Display images (in true color!) in a compatible terminal emulator using block characters

termimg.c

#include <gd.h>
#include <stdio.h>
#include <string.h>

void process(gdImagePtr img) {
    for (int j = 0; j < img->sy/2; j++) {
        for (int i = 0; i < img->sx; i++) {
            int top    = gdImageGetPixel(img, i, j*2    );
            int bottom = gdImageGetPixel(img, i, j*2 + 1);

            printf(
                "\x1b[48;2;%d;%d;%dm\x1b[38;2;%d;%d;%dm\u2584",
                (top >> 16) & 0xff, (top >> 8) & 0xff, top & 0xff,
                (bottom >> 16) & 0xff, (bottom >> 8) & 0xff, bottom & 0xff
            );
        }

        printf("\x1b[0m\n");
    }
}

int main(int argc, char *argv[]) {
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <input file>\n", argv[0]);
        return 1;
    }

    FILE *in;

    if (strcmp(argv[1], "-") == 0) {
        in = stdin;
    } else {
        in = fopen(argv[1], "rb");
    }

    gdImagePtr img = gdImageCreateFromPng(in);

    if (img == NULL) {
        fprintf(stderr, "Cannot load %s: not a PNG\n", argv[1]);
        return 1;
    }

    process(img);
    fclose(in);

    gdImageDestroy(img);

    return 0;
}