GRASS 8 Programmer's Manual
8.6.0dev(2026)-f6f2c534ea
Loading...
Searching...
No Matches
findzc.c
Go to the documentation of this file.
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
*/
52
int
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
}
AMI_STREAM
Definition
ami_stream.h:153
G_math_findzc
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
TINY
#define TINY
Definition
findzc.c:30
M_PI_2
#define M_PI_2
Definition
gis.h:160
M_PI
#define M_PI
Definition
gis.h:157
stdio.h
lib
gmath
findzc.c
Generated on Sun Apr 12 2026 06:59:15 for GRASS 8 Programmer's Manual by
1.9.8