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📁 数字图象处理入门,非常好的书！！！！推荐！
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<p align=center style='margin:0cm;margin-bottom:.0001pt;text-align:center;
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<p align=center style='margin:0cm;margin-bottom:.0001pt;text-align:center;
line-height:18.0pt'><b><span lang=ZH-CN style='font-size:10.5pt'>图</span></b><b><span
style='font-size:10.5pt;font-family:"Times New Roman"'>4.1 </span></b><b><span
lang=ZH-CN style='font-size:10.5pt'>用黑白两种颜色打印出灰度效果</span></b><span
style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>

<p style='margin:0cm;margin-bottom:.0001pt;text-align:justify;text-justify:
inter-ideograph;line-height:18.0pt'><span lang=ZH-CN style='font-size:10.5pt'>图</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>4.1</span><span
lang=ZH-CN style='font-size:10.5pt'>中最左边的是原图，是一幅真正的灰度图，另外三张图都是黑白二值图。容易看出，最左的那幅和原图最接近。</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>

<p style='margin:0cm;margin-bottom:.0001pt;text-align:justify;text-justify:
inter-ideograph;line-height:18.0pt'><span lang=ZH-CN style='font-size:10.5pt'>由二值图象显示出灰度效果的方法，就是我们今天要讲的半影调</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>(halftone)</span><span
lang=ZH-CN style='font-size:10.5pt'>技术，它的一个主要用途就是在只有二值输出的打印机上打印图象。我们介绍两种方法：图案法和抖动法。</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>

<h2 style='text-align:justify;text-justify:inter-ideograph'><span
style='font-family:"Times New Roman"'>4.1 </span><span lang=ZH-CN
style='mso-ascii-font-family:"Times New Roman";mso-hansi-font-family:"Times New Roman"'>图案法</span><span
style='font-family:"Times New Roman"'><o:p></o:p></span></h2>

<p style='margin:0cm;margin-bottom:.0001pt;text-align:justify;text-justify:
inter-ideograph;line-height:18.0pt'><span lang=ZH-CN style='font-size:10.5pt'>图案法</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>(patterning)</span><span
lang=ZH-CN style='font-size:10.5pt'>是指灰度可以用一定比例的黑白点组成的区域表示，从而达到整体图象的灰度感。黑白点的位置选择称为图案化。</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>

<p style='margin:0cm;margin-bottom:.0001pt;text-align:justify;text-justify:
inter-ideograph;line-height:18.0pt'><span lang=ZH-CN style='font-size:10.5pt'>在具体介绍图案法之前，先介绍一下分辨率的概念。计算机显示器，打印机，扫描仪等设备的一个重要指标就是分辨率，单位是</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>dpi(dot per inch)</span><span
lang=ZH-CN style='font-size:10.5pt'>，即每英寸点数，点数越多，分辨率就越高，图象就越清晰。让我们来计算一下，计算机显示器的分辨率有多高。设显示器为</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>15</span><span
lang=ZH-CN style='font-size:10.5pt'>英寸</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>(</span><span lang=ZH-CN style='font-size:10.5pt'>指对角线长度</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>)</span><span
lang=ZH-CN style='font-size:10.5pt'>，最多显示</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>1280</span><span lang=ZH-CN style='font-size:
10.5pt'>×</span><span style='font-size:10.5pt;font-family:"Times New Roman"'>1024</span><span
lang=ZH-CN style='font-size:10.5pt'>个点。因为宽高比为</span><span style='font-size:
10.5pt;font-family:"Times New Roman"'>4</span><span lang=ZH-CN
style='font-size:10.5pt'>：</span><span style='font-size:10.5pt;font-family:
"Times New Roman"'>3</span><span lang=ZH-CN style='font-size:10.5pt'>，所以宽有</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>12</span><span
lang=ZH-CN style='font-size:10.5pt'>英寸，高有</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>9</span><span lang=ZH-CN style='font-size:10.5pt'>英寸，则该显示器的水平分辨率为</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>106dpi</span><span
lang=ZH-CN style='font-size:10.5pt'>，垂直分辨率为</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>113.8dpi</span><span lang=ZH-CN
style='font-size:10.5pt'>。一般的激光打印机的分辨率有</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>300dpi</span><span lang=ZH-CN style='font-size:
10.5pt'>×</span><span style='font-size:10.5pt;font-family:"Times New Roman"'>300dpi</span><span
lang=ZH-CN style='font-size:10.5pt'>，</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>600dpi</span><span lang=ZH-CN style='font-size:
10.5pt'>×</span><span style='font-size:10.5pt;font-family:"Times New Roman"'>600dpi</span><span
lang=ZH-CN style='font-size:10.5pt'>，</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>720dpi</span><span lang=ZH-CN style='font-size:
10.5pt'>×</span><span style='font-size:10.5pt;font-family:"Times New Roman"'>720dpi</span><span
lang=ZH-CN style='font-size:10.5pt'>。所以打出来的图象要比计算机显示出来的清晰的多。扫描仪的分辨率要高一些，数码相机的分辨率更高。</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>

