Color quantization/C

From Rosetta Code
Quantum frog dithered.png
This is a complete program that takes a PPM P6 image and a number, then writes out the image reduced to the number of colors to out.ppm. There is optional dithering, too, which doesn't make a whole lot of difference with say 64 colors or more. And with low colors, the quantization did such a good job of picking average colors that it actually hurts the dithering process.
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <ctype.h>
#include <string.h>
#include <stdint.h>
#include <math.h>
 
typedef struct {
int w, h;
unsigned char *pix;
} image_t, *image;
 
int write_ppm(image im, char *fn)
{
FILE *fp = fopen(fn, "wb");
if (!fp) return 0;
fprintf(fp, "P6\n%d %d\n255\n", im->w, im->h);
fwrite(im->pix, 1, 3 * im->w * im->h, fp);
fclose(fp);
return 1;
}
 
image img_new(int w, int h)
{
image im = malloc(sizeof(image_t) + h * w * 3);
im->w = w; im->h = h;
im->pix = (unsigned char *)(im + 1);
return im;
}
 
int read_num(FILE *f)
{
int n;
while (!fscanf(f, "%d ", &n)) {
if ((n = fgetc(f)) == '#') {
while ((n = fgetc(f)) != '\n')
if (n == EOF) return 0;
} else
return 0;
}
return n;
}
 
image read_ppm(char *fn)
{
FILE *fp = fopen(fn, "rb");
int w, h, maxval;
image im = 0;
if (!fp) return 0;
 
if (fgetc(fp) != 'P' || fgetc(fp) != '6' || !isspace(fgetc(fp)))
goto bail;
 
w = read_num(fp);
h = read_num(fp);
maxval = read_num(fp);
if (!w || !h || !maxval) goto bail;
 
im = img_new(w, h);
fread(im->pix, 1, 3 * w * h, fp);
bail:
if (fp) fclose(fp);
return im;
}
 
 
#define ON_INHEAP 1
 
typedef struct oct_node_t oct_node_t, *oct_node;
struct oct_node_t{
int64_t r, g, b; /* sum of all child node colors */
int count, heap_idx;
unsigned char n_kids, kid_idx, flags, depth;
oct_node kids[8], parent;
};
 
typedef struct {
int alloc, n;
oct_node* buf;
} node_heap;
 
inline int cmp_node(oct_node a, oct_node b)
{
if (a->n_kids < b->n_kids) return -1;
if (a->n_kids > b->n_kids) return 1;
 
int ac = a->count >> a->depth;
int bc = b->count >> b->depth;
return ac < bc ? -1 : ac > bc;
}
 
void down_heap(node_heap *h, oct_node p)
{
int n = p->heap_idx, m;
while (1) {
m = n * 2;
if (m >= h->n) break;
if (m + 1 < h->n && cmp_node(h->buf[m], h->buf[m + 1]) > 0) m++;
 
if (cmp_node(p, h->buf[m]) <= 0) break;
 
h->buf[n] = h->buf[m];
h->buf[n]->heap_idx = n;
n = m;
}
h->buf[n] = p;
p->heap_idx = n;
}
 
void up_heap(node_heap *h, oct_node p)
{
int n = p->heap_idx;
oct_node prev;
 
while (n > 1) {
prev = h->buf[n / 2];
if (cmp_node(p, prev) >= 0) break;
 
h->buf[n] = prev;
prev->heap_idx = n;
n /= 2;
}
h->buf[n] = p;
p->heap_idx = n;
}
 
void heap_add(node_heap *h, oct_node p)
{
if ((p->flags & ON_INHEAP)) {
down_heap(h, p);
up_heap(h, p);
return;
}
 
p->flags |= ON_INHEAP;
if (!h->n) h->n = 1;
if (h->n >= h->alloc) {
while (h->n >= h->alloc) h->alloc += 1024;
h->buf = realloc(h->buf, sizeof(oct_node) * h->alloc);
}
 
p->heap_idx = h->n;
h->buf[h->n++] = p;
up_heap(h, p);
}
 
oct_node pop_heap(node_heap *h)
{
if (h->n <= 1) return 0;
 
oct_node ret = h->buf[1];
h->buf[1] = h->buf[--h->n];
 
h->buf[h->n] = 0;
 
h->buf[1]->heap_idx = 1;
down_heap(h, h->buf[1]);
 
return ret;
}
 
static oct_node pool = 0;
oct_node node_new(unsigned char idx, unsigned char depth, oct_node p)
{
static int len = 0;
if (len <= 1) {
oct_node p = calloc(sizeof(oct_node_t), 2048);
p->parent = pool;
pool = p;
len = 2047;
}
 
oct_node x = pool + len--;
x->kid_idx = idx;
x->depth = depth;
x->parent = p;
if (p) p->n_kids++;
return x;
}
 
void node_free()
{
oct_node p;
while (pool) {
p = pool->parent;
free(pool);
pool = p;
}
}
 
oct_node node_insert(oct_node root, unsigned char *pix)
{
unsigned char i, bit, depth = 0;
 
