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Answer 1

+2 A:

1) Can't you calculate scaleMod / initialDistance once while currentDistance is changing. That way you only have do that value times currentDistance, which removes a divide.

2) Make sure that this is actually the bottleneck. It most likely isn't, unless your doing something really wrong.

Dan McGrath 2010-01-27 19:45:16

Answer 2

A:

You could store scaleMod / initialDistance. When scaling is active (the user's fingers are still on the screen), multiply that value by currentDistance as needed.

Once the user has finished pinching, store the new scaleMod / initialDistance value for the next time pinching happens.

Aaron 2010-01-27 19:46:10

Answer 3

+1 A:

If you fix the scaleMod and initialDistance to powers of 2 you could use shifts for faster multiplication and division.

See here for reference.

Cesar 2010-01-27 19:54:19

Answer 4

A:

if you are doing computation with int (or other integer), see if he can do it using float precision. Floating-point divide is faster than integer (fewer bits to divide, assuming your CPU) has floating-point unit).

also, try to factor out division as multiplication by a reciprocal.

aaa 2010-01-27 19:59:50

In what universe is floating point faster than integer for a divide? Granted, there are pathological sets of floats and degenerate sets of integers for which this will be true. You seem to be making a general statement.

gary 2010-01-27 21:15:42

@gary it is generally true. Look up how floating-point numbers are represented (hint: recall division of powers). Compare instruction latency of IDIV and FDIV, http://siyobik.info/index.php?module=x86

aaa 2010-01-27 21:37:15

The latency is just pipeline overhead, and isn't useful in defining "faster" for a real application. To declare an instruction "faster" because of lower latency is like defining the winner of an auto race according to his travel time to reach the track. Sure, it has some meaning, but not much. When you want to see how an instruction performs in an actual application, you need to look at throughput. In that respect, integers are always as fast or faster.

gary 2010-02-01 21:19:22

@gary throughput is the same for integer and floating-point DIV.it may be different for SSE instructions, but I am not familiar with integer SSE instructions.

aaa 2010-02-01 21:49:20

So we agree - integers are "as fast or faster", not slower

gary 2010-02-01 23:51:53

Answer 5

+2 A:

For any type of the three vars, this calculation can easily be done millions of times per second with little performance impact. Your problem is elsewhere.

Pavel Radzivilovsky 2010-01-27 20:02:38

Answer 6

A:

Check that InitialDistance != 0 first! :)

JasonBlackwoodStone 2010-01-28 18:20:01

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