<p style='margin:0cm;margin-bottom:.0001pt;text-align:justify;text-justify:
inter-ideograph;line-height:18.0pt'><span lang=ZH-CN style='font-size:10.5pt'>言归正传，前面讲了，图案化使用图案来表示象素的灰度，那么我们来做一道计算题。假设有一幅</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>240</span><span
lang=ZH-CN style='font-size:10.5pt'>×</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>180</span><span lang=ZH-CN style='font-size:
10.5pt'>×</span><span style='font-size:10.5pt;font-family:"Times New Roman"'>8bit</span><span
lang=ZH-CN style='font-size:10.5pt'>的灰度图，当用分辨率为</span><span style='font-size:
10.5pt;font-family:"Times New Roman"'>300dpi</span><span lang=ZH-CN
style='font-size:10.5pt'>×</span><span style='font-size:10.5pt;font-family:
"Times New Roman"'>300dpi</span><span lang=ZH-CN style='font-size:10.5pt'>的激光打印机将其打印到</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>12.8</span><span
lang=ZH-CN style='font-size:10.5pt'>×</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>9.6</span><span lang=ZH-CN style='font-size:
10.5pt'>英寸的纸上时，每个象素的图案有多大？</span><span style='font-size:10.5pt;font-family:
"Times New Roman"'><o:p></o:p></span></p>

<p style='margin:0cm;margin-bottom:.0001pt;text-align:justify;text-justify:
inter-ideograph;line-height:18.0pt'><span lang=ZH-CN style='font-size:10.5pt'>这道题很简单，这张纸最多可以打</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>(300</span><span
lang=ZH-CN style='font-size:10.5pt'>×</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>12.8) </span><span lang=ZH-CN style='font-size:
10.5pt'>×</span><span style='font-size:10.5pt;font-family:"Times New Roman"'>(300</span><span
lang=ZH-CN style='font-size:10.5pt'>×</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>9.6)=3840</span><span lang=ZH-CN
style='font-size:10.5pt'>×</span><span style='font-size:10.5pt;font-family:
"Times New Roman"'>2880</span><span lang=ZH-CN style='font-size:10.5pt'>个点，所以每个象素可以用</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>(3840/240)</span><span
lang=ZH-CN style='font-size:10.5pt'>×</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>(2880/180)=16</span><span lang=ZH-CN
style='font-size:10.5pt'>×</span><span style='font-size:10.5pt;font-family:
"Times New Roman"'>16</span><span lang=ZH-CN style='font-size:10.5pt'>个点大小的图案来表示，即一个象素</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>256</span><span
lang=ZH-CN style='font-size:10.5pt'>个点。如果这</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>16</span><span lang=ZH-CN style='font-size:10.5pt'>×</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>16</span><span
lang=ZH-CN style='font-size:10.5pt'>的方块中一个黑点也没有，就可以表示灰度</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>256</span><span
lang=ZH-CN style='font-size:10.5pt'>；有一个黑点，就表示灰度</span><span style='font-size:
10.5pt;font-family:"Times New Roman"'>255</span><span lang=ZH-CN
style='font-size:10.5pt'>；依次类推，当都是黑点时，表示灰度</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>0</span><span lang=ZH-CN style='font-size:10.5pt'>。这样，</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>16</span><span
lang=ZH-CN style='font-size:10.5pt'>×</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>16</span><span lang=ZH-CN style='font-size:10.5pt'>的方块可以表示</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>257</span><span
lang=ZH-CN style='font-size:10.5pt'>级灰度，比要求的</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>8bit</span><span lang=ZH-CN style='font-size:
10.5pt'>共</span><span style='font-size:10.5pt;font-family:"Times New Roman"'>256</span><span
lang=ZH-CN style='font-size:10.5pt'>级灰度还多了一个。所以上面的那幅图的灰度级别完全能够打印出来。</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>

