Misc/abrash-black-book
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James Gregory 2013-12-30 12:25:34 +11:00
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<P>One point I&#146;d like to make is that although the system-memory buffer in Listing 45.1 has exactly the same dimensions as the screen bitmap, that&#146;s not a requirement, and there are some good reasons not to make the two the same size. For example, if the system buffer is bigger than the area displayed on the screen, it&#146;s possible to pan the visible area around the system buffer. Or, alternatively, the system buffer can be just the size of a desired window, representing a window into a larger, virtual buffer. We could then draw the desired portion of the virtual bitmap into the system-memory buffer, then copy the buffer to the screen, and the effect will be of having panned the window to the new location.
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<TABLE WIDTH="100%"><TD VALIGN="TOP" ALIGN="LEFT" WIDTH="5%"><IMG SRC="images/45-02i.jpg"><TD VALIGN="TOP" ALIGN="LEFT" WIDTH="95%"><SMALL><I>Another argument in favor of a small viewing window is that it restricts the amount of display memory actually drawn to. Restricting the display memory used for animation reduces the total number of display-memory accesses, which in turn boosts overall performance; it also improves the performance and appearance of panning, in which the whole window has to be redrawn or copied.</I></SMALL>
<TABLE WIDTH="100%"><TD VALIGN="TOP" ALIGN="LEFT" WIDTH="5%"><IMG SRC="images/i.jpg"><TD VALIGN="TOP" ALIGN="LEFT" WIDTH="95%"><SMALL><I>Another argument in favor of a small viewing window is that it restricts the amount of display memory actually drawn to. Restricting the display memory used for animation reduces the total number of display-memory accesses, which in turn boosts overall performance; it also improves the performance and appearance of panning, in which the whole window has to be redrawn or copied.</I></SMALL>
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<P>If you keep a close watch, you&#146;ll notice that many high-performance animation games similarly restrict their full-featured animation area to a relatively small region. Often, it&#146;s hard to tell that this is the case, because the animation region is surrounded by flashy digitized graphics and by items such as scoreboards and status screens, but look closely and see if the animation region in your favorite game isn&#146;t smaller than you thought.
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