GRASS 8 Programmer's Manual 8.6.0dev(2026)-f6f2c534ea
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findzc.c
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1/**
2 * \file findzc.c
3 *
4 * \brief Zero Crossing functions.
5 *
6 * This program is free software; you can redistribute it and/or modify
7 * it under the terms of the GNU General Public License as published by
8 * the Free Software Foundation; either version 2 of the License, or (at
9 * your option) any later version.
10 *
11 * This program is distributed in the hope that it will be useful, but
12 * WITHOUT ANY WARRANTY; without even the implied warranty of
13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 * General Public License for more details.
15 *
16 * You should have received a copy of the GNU General Public License
17 * along with this program; if not, write to the Free Software
18 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
19 *
20 * \author GRASS Development Team
21 * \author Brad Douglas - rez at touchofmadness com
22 *
23 * \date 2006
24 */
25
26#include <stdio.h>
27#include <math.h>
28
29/** \def TINY Defined as 1.0e-3 */
30#define TINY 1.0e-3
31
32/**
33 * \fn int G_math_findzc (double conv[], int size, double zc[], double thresh,
34 * int num_orients)
35 *
36 * \brief Finds locations and orientations of zero crossings.
37 *
38 * Finds the locations and orientations of zero crossings in the input
39 * array <b>conv</b>, which is the result of the convolution of the
40 * Marr-Hildreth operator with the image. The output array is <b>zc</b>,
41 * which is non-zero only at zero crossing pixels. At those pixels, the
42 * value is 1 + (orientation), where orientation is a value from 0 to
43 * <b>num_orients</b>.
44 *
45 * \param[in] conv input
46 * \param[in] size size of largest matrix column or row
47 * \param[out] zc output
48 * \param[in] thresh magnitude threshold
49 * \param[in] num_orients
50 * \return int always returns 0
51 */
52int G_math_findzc(double conv[], int size, double zc[], double thresh,
53 int num_orients)
54{
55 int i, j, p;
56
57 /* go through entire conv image - but skip border rows and cols */
58 for (i = 1; i < size - 1; i++) {
59 for (p = i * size + 1, j = 1; j < size - 1; j++, p++) {
60 int nbr[4];
61 int ni;
62
63 /* examine the 4-neighbors of position p */
64 nbr[0] = p - 1; /* left */
65 nbr[1] = p + 1; /* right */
66 nbr[2] = p - size; /* up */
67 nbr[3] = p + size; /* down */
68
69 zc[p] = 0;
70
71 for (ni = 0; ni < 4; ni++) {
72 /* condition for a zc: sign is different than a neighbor
73 * and the absolute value is less than that neighbor.
74 * Also, threshold magnitudes to eliminate noise
75 */
76 if ((((conv[p] > 0) && (conv[nbr[ni]] < 0)) ||
77 ((conv[p] < 0) && (conv[nbr[ni]] > 0))) &&
78 (fabs(conv[p]) < fabs(conv[nbr[ni]])) &&
79 (fabs(conv[p] - conv[nbr[ni]]) > thresh)) {
80 double ang;
81 int dir;
82
83 /* found a zc here, get angle of gradient */
84 if (fabs(conv[nbr[1]] - conv[nbr[0]]) < TINY) {
85 ang = M_PI_2;
86
87 if (conv[nbr[2]] - conv[nbr[3]] < 0)
88 ang = -ang;
89 }
90 else
91 ang = atan2(conv[nbr[2]] - conv[nbr[3]],
92 conv[nbr[1]] - conv[nbr[0]]);
93
94 /* scale -PI..PI to 0..num_orients - 1 */
95 dir = num_orients * ((ang + M_PI) / (M_PI * 2.0)) + 0.4999;
96
97 /* shift scale so that 0 (not 8) is straight down */
98 dir = (3 * num_orients / 4 + dir) % num_orients;
99
100 /* add to differentiate between no zc and an orientation */
101 zc[p] = 1 + dir;
102 break; /* quit looking at neighbors */
103 }
104 } /* for ni */
105 } /* for p */
106 }
107
108 return 0;
109}
int G_math_findzc(double conv[], int size, double zc[], double thresh, int num_orients)
Finds locations and orientations of zero crossings.
Definition findzc.c:52
#define TINY
Definition findzc.c:30
#define M_PI_2
Definition gis.h:160
#define M_PI
Definition gis.h:157