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Deduped images

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James Gregory 2013-12-30 12:25:34 +11:00
commit 9e52de9586
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49-03.html
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@ -134,7 +134,7 @@ typedef struct {
<P>Listings 49.1 and 49.2, like all Mode X code I&#146;ve presented, perform no clipping, because clipping code would complicate the listings too much. While clipping can be implemented directly in the low-level Mode X routines (at the beginning of Listing 49.1, for instance), another, potentially simpler approach would be to perform clipping at a higher level, modifying the coordinates and dimensions passed to low-level routines such as Listings 49.1 and 49.2 as necessary to accomplish the desired clipping. It is for precisely this reason that the low-level Mode X routines support programmable start coordinates in the source images, rather than assuming (0,0); likewise for the distinction between the width of the image and the width of the area of the image to draw.
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<P>Also, it would be more efficient to make up structures that describe the source and destination bitmaps, with dimensions and coordinates built in, and simply pass pointers to these structures to the low level, rather than passing many separate parameters, as is now the case. I&#146;ve used separate parameters for simplicity and flexibility.</P>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/49-01i.jpg"><TD WIDTH="95%"><SMALL><I>Be aware that as nifty as Mode X hardware-assisted masked copying is, whether or not it&#146;s actually faster than software-only masked or transparent copying depends upon the processor and the video adapter. The advantage of Mode X masked copying is the 32-bit parallelism; the disadvantages are the need to read display memory and the need to perform an <B>OUT</B> for every four pixels. (<B>OUT</B> is a slow 486/Pentium instruction, and most VGAs respond to <B>OUT</B>s much more slowly than to display memory writes.)</I></SMALL>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/i.jpg"><TD WIDTH="95%"><SMALL><I>Be aware that as nifty as Mode X hardware-assisted masked copying is, whether or not it&#146;s actually faster than software-only masked or transparent copying depends upon the processor and the video adapter. The advantage of Mode X masked copying is the 32-bit parallelism; the disadvantages are the need to read display memory and the need to perform an <B>OUT</B> for every four pixels. (<B>OUT</B> is a slow 486/Pentium instruction, and most VGAs respond to <B>OUT</B>s much more slowly than to display memory writes.)</I></SMALL>
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