GRASS GIS 7 Programmer's Manual  7.5.svn(2018)-r72990
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histo_eq.c
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1 
2 /**************************************************************
3 * Rast_histogram_eq (histo, map, min, max)
4 *
5 * struct Histogram *histo; histogram as returned by Rast_read_histogram()
6 * unsigned char **map; equalized category mapping
7 * CELL *min, *max; min,max category for map
8 *
9 * perform histogram equalization
10 * inputs are histo, output is map,min,max
11 ****************************************************************/
12 #include <grass/gis.h>
13 #include <grass/raster.h>
14 
15 void Rast_histogram_eq(const struct Histogram *histo,
16  unsigned char **map, CELL * min, CELL * max)
17 {
18  int i;
19  int x;
20  CELL cat, prev;
21  double total;
22  double sum;
23  double span;
24  int ncats;
25  long count;
26  unsigned char *xmap;
27  int len;
28  int first, last;
29 
30  ncats = Rast_get_histogram_num(histo);
31  if (ncats == 1) {
32  *min = *max = Rast_get_histogram_cat(0, histo);
33  *map = xmap = (unsigned char *)G_malloc(1);
34  *xmap = 0;
35  return;
36  }
37  if ((*min = Rast_get_histogram_cat(first = 0, histo)) == 0)
38  *min = Rast_get_histogram_cat(++first, histo);
39  if ((*max = Rast_get_histogram_cat(last = ncats - 1, histo)) == 0)
40  *max = Rast_get_histogram_cat(--last, histo);
41  len = *max - *min + 1;
42  *map = xmap = (unsigned char *)G_malloc(len);
43 
44  total = 0;
45  for (i = first; i <= last; i++) {
46  if (Rast_get_histogram_cat(i, histo) == 0)
47  continue;
48  count = Rast_get_histogram_count(i, histo);
49  if (count > 0)
50  total += count;
51  }
52  if (total <= 0) {
53  for (i = 0; i < len; i++)
54  xmap[i] = 0;
55  return;
56  }
57 
58  span = total / 256;
59 
60  sum = 0.0;
61  cat = *min - 1;
62  for (i = first; i <= last; i++) {
63  prev = cat + 1;
64  cat = Rast_get_histogram_cat(i, histo);
65  count = Rast_get_histogram_count(i, histo);
66  if (count < 0 || cat == 0)
67  count = 0;
68  x = (sum + (count / 2.0)) / span;
69  if (x < 0)
70  x = 0;
71  else if (x > 255)
72  x = 255;
73  sum += count;
74 
75  while (prev++ <= cat)
76  *xmap++ = x;
77  }
78 }
CELL cat
Definition: raster3d/cats.c:82
CELL Rast_get_histogram_cat(int n, const struct Histogram *histogram)
Returns cat for the nth element in the histogram.
Definition: histogram.c:178
#define min(x, y)
Definition: draw2.c:31
int count
#define x
#define max(x, y)
Definition: draw2.c:32
void Rast_histogram_eq(const struct Histogram *histo, unsigned char **map, CELL *min, CELL *max)
Definition: histo_eq.c:15
int CELL
Definition: gis.h:580
int Rast_get_histogram_num(const struct Histogram *histogram)
Sorts the histogram in ascending order by counts then category.
Definition: histogram.c:165
long Rast_get_histogram_count(int n, const struct Histogram *histogram)
Returns count for the nth element in the histogram.
Definition: histogram.c:195