PocketMatrix

I've noticed a few people stating that recip tables are faster than doing a divide. While I understand the obvious here, I would have thought that the cache hit would negate much of the advantage.

What size recip tables are people using? Is there a routine out there that uses a small recip table and then some code to get a high bit answer? We usually are looking for about 14->16 bits for the recip. ie;

long invW= ((1<<30)/w)<<2; //invW is 16:16
verts->x=imul16(verts->x*invW);
etc...

Actually, everthing is done is ASM to take advantage of the 32x32->64 bit multiply (and accumulate) functions of the ARM.... but you get the idea.

Cheers,

rcp

a simple use of logarithms in optimization is to make use of this property:

log(a*b) = log(a) + log(b)

That way, you can code a multiplication (or division using log(a) - log(b)) with 3 table lookups. One for log(a), one for log(b) and one to execute the exponential function.

The advantage of such a system is that you can lookup any multiplication between two integers between 0 and n with log tables that have only n entries.

cryo