Misc/abrash-black-book
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Remove colour attributes from body and strip most of the font tags out

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James Gregory 2013-12-30 13:57:02 +11:00
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<P>Getting proper front-to-back drawing order is a little more complicated with polygons on nodes. As we walk the BSP tree front-to-back, in each leaf we mark the polygons that are at least partially in that leaf, and then after we&rsquo;ve recursed and processed everything in front of a node, we then process all the marked polygons on that node, after which we recurse to process the polygons behind the node. So putting the polygons on the nodes saves memory and improves performance significantly, but loses the simple approach of simply recursing the tree and processing the polygons in each leaf as we come to it, in favor of recursing and marking in front of a node, processing marked polygons on the node, then recursing behind the node.
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<P>After the BSP is built, the outer surfaces of the level, which no one can ever see (because levels are sealed spaces), are removed, so the interior of the level, containing all the empty space through which a player can move, is completely surrounded by a solid region. This eliminates a great many irrelevant polygons, and reduces the complexity of the next step, calculating the potentially visible set.</P>
<H3><A NAME="Heading3"></A><FONT COLOR="#000077">The Potentially Visible Set (PVS)</FONT></H3>
<H3><A NAME="Heading3"></A>The Potentially Visible Set (PVS)</H3>
<P>After the BSP tree is built, the potentially visible set (PVS) for each leaf is calculated. The PVS for a leaf consists of all the leaves that can be seen from anywhere in that leaf, and is used to reduce to a near-minimum the polygons that have to be considered for drawing from a given viewpoint, as well as the entities that have to be updated over the network (for multiplayer games) and drawn. Calculating the PVS is expensive; Quake levels take 10 to 30 minutes to process on a four-processor Alpha, and even with speedup tweaks to the BSPer (the most effective of which was replacing many calls to <B>malloc()</B> with stack-based structures&mdash;beware of <B>malloc()</B> in performance-sensitive code), Quake 2 levels are taking up to an hour to process. (Note, however, that that includes BSPing, PVS calculations, and radiosity lighting, which I&rsquo;ll discuss later.)</P>
<P>Some good news, though, is that in the nearly two years since we got the Alpha, Pentium Pros have become as fast as that generation of Alphas, so it is now possible to calculate the PVS on an affordable machine. On the other hand, even 10 minutes of BSPing does hurt designer productivity. John has always been a big advocate of moving code out of the runtime program into utilities, and of preprocessing for performance and runtime simplicity, but even he thinks that in Quake, we may have pushed that to the point where it interfered too much with workflow. The real problem, of course, is that even a huge amount of money can&rsquo;t buy orders of magnitude more performance than commodity computers; we are getting an eight-R10000 SGI compute server, but that&rsquo;s only about twice as fast as an off-the-shelf four-processor Pentium Pro.</P>
<P>The size of the PVS for each leaf is manageable because it is stored as a bit vector, with a 1-bit for the position in the overall leaf array of each leaf that&rsquo;s visible from the current leaf. Most leaves are invisible from any one leaf, so the PVS for each leaf consists mostly of zeros, and compacts nicely with run-length encoding.</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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