<p style='margin:0cm;margin-bottom:.0001pt;text-align:justify;text-justify:
inter-ideograph;line-height:18.0pt'><span lang=ZH-CN style='font-size:10.5pt'>这里有一个图案构成的问题，即黑点打在哪里？比如说，只有一个黑点时，我们可以打在正中央，也可以打</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>16</span><span
lang=ZH-CN style='font-size:10.5pt'>×</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>16</span><span lang=ZH-CN style='font-size:10.5pt'>的左上角。图案可以是规则的，也可以是不规则的。一般情况下，有规则的图案比随即图案能够避免点的丛集，但有时会导致图象中有明显的线条。</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>

<p style='margin:0cm;margin-bottom:.0001pt;text-align:justify;text-justify:
inter-ideograph;line-height:18.0pt'><span lang=ZH-CN style='font-size:10.5pt'>如图</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>4.1</span><span
lang=ZH-CN style='font-size:10.5pt'>中，</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>2</span><span lang=ZH-CN style='font-size:10.5pt'>×</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>2</span><span
lang=ZH-CN style='font-size:10.5pt'>的图案可以表示</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>5</span><span lang=ZH-CN style='font-size:10.5pt'>级灰度，当图象中有一片灰度为的</span><span
style='font-size:10.5pt;font-family:"Times New Roman"'>1</span><span
lang=ZH-CN style='font-size:10.5pt'>的区域时，如图</span><span style='font-size:10.5pt;
font-family:"Times New Roman"'>4.2</span><span lang=ZH-CN style='font-size:
10.5pt'>所示，有明显的水平和垂直线条。</span><span style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>

<div align=center>

<table border=0 cellspacing=0 cellpadding=0 style='mso-cellspacing:0cm;
 mso-padding-alt:0cm 0cm 0cm 0cm'>
 <tr>
  <td width=276 valign=bottom style='width:207.0pt;padding:0cm 0cm 0cm 0cm'>
  <p align=center style='text-align:center;line-height:18.0pt'><b><span
  style='font-size:10.5pt;font-family:"Times New Roman"'><!--[if gte vml 1]><v:shape
   id="_x0000_i1026" type="#_x0000_t75" alt="" style='width:237pt;height:66.75pt'>
   <v:imagedata src="./chp4.files/image002.gif" o:href="http://www-scf.usc.edu/~flv/ipbook/chap04.files/image002.gif"/>
  </v:shape><![endif]--><![if !vml]><img width=316 height=89
  src="./chp4.files/image002.gif" v:shapes="_x0000_i1026"><![endif]></span></b><span
  style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>
  <p align=center style='text-align:center;line-height:18.0pt'><b><span
  lang=ZH-CN style='font-size:10.5pt'>图</span></b><b><span style='font-size:
  10.5pt;font-family:"Times New Roman"'>4.2&nbsp;&nbsp;&nbsp;&nbsp; 2</span></b><b><span
  lang=ZH-CN style='font-size:10.5pt'>×</span></b><b><span style='font-size:
  10.5pt;font-family:"Times New Roman"'>2</span></b><b><span lang=ZH-CN
  style='font-size:10.5pt'>的图案</span></b><span style='font-size:10.5pt;
  font-family:"Times New Roman"'><o:p></o:p></span></p>
  </td>
  <td width=276 valign=bottom style='width:207.0pt;padding:0cm 0cm 0cm 0cm'>
  <p align=center style='text-align:center;line-height:18.0pt'><b><span
  style='font-size:10.5pt;font-family:"Times New Roman"'><!--[if gte vml 1]><v:shape
   id="_x0000_i1027" type="#_x0000_t75" alt="" style='width:121.5pt;height:73.5pt'>
   <v:imagedata src="./chp4.files/image003.gif" o:href="http://www-scf.usc.edu/~flv/ipbook/chap04.files/image003.gif"/>
  </v:shape><![endif]--><![if !vml]><img width=162 height=98
  src="./chp4.files/image003.gif" v:shapes="_x0000_i1027"><![endif]></span></b><span
  style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>
  <p align=center style='text-align:center;line-height:18.0pt'><b><span
  lang=ZH-CN style='font-size:10.5pt'>图</span></b><b><span style='font-size:
  10.5pt;font-family:"Times New Roman"'>4.3&nbsp;&nbsp;&nbsp;&nbsp; </span></b><b><span
  lang=ZH-CN style='font-size:10.5pt'>规则图案导致线条</span></b><span
  style='font-size:10.5pt;font-family:"Times New Roman"'><o:p></o:p></span></p>
  </td>
 </tr>
</table>
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