Path: chuka.playstation.co.uk!news
From: "Tom" <Tom.Madams@cwcom.net>
Newsgroups: scee.yaroze.programming.2d_graphics
Subject: Re: Inverse Tangent
Date: Thu, 23 Sep 1999 18:53:50 +0100
Organization: PlayStation Net Yaroze (SCEE)
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Message-ID: <7sdpff$7b1@chuka.playstation.co.uk>
References: <7satfu$6r1@chuka.playstation.co.uk>
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Hi Matt,

    I hope this works. We've just got a network set up in our house and this
is my first stab at sending a post via the file server.

Anyway, the answer to your question is that there isn't one (a function I
mean, not an answer). I found some code from someone elses demo (sorry
whoever you are I can't remember where I got it from) which can either be
found in math.c of my latest project, or listed down below. The original
code cocked up when the angle was exactly 45 degrees, but I think this works
in all cases.

Cheers,

    Tom


// table for inverse tangent (of form 512 = 90 degrees)
short InvTanLut[] = {
   0,   3,   5,   8,  10,  13,  15,  18,  20,  23,  25,  28,  31,  33,  36,
38,
  41,  43,  46,  48,  51,  53,  56,  58,  61,  63,  66,  69,  71,  74,  76,
79,
  81,  84,  86,  89,  91,  94,  96,  99, 101, 104, 106, 108, 111, 113, 116,
118,
 121, 123, 126, 128, 131, 133, 136, 138, 140, 143, 145, 148, 150, 152, 155,
157,
 160, 162, 164, 167, 169, 172, 174, 176, 179, 181, 183, 186, 188, 190, 193,
195,
 197, 200, 202, 204, 207, 209, 211, 214, 216, 218, 220, 223, 225, 227, 229,
232,
 234, 236, 238, 241, 243, 245, 247, 249, 252, 254, 256, 258, 260, 262, 265,
267,
 269, 271, 273, 275, 277, 279, 282, 284, 286, 288, 290, 292, 294, 296, 298,
300,
 302, 304, 306, 308, 310, 312, 314, 316, 318, 320, 322, 324, 326, 328, 330,
332,
 334, 336, 338, 340, 342, 344, 346, 347, 349, 351, 353, 355, 357, 359, 360,
362,
 364, 366, 368, 370, 371, 373, 375, 377, 379, 380, 382, 384, 386, 387, 389,
391,
 393, 394, 396, 398, 399, 401, 403, 405, 406, 408, 410, 411, 413, 415, 416,
418,
 419, 421, 423, 424, 426, 428, 429, 431, 432, 434, 435, 437, 439, 440, 442,
443,
 445, 446, 448, 449, 451, 452, 454, 455, 457, 458, 460, 461, 463, 464, 466,
467,
 469, 470, 471, 473, 474, 476, 477, 479, 480, 481, 483, 484, 486, 487, 488,
490,
 491, 492, 494, 495, 496, 498, 499, 500, 502, 503, 504, 506, 507, 508, 509,
511
};

// A modified version of some code that I found
long InvTan(long x, long y) {
 long t;

 if (x==0 && y==0)
  return 0;

 if (Abs(x) == Abs(y)) {
  if (x > 0)
   return (y<0 ? 3584 : 512);
  return (y<0 ? 2560 : 1536);
 }

 if (Abs(x) > Abs(y)) {
  t = (y<<8)/x;

  if (t>=0)
   return InvTanLut[t&255] + (x<0 ? 2048 : 0);

  return (x<0 ? 2048 : 4096) - InvTanLut[(-t)&255];
 }

 t = (x<<8)/y;
 if (t>=0)
  return (y<0 ? 3072 : 1024) - InvTanLut[t&255];

 return InvTanLut[(-t) & 255] + (y < 0 ? 3072 : 1024);
}