Line data    Source code

       1             : /* cairo - a vector graphics library with display and print output
       2             :  *
       3             :  * Copyright © 2002 University of Southern California
       4             :  *
       5             :  * This library is free software; you can redistribute it and/or
       6             :  * modify it either under the terms of the GNU Lesser General Public
       7             :  * License version 2.1 as published by the Free Software Foundation
       8             :  * (the "LGPL") or, at your option, under the terms of the Mozilla
       9             :  * Public License Version 1.1 (the "MPL"). If you do not alter this
      10             :  * notice, a recipient may use your version of this file under either
      11             :  * the MPL or the LGPL.
      12             :  *
      13             :  * You should have received a copy of the LGPL along with this library
      14             :  * in the file COPYING-LGPL-2.1; if not, write to the Free Software
      15             :  * Foundation, Inc., 51 Franklin Street, Suite 500, Boston, MA 02110-1335, USA
      16             :  * You should have received a copy of the MPL along with this library
      17             :  * in the file COPYING-MPL-1.1
      18             :  *
      19             :  * The contents of this file are subject to the Mozilla Public License
      20             :  * Version 1.1 (the "License"); you may not use this file except in
      21             :  * compliance with the License. You may obtain a copy of the License at
      22             :  * http://www.mozilla.org/MPL/
      23             :  *
      24             :  * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY
      25             :  * OF ANY KIND, either express or implied. See the LGPL or the MPL for
      26             :  * the specific language governing rights and limitations.
      27             :  *
      28             :  * The Original Code is the cairo graphics library.
      29             :  *
      30             :  * The Initial Developer of the Original Code is University of Southern
      31             :  * California.
      32             :  *
      33             :  * Contributor(s):
      34             :  *      Carl D. Worth <cworth@cworth.org>
      35             :  */
      36             : 
      37             : #include "cairoint.h"
      38             : 
      39             : #include "cairo-slope-private.h"
      40             : 
      41             : /* Compare two slopes. Slope angles begin at 0 in the direction of the
      42             :    positive X axis and increase in the direction of the positive Y
      43             :    axis.
      44             : 
      45             :    This function always compares the slope vectors based on the
      46             :    smaller angular difference between them, (that is based on an
      47             :    angular difference that is strictly less than pi). To break ties
      48             :    when comparing slope vectors with an angular difference of exactly
      49             :    pi, the vector with a positive dx (or positive dy if dx's are zero)
      50             :    is considered to be more positive than the other.
      51             : 
      52             :    Also, all slope vectors with both dx==0 and dy==0 are considered
      53             :    equal and more positive than any non-zero vector.
      54             : 
      55             :    <  0 => a less positive than b
      56             :    == 0 => a equal to b
      57             :    >  0 => a more positive than b
      58             : */
      59             : int
      60           0 : _cairo_slope_compare (const cairo_slope_t *a, const cairo_slope_t *b)
      61             : {
      62           0 :     cairo_int64_t ady_bdx = _cairo_int32x32_64_mul (a->dy, b->dx);
      63           0 :     cairo_int64_t bdy_adx = _cairo_int32x32_64_mul (b->dy, a->dx);
      64             :     int cmp;
      65             : 
      66           0 :     cmp = _cairo_int64_cmp (ady_bdx, bdy_adx);
      67           0 :     if (cmp)
      68           0 :         return cmp;
      69             : 
      70             :     /* special-case zero vectors.  the intended logic here is:
      71             :      * zero vectors all compare equal, and more positive than any
      72             :      * non-zero vector.
      73             :      */
      74           0 :     if (a->dx == 0 && a->dy == 0 && b->dx == 0 && b->dy ==0)
      75           0 :         return 0;
      76           0 :     if (a->dx == 0 && a->dy == 0)
      77           0 :         return 1;
      78           0 :     if (b->dx == 0 && b->dy ==0)
      79           0 :         return -1;
      80             : 
      81             :     /* Finally, we're looking at two vectors that are either equal or
      82             :      * that differ by exactly pi. We can identify the "differ by pi"
      83             :      * case by looking for a change in sign in either dx or dy between
      84             :      * a and b.
      85             :      *
      86             :      * And in these cases, we eliminate the ambiguity by reducing the angle
      87             :      * of b by an infinitesimally small amount, (that is, 'a' will
      88             :      * always be considered less than 'b').
      89             :      */
      90           0 :     if ((a->dx ^ b->dx) < 0 || (a->dy ^ b->dy) < 0) {
      91           0 :         if (a->dx > 0 || (a->dx == 0 && a->dy > 0))
      92           0 :             return +1;
      93             :         else
      94           0 :             return -1;
      95             :     }
      96             : 
      97             :     /* Finally, for identical slopes, we obviously return 0. */
      98           0 :     return 0;
      99             : }