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| 1892 | serge | 1 | /* cairo - a vector graphics library with display and print output |
| 2 | * |
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| 3 | * Copyright © 2002 University of Southern California |
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| 4 | * |
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| 5 | * This library is free software; you can redistribute it and/or |
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| 6 | * modify it either under the terms of the GNU Lesser General Public |
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| 7 | * License version 2.1 as published by the Free Software Foundation |
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| 8 | * (the "LGPL") or, at your option, under the terms of the Mozilla |
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| 9 | * Public License Version 1.1 (the "MPL"). If you do not alter this |
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| 10 | * notice, a recipient may use your version of this file under either |
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| 11 | * the MPL or the LGPL. |
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| 12 | * |
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| 13 | * You should have received a copy of the LGPL along with this library |
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| 14 | * in the file COPYING-LGPL-2.1; if not, write to the Free Software |
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| 15 | * Foundation, Inc., 51 Franklin Street, Suite 500, Boston, MA 02110-1335, USA |
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| 16 | * You should have received a copy of the MPL along with this library |
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| 17 | * in the file COPYING-MPL-1.1 |
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| 18 | * |
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| 19 | * The contents of this file are subject to the Mozilla Public License |
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| 20 | * Version 1.1 (the "License"); you may not use this file except in |
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| 21 | * compliance with the License. You may obtain a copy of the License at |
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| 22 | * http://www.mozilla.org/MPL/ |
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| 23 | * |
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| 24 | * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY |
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| 25 | * OF ANY KIND, either express or implied. See the LGPL or the MPL for |
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| 26 | * the specific language governing rights and limitations. |
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| 27 | * |
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| 28 | * The Original Code is the cairo graphics library. |
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| 29 | * |
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| 30 | * The Initial Developer of the Original Code is University of Southern |
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| 31 | * California. |
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| 32 | * |
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| 33 | * Contributor(s): |
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| 34 | * Carl D. Worth |
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| 35 | */ |
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| 36 | |||
| 37 | #include "cairoint.h" |
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| 38 | |||
| 39 | #include "cairo-slope-private.h" |
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| 40 | |||
| 41 | /* Compare two slopes. Slope angles begin at 0 in the direction of the |
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| 42 | positive X axis and increase in the direction of the positive Y |
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| 43 | axis. |
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| 44 | |||
| 45 | This function always compares the slope vectors based on the |
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| 46 | smaller angular difference between them, (that is based on an |
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| 47 | angular difference that is strictly less than pi). To break ties |
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| 48 | when comparing slope vectors with an angular difference of exactly |
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| 49 | pi, the vector with a positive dx (or positive dy if dx's are zero) |
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| 50 | is considered to be more positive than the other. |
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| 51 | |||
| 52 | Also, all slope vectors with both dx==0 and dy==0 are considered |
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| 53 | equal and more positive than any non-zero vector. |
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| 54 | |||
| 55 | < 0 => a less positive than b |
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| 56 | == 0 => a equal to b |
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| 57 | > 0 => a more positive than b |
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| 58 | */ |
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| 59 | int |
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| 60 | _cairo_slope_compare (const cairo_slope_t *a, const cairo_slope_t *b) |
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| 61 | { |
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| 62 | cairo_int64_t ady_bdx = _cairo_int32x32_64_mul (a->dy, b->dx); |
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| 63 | cairo_int64_t bdy_adx = _cairo_int32x32_64_mul (b->dy, a->dx); |
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| 64 | int cmp; |
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| 65 | |||
| 66 | cmp = _cairo_int64_cmp (ady_bdx, bdy_adx); |
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| 67 | if (cmp) |
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| 68 | return cmp; |
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| 69 | |||
| 70 | /* special-case zero vectors. the intended logic here is: |
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| 71 | * zero vectors all compare equal, and more positive than any |
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| 72 | * non-zero vector. |
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| 73 | */ |
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| 74 | if (a->dx == 0 && a->dy == 0 && b->dx == 0 && b->dy ==0) |
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| 75 | return 0; |
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| 76 | if (a->dx == 0 && a->dy == 0) |
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| 77 | return 1; |
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| 78 | if (b->dx == 0 && b->dy ==0) |
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| 79 | return -1; |
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| 80 | |||
| 81 | /* Finally, we're looking at two vectors that are either equal or |
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| 82 | * that differ by exactly pi. We can identify the "differ by pi" |
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| 83 | * case by looking for a change in sign in either dx or dy between |
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| 84 | * a and b. |
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| 85 | * |
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| 86 | * And in these cases, we eliminate the ambiguity by reducing the angle |
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| 87 | * of b by an infinitesimally small amount, (that is, 'a' will |
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| 88 | * always be considered less than 'b'). |
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| 89 | */ |
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| 90 | if ((a->dx ^ b->dx) < 0 || (a->dy ^ b->dy) < 0) { |
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| 91 | if (a->dx > 0 || (a->dx == 0 && a->dy > 0)) |
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| 3959 | Serge | 92 | return -1; |
| 93 | else |
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| 1892 | serge | 94 | return +1; |
| 95 | } |
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| 96 | |||
| 97 | /* Finally, for identical slopes, we obviously return 0. */ |
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| 98 | return 0; |
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| 99 | } |