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James Gregory 2013-12-30 18:34:31 +11:00
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@ -11,13 +11,13 @@ regarding range and precision. Floating-point is much easier—and, with
the Pentium generation of processors, is generally comparable in speed.
In fact, for some operations, such as multiplication in general and
division when the floating-point unit is in single-precision mode,
floating-point is much faster. Check out Chris Hecker’s column in the
floating-point is much faster. Check out Chris Hecker's column in the
February 1996 *Game Developer* for an interesting discussion along these
lines.
Second, the planes that form the frustum are shifted ever so slightly
inward from their proper positions at the edge of the field of view.
This guarantees that it’s never possible to generate a visible vertex
This guarantees that it's never possible to generate a visible vertex
exactly at the eyepoint, averting the divide-by-zero error that such a
vertex would cause when projected and at no performance cost.
@ -40,12 +40,12 @@ objects began to warp visibly after many rotations.
Fourth, Listing 65.3 processes each input polygon into a clipped
polygon, one line segment at a time. It would be more efficient to
process all the vertices, categorizing whether and how they’re clipped,
process all the vertices, categorizing whether and how they're clipped,
and then perform a test such as the Cohen-Sutherland outcode test to
detect trivial acceptance (the polygon is entirely inside) and sometimes
trivial rejection (the polygon is fully outside) without ever dealing
with the edges, and to identify which planes actually need to be clipped
against, as discussed in “Line-Segment Clipping Revisited,” *Dr. Dobb’s
against, as discussed in "Line-Segment Clipping Revisited," *Dr. Dobb's
Journal*, January 1996. Some clipping approaches also minimize the
number of intersection calculations when a segment is clipped by
multiple planes. Further, Listing 65.3 clips a polygon against each
@ -88,7 +88,7 @@ For a game, 90 degrees is a pretty good field of view, but can we get
the same sort of efficient clipping if we need some other field of view?
Sure. All we have to do is scale the x and y results of the
world-to-view transformation to account for the field of view, so that
the coordinates lie in a viewspace that’s normalized such that the
the coordinates lie in a viewspace that's normalized such that the
frustum planes extend along lines of x==z and y==z. The resulting
visible projected points span the range -1 to 1 (before scaling up to
get pixel coordinates), just as with a 90-degree field of view, so the
@ -96,27 +96,27 @@ rest of the drawing pipeline remains unchanged. Better yet, there is no
cost in performance because the adjustment can be added to the
transformation matrix.
I didn’t implement normalized clipping in Listing 65.3 because I wanted
I didn't implement normalized clipping in Listing 65.3 because I wanted
to illustrate the general 3-D clipping mechanism without additional
complications, and because for many applications the dot product (which,
after all, takes only 10-20 cycles on a Pentium) is sufficient. However,
the more frustum clipping you’re doing, especially if most of the
the more frustum clipping you're doing, especially if most of the
polygons are trivially visible, the more attractive the performance
advantages of normalized clipping become.
### Further Reading {#Heading9}
You now have the basics of 3-D clipping, but because fast clipping is
central to high-performance 3-D, there’s a lot more to be learned. One
central to high-performance 3-D, there's a lot more to be learned. One
good place for further reading is Foley and van Dam; another is
*Procedural Elements of Computer Graphics*, by David F. Rogers. Read and
understand either of these books, and you’ll know everything you need
understand either of these books, and you'll know everything you need
for world-class clipping.
And, as you read, you might take a moment to consider how wonderful it
is that anyone who’s interested can tap into so much expert knowledge
is that anyone who's interested can tap into so much expert knowledge
for the price of a book—or, on the Internet, for free—with no strings
attached. Our part of the world is a pretty good place right now, isn’t
attached. Our part of the world is a pretty good place right now, isn't
it?
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