arctan2.h

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00001 /*
00002  * SpanDSP - a series of DSP components for telephony
00003  *
00004  * arctan2.h - A quick rough approximate arc tan
00005  *
00006  * Written by Steve Underwood <steveu@coppice.org>
00007  *
00008  * Copyright (C) 2003 Steve Underwood
00009  *
00010  * All rights reserved.
00011  *
00012  * This program is free software; you can redistribute it and/or modify
00013  * it under the terms of the GNU General Public License as published by
00014  * the Free Software Foundation; either version 2 of the License, or
00015  * (at your option) any later version.
00016  *
00017  * This program is distributed in the hope that it will be useful,
00018  * but WITHOUT ANY WARRANTY; without even the implied warranty of
00019  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
00020  * GNU General Public License for more details.
00021  *
00022  * You should have received a copy of the GNU General Public License
00023  * along with this program; if not, write to the Free Software
00024  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
00025  *
00026  * $Id: arctan2.h,v 1.5 2005/11/28 13:43:34 steveu Exp $
00027  */
00028 
00029 /*! \file */
00030 
00031 #if !defined(_ARCTAN2_H_)
00032 #define _ARCTAN2_H_
00033 
00034 /*! \page arctan2_page Fast approximate four quadrant arc-tangent
00035 \section arctan2_page_sec_1 What does it do?
00036 This module provides a fast approximate 4-quadrant arc tangent function,
00037 based on something at dspguru.com. The worst case error is about 4.07 degrees.
00038 This is fine for many "where am I" type evaluations in comms. work.
00039 
00040 \section arctan2_page_sec_2 How does it work?
00041 ???.
00042 */
00043 
00044 #ifdef __cplusplus
00045 extern "C" {
00046 #endif
00047 
00048 /* This returns its answer as a signed 32 bit integer phase value. */
00049 static __inline__ int32_t arctan2(float y, float x)
00050 {
00051     float abs_y;
00052     float angle;
00053 
00054     if (x == 0.0  ||  y == 0.0)
00055         return 0;
00056     
00057     abs_y = fabsf(y);
00058 
00059     /* If we are in quadrant II or III, flip things around */
00060     if (x < 0.0)
00061         angle = 3.0 - (x + abs_y)/(abs_y - x);
00062     else
00063         angle = 1.0 - (x - abs_y)/(abs_y + x);
00064     angle *= 536870912.0;
00065 
00066     /* If we are in quadrant III or IV, negate to return an
00067        answer in the range +-pi */
00068     if (y < 0.0)
00069         angle = -angle;
00070     return (int32_t) angle;
00071 }
00072 /*- End of function --------------------------------------------------------*/
00073 
00074 #ifdef __cplusplus
00075 }
00076 #endif
00077 
00078 #endif
00079 /*- End of file ------------------------------------------------------------*/

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