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 <P>After the PVS is marked, the BSP is walked front-to-back. At each node, the bounding box of the node&#146;s subspace is clipped against the view frustum; if the bounding box is fully clipped, then that node and all its children are ignored. Likewise, if the node is not in the PVS for the current viewpoint leaf, the node and all its children are ignored. If the bounding box is partially clipped or not clipped at all, that information is passed to the children so that any unnecessary clip tests can be avoided. The children in front of the node are then processed recursively. When a leaf is reached, polygons that touch that leaf are marked as potentially drawable. When recursion in front of a node is finished, all polygons on the front side of the node that are marked as potentially drawable are added to the edge list, and then the children on the back side of that node are similarly processed recursively.</P>
 <P>The edge list is a special, intermediate step between polygons and drawing. Each polygon is clipped, transformed, and projected, and its non-horizontal edges are added to a global list of potentially drawable edges. After all the potentially drawable edges in the world have been added, the global edge list is scanned out all at once, and all the visible spans (the nearest spans, as determined by sorting on BSP-walk order) in the world are emitted into span lists linked off the respective surface descriptors (for now, you can think of a surface as being the same as a polygon). Taken together, these spans cover every pixel on the screen once and only once, resulting in zero overdraw; surfaces that are completely hidden by nearer surfaces generate no spans at all. The spans are then drawn; all the spans for one surface are drawn, and then all the spans for the next, so that there&#146;s texture coherency between spans, which is very helpful for processor cache coherency, and also to reduce setup overhead.</P>
 <P>The primary purpose of the edge list is to make Quake&#146;s performance as level&#151;that is, as consistent&#151;as possible. Compared to simply drawing all potentially drawable polygons front-to-back, the edge list certainly slows down the best case, that is, when there&#146;s no overdraw. However, by eliminating overdraw, the worst case is helped considerably; in Quake, there&#146;s a ratio of perhaps 4:1 between worst and best case drawing time, versus the 10:1 or more that can happen with straight polygon drawing. Leveling is very important, because cases where a game slows down to the point of being unplayable dictate game and level design, and the fewer constraints placed on design, the better.</P>
-<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/70-01i.jpg"><TD WIDTH="95%"><SMALL><I>A corollary is that best case performance can be seductively misleading; it&#146;s a great feeling to see a scene running at 30 or even 60 frames per second, but if the bulk of the game runs at 15 fps, those best cases are just going to make the rest of the game look worse.</I></SMALL>
+<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/i.jpg"><TD WIDTH="95%"><SMALL><I>A corollary is that best case performance can be seductively misleading; it&#146;s a great feeling to see a scene running at 30 or even 60 frames per second, but if the bulk of the game runs at 15 fps, those best cases are just going to make the rest of the game look worse.</I></SMALL>
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