Raw Image (RGB) Compositor

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
void usage(char *prog_name) {
  fprintf(
    stderr,
    "Usage: %s orig.raw compose1.raw compose2.raw out.raw",
    prog_name);
}
int main(int argc, char **argv) {
  if (argc != 5) {
    usage(argv[0]);
    return EXIT_FAILURE;
  }
  FILE *orig = fopen(argv[1], "rb");
  FILE *compose1 = fopen(argv[2], "rb");
  FILE *compose2 = fopen(argv[3], "rb");
  FILE *out = fopen(argv[4], "wb");
  while (true) {
    unsigned char ori[3];
    unsigned char comp1[3];
    unsigned char comp2[3];
    if (fscanf(orig, "%3c", ori) == EOF) break;
    fscanf(compose1, "%3c", comp1);
    fscanf(compose2, "%3c", comp2);
    unsigned char ou[3];
    for (int i = 0; i < 3; i++) {
      ou[i] = 255 * ori[i] / (ori[i] + comp1[i] + comp2[i]);
    }
    fprintf(out, "%c%c%c", ou[0], ou[1], ou[2]);
  }
  fclose(orig);
  fclose(compose1);
  fclose(compose2);
  fclose(out);
  return EXIT_SUCCESS;
}