for (bit = 1 << 7; ++depth < 8; bit >>= 1) {
i = !!(pix[1] & bit) * 4 + !!(pix[0] & bit) * 2 + !!(pix[2] & bit);
if (!root->kids[i])
root->kids[i] = node_new(i, depth, root);
 
root = root->kids[i];
}
 
root->r += pix[0];
root->g += pix[1];
root->b += pix[2];
root->count++;
return root;
}
 
oct_node node_fold(oct_node p)
{
if (p->n_kids) abort();
oct_node q = p->parent;
q->count += p->count;
 
q->r += p->r;
q->g += p->g;
q->b += p->b;
q->n_kids --;
q->kids[p->kid_idx] = 0;
return q;
}
 
void color_replace(oct_node root, unsigned char *pix)
{
unsigned char i, bit;
 
for (bit = 1 << 7; bit; bit >>= 1) {
i = !!(pix[1] & bit) * 4 + !!(pix[0] & bit) * 2 + !!(pix[2] & bit);
if (!root->kids[i]) break;
root = root->kids[i];
}
 
pix[0] = root->r;
pix[1] = root->g;
pix[2] = root->b;
}
 
void error_diffuse(image im, node_heap *h)
{
oct_node nearest_color(int *v) {
int i;
int diff, max = 100000000;
oct_node o = 0;
for (i = 1; i < h->n; i++) {
diff = 3 * abs(h->buf[i]->r - v[0])
+ 5 * abs(h->buf[i]->g - v[1])
+ 2 * abs(h->buf[i]->b - v[2]);
if (diff < max) {
max = diff;
o = h->buf[i];
}
}
return o;
}
 
# define POS(i, j) (3 * ((i) * im->w + (j)))
int i, j;
int *npx = calloc(sizeof(int), im->h * im->w * 3), *px;
int v[3];
unsigned char *pix = im->pix;
oct_node nd;
 
#define C10 7
#define C01 5
#define C11 2
#define C00 1
#define CTOTAL (C00 + C11 + C10 + C01)
 
for (px = npx, i = 0; i < im->h; i++) {
for (j = 0; j < im->w; j++, pix += 3, px += 3) {
px[0] = (int)pix[0] * CTOTAL;
px[1] = (int)pix[1] * CTOTAL;
px[2] = (int)pix[2] * CTOTAL;
}
}
#define clamp(x, i) if (x[i] > 255) x[i] = 255; if (x[i] < 0) x[i] = 0
pix = im->pix;
for (px = npx, i = 0; i < im->h; i++) {
for (j = 0; j < im->w; j++, pix += 3, px += 3) {
px[0] /= CTOTAL;
px[1] /= CTOTAL;
px[2] /= CTOTAL;
clamp(px, 0); clamp(px, 1); clamp(px, 2);
 
nd = nearest_color(px);
 
v[0] = px[0] - nd->r;
v[1] = px[1] - nd->g;
v[2] = px[2] - nd->b;
 
pix[0] = nd->r; pix[1] = nd->g; pix[2] = nd->b;
if (j < im->w - 1) {
npx[POS(i, j+1) + 0] += v[0] * C10;
npx[POS(i, j+1) + 1] += v[1] * C10;
npx[POS(i, j+1) + 2] += v[2] * C10;
}
if (i >= im->h - 1) continue;
 
npx[POS(i+1, j) + 0] += v[0] * C01;
npx[POS(i+1, j) + 1] += v[1] * C01;
npx[POS(i+1, j) + 2] += v[2] * C01;
 
if (j < im->w - 1) {
npx[POS(i+1, j+1) + 0] += v[0] * C11;
npx[POS(i+1, j+1) + 1] += v[1] * C11;
npx[POS(i+1, j+1) + 2] += v[2] * C11;
}
if (j) {
npx[POS(i+1, j-1) + 0] += v[0] * C00;
npx[POS(i+1, j-1) + 1] += v[1] * C00;
npx[POS(i+1, j-1) + 2] += v[2] * C00;
}
}
}
free(npx);
}
 
void color_quant(image im, int n_colors, int dither)
{
int i;
unsigned char *pix = im->pix;
node_heap heap = { 0, 0, 0 };
 
oct_node root = node_new(0, 0, 0), got;
for (i = 0; i < im->w * im->h; i++, pix += 3)
heap_add(&heap, node_insert(root, pix));
 
while (heap.n > n_colors + 1)
heap_add(&heap, node_fold(pop_heap(&heap)));
 
double c;
for (i = 1; i < heap.n; i++) {
got = heap.buf[i];
c = got->count;
got->r = got->r / c + .5;
got->g = got->g / c + .5;
got->b = got->b / c + .5;
}
 
if (dither) error_diffuse(im, &heap);
else
for (i = 0, pix = im->pix; i < im->w * im->h; i++, pix += 3)
color_replace(root, pix);
 
node_free();
free(heap.buf);
}
 
int main(int c, char *v[])
{
if (c < 3) {
fprintf(stderr, "usage: %s ppm_file n_colors\n", v[0]);
return 0;
}
 
c = atoi(v[2]) ? : 16; /* GCC extension */
 
image im = read_ppm(v[1]);
color_quant(im, c, 0);
write_ppm(im, "out.pnm");
free(im);
 
return 0;
}