Misc/abrash-black-book
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James Gregory 2013-12-30 18:34:31 +11:00
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@ -7,13 +7,13 @@ draws across each destination scan line, near the end of the listing.
One optimization is elimination of the call to the set-pixel routine
used to draw each pixel in Listing 57.1. Function calls are expensive
operations, to be avoided when performance matters. Also, although Mode
X (the undocumented 320x240 256-color VGA mode X-Sharp runs in) doesn’t
X (the undocumented 320x240 256-color VGA mode X-Sharp runs in) doesn't
lend itself well to pixel-oriented operations like line drawing or
texture mapping, the inner loop has been set up to minimize Mode X’s
texture mapping, the inner loop has been set up to minimize Mode X's
overhead. A rotating plane mask is maintained in AL, with DX pointing to
the Map Mask register; thus, only a rotate and an **OUT** are required
to select the plane to which to write, cycling from plane 0 through
plane 3 and wrapping back to 0. Better yet, because we know that we’re
plane 3 and wrapping back to 0. Better yet, because we know that we're
simply stepping horizontally across the destination scan line, we can
use a clever optimization to both step the destination and reduce the
overhead of maintaining the mask. Two copies of the current plane mask
@ -42,7 +42,7 @@ efficient in Listing 57.1, consisting of only two adds and the macro
take four instructions apiece, and the macro hides not only conversion
from fixed-point to integer, but also a time-consuming multiplication.
Incremental approaches are excellent at avoiding multiplication, because
cumulative additions can often replace multiplication. That’s the case
cumulative additions can often replace multiplication. That's the case
with stepping through the source texture in Listing 57.2; ten
instructions, with a maximum of two branches, replace all the texture
calculations of Listing 57.1. Listing 57.2 simply detects when the
@ -55,14 +55,14 @@ worth the trouble? Indeed it is. Listing 57.2 is more than twice as fast
as Listing 57.1, and the difference is very noticeable when large,
texture-mapped areas are animated. Whether more than doubling
performance is significant is a matter of opinion, I suppose, but
imagine that you’re in William Gibson’s *Neuromancer*, trying to crack a
imagine that you're in William Gibson's *Neuromancer*, trying to crack a
corporate database. Which texture-mapping routine would you rather have
interfacing you to Cyberspace?
I’m always interested in getting your feedback on and hearing about
I'm always interested in getting your feedback on and hearing about
potential improvements to X-Sharp. Contact me through the publisher.
There is no truth to the rumor that I can be reached under the alias
“sheep-shearer,” at least not for another 9,999 sheep.
"sheep-shearer," at least not for another 9,999 sheep.
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