><span style='font-family:宋体;"Times New Roman";"Times New Roman"'>≤</span><span
lang=EN-US>-width</span><span style='font-family:宋体;
"Times New Roman"'>：很显然，图象完全移出了屏幕，不用做任何处理；</span></p>
<p style='line-height:
18.0pt;'> <span
lang=EN-US>(2)<span style='font:7.0pt "Times New Roman"'> </span></span>
<span
lang=EN-US>-width<tx</span><span style='font-family:宋体;"Times New Roman";"Times New Roman"'>≤</span><span
lang=EN-US>0</span><span style='font-family:宋体;
"Times New Roman"'>：如图</span><span lang=EN-US>2.5</span><span
style='font-family:宋体;"Times New Roman"'>所示。容易看出，图象区域的</span><span lang=EN-US>x</span><span
style='font-family:宋体;"Times New Roman"'>范围从</span><span lang=EN-US>0</span><span style='font-family:
宋体;"Times New Roman"'>到</span><span
lang=EN-US>width-|tx|,</span><span style='font-family:宋体;"Times New Roman";"Times New Roman"'>对应原图的范围从</span><span
lang=EN-US>|tx|</span><span style='font-family:宋体;
"Times New Roman"'>到</span><span lang=EN-US>width</span><span
style='font-family:宋体;"Times New Roman"'>；</span></p>
<p class=a style='line-height:18.0pt'><span lang=EN-US> <img width=397 height=364
src="chap02.files/image018.jpg" v:shapes="_x0000_i1026"> </span></p>
<p align=center style='text-align:center;line-height:18.0pt'><b><span
style='font-family:宋体;"Times New Roman"'>图</span>2.5 tx</b><b><span
style='font-family:宋体;"Times New Roman"'>≤</span><span lang=EN-US>0</span></b><b><span
style='font-family:宋体;"Times New Roman"'>，</span><span lang=EN-US>ty</span></b><b><span
style='font-family:宋体;"Times New Roman"'>≤</span><span lang=EN-US>0</span></b><b><span
style='font-family:宋体;"Times New Roman"'>的情况</span><span lang=EN-US></span></b></p>
<p style='line-height:18.0pt;
'> <span
lang=EN-US>(3)<span style='font:7.0pt "Times New Roman"'> </span></span>
<span
lang=EN-US>0< t<sub>x</sub> <width</span><span style='font-family:宋体;
"Times New Roman"'>：如图</span><span
lang=EN-US>2.6</span><span style='font-family:宋体;
"Times New Roman"'>所示。容易看出，图象区域的</span><span lang=EN-US>x</span><span
style='font-family:宋体;"Times New Roman"'>范围从</span><span lang=EN-US>t<sub>x</sub>
</span><span
style='font-family:宋体;"Times New Roman"'>到</span><span lang=EN-US>width</span><span style='font-family:
宋体;"Times New Roman"'>，对应原图的范围从</span><span
lang=EN-US>0</span><span style='font-family:宋体;
"Times New Roman"'>到</span><span lang=EN-US>width - t<sub>x </sub></span><span style='font-family:宋体;
"Times New Roman"'>；</span></p>
<p class=a style='line-height:18.0pt'><span lang=EN-US> <img width=417 height=342
src="chap02.files/image020.jpg" v:shapes="_x0000_i1039"> </span></p>
<p align=center style='text-align:center;line-height:18.0pt'><b><span
style='font-family:宋体;"Times New Roman"'>图</span>2.6 0<
tx<width</b><b><span style='font-family:宋体;"Times New Roman";"Times New Roman"'>，</span><span
lang=EN-US>0<ty<height</span></b><b><span style='font-family:宋体;
"Times New Roman"'>的情况</span><span
lang=EN-US></span></b></p>
<p style='line-height:18.0pt;
'> <span
lang=EN-US>(4)<span style='font:7.0pt "Times New Roman"'> </span></span>
<span
lang=EN-US>t<sub>x</sub> </span><span style='font-family:宋体;"Times New Roman";"Times New Roman"'>≥</span><span
lang=EN-US>width</span><span style='font-family:宋体;
"Times New Roman"'>：很显然，图象完全移出了屏幕，不用做任何处理。</span></p>
<p style='line-height:18.0pt'><span
lang=EN-US>y</span><span style='font-family:宋体;
"Times New Roman"'>方向是对应的</span><span lang=EN-US>(height</span><span
style='font-family:宋体;"Times New Roman"'>表示图象的高度</span><span lang=EN-US>)</span><span
style='font-family:宋体;"Times New Roman"'>：</span></p>
<p style='line-height:18.0pt;
'> <span
lang=EN-US>(1)<span style='font:7.0pt "Times New Roman"'> </span></span>
<span
lang=EN-US>t<sub>y</sub></span><span style='font-family:宋体;"Times New Roman";"Times New Roman"'>≤</span><span
lang=EN-US>-height</span><span style='font-family:宋体;"Times New Roman";"Times New Roman"'>，图象完全移出了屏幕，不用做任何处理；</span></p>
<p style='line-height:18.0pt;
'> <span
lang=EN-US>(2)<span style='font:7.0pt "Times New Roman"'> </span></span>
<span
lang=EN-US>-height<t<sub>y</sub></span><span style='font-family:宋体;
"Times New Roman"'>≤</span><span
lang=EN-US>0</span><span style='font-family:宋体;
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