Question

right now i have a bottle neck in my program I m trying to write. i am trying to use a pinch gesture to control the scale of a UIImage. its the calculation of the scale that is causing the program to slow down and become choppy. below is the equation.

currentScale = (currentDistance / initialDistance) * scaleMod;

scaleMod is what ever the current scale was the user took their fingers off the screen. so the next time the user does a pinch the old scale is essentially the starting point of the new scaling action.

Answer 1

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.

Answer 2

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.

Answer 3

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

See here for reference.

Answer 4

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.

Answer 5

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.

Answer 6

Check that InitialDistance != 0 first! :)

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