GRASS GIS 8 Programmer's Manual 8.2.1RC1(2022)-exported
findzc.c
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1
2/**
3 * \file findzc.c
4 *
5 * \brief Zero Crossing functions.
6 *
7 * This program is free software; you can redistribute it and/or modify
8 * it under the terms of the GNU General Public License as published by
9 * the Free Software Foundation; either version 2 of the License, or (at
10 * your option) any later version.
11 *
12 * This program is distributed in the hope that it will be useful, but
13 * WITHOUT ANY WARRANTY; without even the implied warranty of
14 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15 * General Public License for more details.
16 *
17 * You should have received a copy of the GNU General Public License
18 * along with this program; if not, write to the Free Software
19 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
20 *
21 * \author GRASS GIS Development Team
22 * \author Brad Douglas - rez at touchofmadness com
23 *
24 * \date 2006
25 */
26
27#include <stdio.h>
28#include <math.h>
29
30
31/** \def TINY Defined as 1.0e-3 */
32#define TINY 1.0e-3
33
34
35/**
36 * \fn int G_math_findzc (double conv[], int size, double zc[], double thresh, int num_orients)
37 *
38 * \brief Finds locations and orientations of zero crossings.
39 *
40 * Finds the locations and orientations of zero crossings in the input
41 * array <b>conv</b>, which is the result of the convolution of the
42 * Marr-Hildreth operator with the image. The output array is <b>zc</b>,
43 * which is non-zero only at zero crossing pixels. At those pixels, the
44 * value is 1 + (orientation), where orientation is a value from 0 to
45 * <b>num_orients</b>.
46 *
47 * \param[in] conv input
48 * \param[in] size size of largest matrix column or row
49 * \param[out] zc output
50 * \param[in] thresh magnitude threshold
51 * \param[in] num_orients
52 * \return int always returns 0
53 */
54
55int
56G_math_findzc(double conv[], int size, double zc[], double thresh,
57 int num_orients)
58{
59 int i, j, p;
60
61 /* go through entire conv image - but skip border rows and cols */
62 for (i = 1; i < size - 1; i++) {
63 for (p = i * size + 1, j = 1; j < size - 1; j++, p++) {
64 int nbr[4];
65 int ni;
66
67 /* examine the 4-neighbors of position p */
68 nbr[0] = p - 1; /* left */
69 nbr[1] = p + 1; /* right */
70 nbr[2] = p - size; /* up */
71 nbr[3] = p + size; /* down */
72
73 zc[p] = 0;
74
75 for (ni = 0; ni < 4; ni++) {
76 /* condition for a zc: sign is different than a neighbor
77 * and the absolute value is less than that neighbor.
78 * Also, threshold magnitudes to eliminate noise
79 */
80 if ((((conv[p] > 0) && (conv[nbr[ni]] < 0)) ||
81 ((conv[p] < 0) && (conv[nbr[ni]] > 0))) &&
82 (fabs(conv[p]) < fabs(conv[nbr[ni]])) &&
83 (fabs(conv[p] - conv[nbr[ni]]) > thresh)) {
84 double ang;
85 int dir;
86
87 /* found a zc here, get angle of gradient */
88 if (fabs(conv[nbr[1]] - conv[nbr[0]]) < TINY) {
89 ang = M_PI_2;
90
91 if (conv[nbr[2]] - conv[nbr[3]] < 0)
92 ang = -ang;
93 }
94 else
95 ang = atan2(conv[nbr[2]] - conv[nbr[3]],
96 conv[nbr[1]] - conv[nbr[0]]);
97
98 /* scale -PI..PI to 0..num_orients - 1 */
99 dir =
100 num_orients * ((ang + M_PI) / (M_PI * 2.0)) + 0.4999;
101
102 /* shift scale so that 0 (not 8) is straight down */
103 dir = (3 * num_orients / 4 + dir) % num_orients;
104
105 /* add to differentiate between no zc and an orientation */
106 zc[p] = 1 + dir;
107 break; /* quit looking at neighbors */
108 }
109 } /* for ni */
110 } /* for p */
111 }
112
113 return 0;
114}
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:56
#define TINY
Definition: findzc.c:32