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 <H3><A NAME="Heading2"></A><FONT COLOR="#000077">Using Backface Removal to Eliminate Hidden Surfaces</FONT></H3>
 <P>As I&#146;m fond of pointing out, computer animation isn&#146;t a matter of mathematically exact modeling or raw technical prowess, but rather of fooling the eye and the mind. That&#146;s especially true for 3-D animation, where we&#146;re not only trying to convince viewers that they&#146;re seeing objects on a screen&#151;when in truth that screen contains no objects at all, only gaggles of pixels&#151;but we&#146;re also trying to create the illusion that the objects exist in three-space, possessing four dimensions (counting movement over time as a fourth dimension) of their own. To make this magic happen, we must provide cues for the eye not only to pick out boundaries, but also to detect depth, orientation, and motion. This involves perspective, shading, proper handling of hidden surfaces, and rapid and smooth screen updates; the whole deal is considerably more difficult to pull off on a PC than 2-D animation.
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-<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/51-01i.jpg"><TD WIDTH="95%" VALIGN="TOP"><SMALL><I>In some senses, however, 3-D animation is easier than 2-D. Because there&#146;s more going on in 3-D animation, the eye and brain tend to make more assumptions, and so are more apt to see what they expect to see, rather than what&#146;s actually there.</I></SMALL>
+<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/i.jpg"><TD WIDTH="95%" VALIGN="TOP"><SMALL><I>In some senses, however, 3-D animation is easier than 2-D. Because there&#146;s more going on in 3-D animation, the eye and brain tend to make more assumptions, and so are more apt to see what they expect to see, rather than what&#146;s actually there.</I></SMALL>
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 <P>If you&#146;re piloting a (virtual) ship through a field of thousands of asteroids at high speed, you&#146;re unlikely to notice if the more distant asteroids occasionally seem to go right through each other, or if the topographic detail on the asteroids&#146; surfaces sometimes shifts about a bit. You&#146;ll be busy viewing the asteroids in their primary role, as objects to be navigated around, and the mere presence of topographic detail will suffice; without being aware of it, you&#146;ll fill in the blanks. Your mind will see the topography peripherally, recognize it for what it is supposed to be, and, unless the landscape does something really obtrusive such as vanishing altogether or suddenly shooting a spike miles into space, you will see what you expect to see: a bunch of nicely detailed asteroids tumbling around you.
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