#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>


static int getmagic(FILE *f)
{
    int ic1, ic2;

    if ((ic1 = getc(f)) == EOF) {
	fprintf(stderr, "Premature end of file\n");
	return -1;
    }
    if ((ic2 = getc(f)) == EOF) {
	fprintf(stderr, "Premature end of file\n");
	return -1;
    }
    return ic1 * 256 + ic2;
}

static char pgetc(FILE *f)
{
    register int ic;
    register char c;

    if ((ic = getc(f)) == EOF) {
	fprintf(stderr, "Premature end of file\n");
	return -1;
    }
    c = (char)ic;

    if (c == '#') {
	do {
	    if ((ic = getc(f)) == EOF) {
		fprintf(stderr, "Premature end of file\n");
		return -1;
	    }
	    c = (char)ic;
	} while (c != '\n' && c != '\r');
    }

    return c;
}

static int getint(FILE *f)
{
    register char c;
    register int i;

    do {
	if ((c = pgetc(f)) == -1)
	    return -1;
    } while (c == ' ' || c == '\t' || c == '\n' || c == '\r');

    if (c < '0' || c > '9') {
	fprintf(stderr, "Junk in file where an integer should be\n");
	return -1;
    }

    i = 0;
    do {
	i = i * 10 + c - '0';
	if ((c = pgetc(f)) == -1)
	    return -1;
    } while (c >= '0' && c <= '9');

    return i;
}

static void readhdr(FILE *f, int *w, int *h)
{
    int magic, maxval;
    static int lastw = -1, lasth = -1;

    if ((magic = getmagic(f)) == -1) exit(1);
    if ((*w = getint(f)) == -1) exit(1);
    if ((*h = getint(f)) == -1) exit(1);
    if ((maxval = getint(f)) == -1) exit(1);

    if (magic != 'P' * 256 + '5') {
	fprintf(stderr, "yuv2rgb: Not a PGM file\n");
	exit(1);
    }
    if (maxval != 255) {
	fprintf(stderr, "yuv2rgb: Maxval != 255\n");
	exit(1);
    }
    if (lastw != -1) {
	if (*w != lastw || *h != lasth) {
	    fprintf(stderr, "yuv2rgb: Inconsistent dimensions\n");
	    exit(1);
	}
    }
    lastw = *w; lasth = *h;
}

int main(int argc, char **argv)
{
    FILE *fy, *fu, *fv;
    int w, h, x, y;

    if (argc < 4) {
	fprintf(stderr, "usage: yuv2rgb yfile ufile vfile\n");
	exit(1);
    }

    fy = fopen(argv[1], "r");
    fu = fopen(argv[2], "r");
    fv = fopen(argv[3], "r");

    readhdr(fy, &w, &h);
    readhdr(fu, &w, &h);
    readhdr(fv, &w, &h);

    printf("P6\n%d %d\n255\n", w, h);

    for (y = 0; y < h; y++) {
	for (x = 0; x < w; x++) {
	    unsigned char y, u, v;
	    double c, d, e, rd, gd, bd;
	    unsigned char r, g, b;

	    fread(&y, 1, 1, fy); fread(&u, 1, 1, fu); fread(&v, 1, 1, fv);

	    c = y - 16; d = u - 128; e = v - 128;

	    rd = 1.164383 * c + 1.596027 * e + .5;
	    gd = 1.164383 * c - 0.391762 * d - 0.812968 * e + .5;
	    bd = 1.164383 * c + 2.017232 * d + .5;
	    rd = (rd < 0 ? 0 : (rd > 255 ? 255 : rd));
	    gd = (gd < 0 ? 0 : (gd > 255 ? 255 : gd));
	    bd = (bd < 0 ? 0 : (bd > 255 ? 255 : bd));
	    r = rd; g = gd; b = bd;

	    fwrite(&r, 1, 1, stdout);
	    fwrite(&g, 1, 1, stdout);
	    fwrite(&b, 1, 1, stdout);
	}
    }

    return 0;
}
.