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Old 07-22-2007, 03:35 PM   #1
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Join Date: Oct 2006
Posts: 38
Default Inverse Square Root

I'm developing a physics engine based somewhat on Flade and the Tony Pa Vector tutorials (they Google well so I don't think I need to link them).

I've ported the engine to Java and found an inverse square root method that was originally written for Quake 3 and optimised by Chris Lomont. This allows you to calculate vector normals much faster.

Here's my Java version:
// Original C code by Chris Lomont <http://www.lomont.org/Math/Papers/2003/InvSqrt.pdf>
  static float invSqrt(float x){
    float xhalf = 0.5f*x;
    int i = Float.floatToIntBits(x); // get bits for floating value
    i = 0x5f375a86- (i>>1); // gives initial guess y0
    x = Float.intBitsToFloat(i); // convert bits back to float
    x = x*(1.5f-xhalf*x*x); // Newton step, repeating increases accuracy
    return x;
My trouble here is before I can test it for speed in Flash I need a method for getting the floating point value of x in and out of integer bit format. Can anyone help?
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Old 11-18-2007, 06:07 PM   #2
daniele guido
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Join Date: Nov 2007
Posts: 1

I know it's a bit late, but..
I think who's searching for inverted sqrt would find it interesting:
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