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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><!-- 这篇文章由Dancingwind翻译，作者的联系方式zhouwei02@mails.tsinghua.edu.cn --><title>NeHe OpenGL教程第四十八课，DancingWind翻译</title>


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    <td class="tinylogo" height="130" width="326"><img alt="NeHe Productions" src="Tutorial_48_files/logo.png" height="130" width="326"></td>
      <td align="center" valign="center"><div align="center"><font class="text"><font class="theme" size="+3"><b><i>第48课</i></b></font></font></div></td>
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            <td width="18%"><img src="Tutorial_48_files/lesson48.jpg" height="120" width="160"></td>
            <td width="82%"><p><font><b>轨迹球实现的鼠标旋转</b></font></p>
              <p><font size="3">使用鼠标旋转物体,很简单也有很多实现方法,这里我们教会你模拟轨迹球来实现它.</font></p></td>
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      <td class="back3" valign="top" width="100%">轨迹球控制<br>
        By Terence J. Grant (tjgrant@tatewake.com) 
        <p>如果只用鼠标来控制你的模型是不是很酷?轨迹球可以帮你做到这一点，我将告诉你我的实现，你可以把它应用在你的工程里。</p>
        <p>我的实现是基于Bretton Wade’s，它是基于Ken Shoemake’s 实现的，最初的版本，你可以从游戏编程指南这本图上找到。但我还是修正了一些错误，并优化了它。</p>
        <p>轨迹球实现的内容就是把二维的鼠标点映射到三维的轨迹球，并基于它完成旋转变化。</p>
        <p>为了完成这个设想，首先我们把鼠标坐标映射到[-1，1]之间，它很简单：<br>
        </p>
        <p>&nbsp;</p>
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<pre>MousePt.X  =  ((MousePt.X / ((Width  -1) / 2)) -1);<br>MousePt.Y  = -((MousePt.Y / ((Height -1) / 2))-1);</pre>
<p></p>
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    <td class="l"><img alt="" src="Tutorial_48_files/blank1.gif" height="28" width="28"></td>
      <td class="back3" valign="top" width="100%">这只是为了数学上的简化，下面我们计算这个长度，如果它大于轨迹球的边界，我们将简单的把z轴设为0，否则我们把z轴设置为这个二维点映射到球面上对应的z值。
        <p>一旦我们有了两个点，就可以计算它的法向量了和旋转角了。</p>
        <p>下面我们从构造函数开始，完整的讲解这个类：</p>
        <p>&nbsp;</p>
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<pre>ArcBall_t::ArcBall_t(GLfloat NewWidth, GLfloat NewHeight)</pre>
<p></p>
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      <td class="back3" valign="top" width="100%">当点击鼠标时，记录点击的位置 </td>
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