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James Gregory 2013-12-30 13:57:02 +11:00
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<P><BR></P>
<H2><A NAME="Heading1"></A><FONT COLOR="#000077">Chapter 65<BR>3-D Clipping and Other Thoughts
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<H3><A NAME="Heading2"></A><FONT COLOR="#000077">Determining What&rsquo;s Inside Your Field of View</FONT></H3>
<H2><A NAME="Heading1"></A>Chapter 65<BR>3-D Clipping and Other Thoughts</H2>
<H3><A NAME="Heading2"></A>Determining What&rsquo;s Inside Your Field of View</H3>
<P>Our part of the world is changing, and I&rsquo;m concerned. By way of explanation, three anecdotes.
</P>
<P>Anecdote the first: In the introduction to one of his books, Frank Herbert, author of <I>Dune</I>, told how he had once been approached by a friend who claimed he (the friend) had a killer idea for an SF story, and offered to tell it to Herbert. In return, Herbert had to agree that if he used the idea in a story, he&rsquo;d split the money from the story with this fellow. Herbert&rsquo;s response was that ideas were a dime a dozen; he had more story ideas than he could ever write in a lifetime. The hard part was the writing, not the ideas.</P>
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<P>Extend that to programming. The way it should work is that a steady flow of information circulates, so that everyone can do the best work they&rsquo;re capable of. The idea is that I don&rsquo;t gain by intellectually impoverishing you, and vice-versa; as we both compete and (intentionally or otherwise) share ideas, both our products become better, so the market grows larger and everyone benefits.</P>
<P>That&rsquo;s the way things have worked with programming for a long time. So far as I can see it has worked remarkably well, and the recent signs of change make me concerned about the future of our profession.</P>
<P>Things aren&rsquo;t changing <I>everywhere</I>, though; over the past year, I&rsquo;ve circulated a good bit of info about 3-D graphics, and plan to keep on doing it as long as I can. Next, we&rsquo;re going to take a look at 3-D clipping.</P>
<H3><A NAME="Heading3"></A><FONT COLOR="#000077">3-D Clipping Basics</FONT></H3>
<H3><A NAME="Heading3"></A>3-D Clipping Basics</H3>
<P>Before I got deeply into 3-D, I kept hearing how difficult 3-D clipping was, so I was pleasantly surprised when I actually got around to doing it and found that it was quite straightforward, after all. At heart, 3-D clipping is nothing more than evaluating whether and where a line intersects a plane; in this context, the plane is considered to have an &ldquo;inside&rdquo; (a side on which points are to be kept) and an &ldquo;outside&rdquo; (a side on which points are to be removed or clipped). We can easily extend this single operation to polygon clipping, working with the line segments that form the edges of a polygon.
</P>
<P>The most common application of 3-D clipping is as part of the process of hidden surface removal. In this application, the four planes that make up the view volume, or view frustum, are used to clip away parts of polygons that aren&rsquo;t visible. Sometimes this process includes clipping to near and far plane, to restrict the depth of the scene. Other applications include clipping to splitting planes while building BSP trees, and clipping moving objects to convex sectors such as BSP leaves. The clipping principles I&rsquo;ll cover apply to any sort of 3-D clipping task, but clipping to the frustum is the specific context in which I&rsquo;ll discuss clipping below.</P>
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<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book &copy; 2001 Michael Abrash</font>
Graphics Programming Black Book &copy; 2001 Michael Abrash
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