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  Fast Math: atan2 lookup table  (Read 2334 times)
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Offline Riven
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« JGO Overlord »


Medals: 1220
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Hand over your head.


« Posted 2011-05-09 20:46:10 »

Math.atan2() is very convenient, yet very slow.
A lookup table will be roughly 8x faster.
You lose accuracy, but heck, that's obvious.

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   public static final float atan2Deg(float y, float x)
   {
      return FastMath.atan2(y, x) * DEG;
   }

   public static final float atan2DegStrict(float y, float x)
   {
      return (float) Math.atan2(y, x) * DEG;
   }

   public static final float atan2(float y, float x)
   {
      float add, mul;

      if (x < 0.0f)
      {
         if (y < 0.0f)
         {
            x = -x;
            y = -y;

            mul = 1.0f;
         }
         else
         {
            x = -x;
            mul = -1.0f;
         }

         add = -3.141592653f;
      }
      else
      {
         if (y < 0.0f)
         {
            y = -y;
            mul = -1.0f;
         }
         else
         {
            mul = 1.0f;
         }

         add = 0.0f;
      }

      float invDiv = ATAN2_DIM_MINUS_1 / ((x < y) ? y : x);

      int xi = (int) (x * invDiv);
      int yi = (int) (y * invDiv);

      return (atan2[yi * ATAN2_DIM + xi] + add) * mul;
   }


   private static final int     ATAN2_BITS        = 7;

   private static final int     ATAN2_BITS2       = ATAN2_BITS << 1;
   private static final int     ATAN2_MASK        = ~(-1 << ATAN2_BITS2);
   private static final int     ATAN2_COUNT       = ATAN2_MASK + 1;
   private static final int     ATAN2_DIM         = (int) Math.sqrt(ATAN2_COUNT);

   private static final float   ATAN2_DIM_MINUS_1 = (ATAN2_DIM - 1);

   private static final float[] atan2             = new float[ATAN2_COUNT];

   static
   {
      for (int i = 0; i < ATAN2_DIM; i++)
      {
         for (int j = 0; j < ATAN2_DIM; j++)
         {
            float x0 = (float) i / ATAN2_DIM;
            float y0 = (float) j / ATAN2_DIM;

            atan2[j * ATAN2_DIM + i] = (float) Math.atan2(y0, x0);
         }
      }
   }

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