Trying to manually decode PGM image file. Where does my code fail? - c

Question

I'm having trouble writing a code that turns an RGB image (in this case a .ppm file) into an image in grayscale (.pgm output file).
The code:
int i;
typedef struct {
unsigned char r, g, b;
} rgb;
FILE *fd;
FILE *gr;
rgb *img;
unsigned width, height;
fd = fopen("rocks.ppm", "r");
fscanf(fd, "%d %d\n", &width, &height);
img = calloc(3, width * height);
fread(img, 3, width * height, fd);
unsigned char *gray;
double len = width * height;
gray = calloc(1, len);
for (i = 0; i < len; i++) {
gray[i] = img[i].r * 0.2126 + img[i].g * 0.7152 + img[i].b * 0.0722;
}
fclose(fd);
gr = fopen("rocks_gray.pgm", "w");
fprintf(gr, "P5\n%d %d\n255\n", width, height);
fwrite(gray, 1, len, gr);
fclose(gr);
When I compile and run the code the image file created comes out completely black, and I can't put my finger on the problem.
Any help would be deeply appreciated.

Answer 1

The problem is here: fscanf(fd, "%d %d\n", &width, &height);
The ppm file starts with P6 so the fscanf() fails to convert anything and returns 0, leaving width and height unchanged, hence uninitialized, causing the rest of the program to have undefined behavior.
You should change the fscanf() and check the return value.
More generally, you should always check for errors that may cause undefined behavior and report the problem with a meaningful error message.
Here is a modified version:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
typedef struct {
unsigned char r, g, b;
} rgb;
int main() {
const char *input_file = "rocks.ppm";
FILE *fd = fopen(input_file, "rb");
if (fd == NULL) {
perror(input_file);
return 1;
}
unsigned width, height;
char newline[2];
// Accept binary pixmaps:
// P6 <whitespace> width <whitespace> height <whitespace> 255 <single space> <raster>
if (fscanf(fd, "P6 %u %u 255%1[ \t\r\n\v\f]", &width, &height, newline) != 3) {
fprintf(stderr, "%s: invalid source format\n", input_file);
fclose(fd);
return 1;
}
unsigned len = width * height;
rgb *img = calloc(len, 3);
if (img == NULL) {
fprintf(stderr, "cannot allocate memory for %u pixels\n", len);
fclose(fd);
return 1;
}
if (fread(img, 3, len, fd) != len) {
fprintf(stderr, "%s: cannot read %u pixels\n", input_file, len);
fclose(fd);
return 1;
}
fclose(fd);
unsigned char *gray = calloc(len, 1);
if (gray == NULL) {
fprintf(stderr, "cannot allocate memory for %u gray scales\n", len);
return 1;
}
for (unsigned i = 0; i < len; i++) {
// using non standard gray scale conversion
gray[i] = img[i].r * 0.2126 + img[i].g * 0.7152 + img[i].b * 0.0722;
}
const char *output_file = "rocks_gray.pgm";
FILE *gr = fopen(output_file, "wb");
if (gr == NULL) {
perror(output_file);
return 1;
}
fprintf(gr, "P5\n%u %u\n255\n", width, height);
fwrite(gray, 1, len, gr);
fclose(gr);
return 0;
}