This doesn't mean that 3D geometry can't be accelerated efficiently in
the vector unit -- even small dot products! Naturally scaling by 4x
(or better!) is easily possible. You "just" need to organize your code
to take advantage of economies of scale. That is, process a bunch of
vertices at once. Unfortunately, that usually means taking a giant
wrecking ball to your application core data structures in order to
solve the structural problems that are holding the vector unit back.
Namely, replace packed data structures such as this:
/*
NON-ACCELERATED EXAMPLE
A Cartesian vector structure and convenience make function.
Let's also add a simple function to calculate the length
of the vector.
*/
typedef struct _Vector
{
float i;
float j;
float k;
} Vector;
...with planar array representations something like this:
#define kMyVectorSize 16 /* should be a multiple of 4 */
This means grouping many vertices together in the same structure. This
can cause its own problems in some cases. For example, common
optimizations like just calculating points that fall in the view
frustrum might have to be thrown out. You'll need to proceed
judiciously here. On the other hand, it can often do wonderful things
for your cache organization (c.f. Judy trees), if you can identify
sets of vertices that "go together", for example, the set of vertices
in an avatar's leg, or 5 consecutive amino acids in a protein. These
are likely to be found near each other, and are therefore likely
subject to similar sets of operations, so can usually be treated as a
single unit.
In any case, once you have your {i,j,k} or {x,y,z} or what-have-yous
in separate arrays, the vector arithmetic starts to look a lot like
the scalar arithmetic done wider, and should speed up by approximately
a factor of 4 on G5/Core 2.
#include <Accelerate/Accelerate.h>
// Calculate the distance of 4 vertices from the origin
vFloat VectorLength( vFloat vi, vFloat vj, vFloat vk )
{
return vsqrtf( vi * vi + vj * vj + vk * vk );
}
or maybe like this for more than four vertices at a time (usually
somewhat more efficient):
< to be continued >
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