Variations on a theme.

gistfile1.txt

$ wget -q http://www.secretareaofvipquality.net/maiddog.png
$ ./imgcat maiddog.png
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imgcat.c

#include <getopt.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <wand/magick_wand.h>

void usage(char *argv_0) {
    fprintf(
        stderr,
        "Usage: %s [OPTIONS] FILENAME\n"
        "\n"
        "Options:\n"
        "\t--width, -w N|auto\n"
        "\t\tSpecify width of the output, or `auto' for terminal width.\n"
        "\t\tPreserves aspect ratio if unspecified. Defaults to 80 if both\n"
        "\t\t-w and -h are omitted.\n"
        "\n"
        "\t--height, -h N|auto\n"
        "\t\tSpecify height of the output, or `auto' for terminal height.\n"
        "\t\tPreserves aspect ratio if unspecified.\n"
        "\n"
        "\t--mode, -m no-white|monochrome\n"
        "\t\tOutput control.\n"
        "\t--normalise, -n\n"
        "\t\tNormalise image.\n"
        "\n"
        "\t--invert, -i\n"
        "\t\tInvert colours.\n"
        "\n",
        argv_0);
    exit(1);
}

int main(int argc, char **argv)
{
    int width = 0, height = 0, mode = 0, normalise = 0, invert = 0;
    char **shades;

    char *sh[3][5] = {
        {" ", "░", "▒", "▓", "█"},
        {"░", "▒", "▓", "█", "."},
        {" ", "█", ".", ".", "."}
    };


    /* Parse options. */

    enum { NURUPO, WIDTH, HEIGHT, MODE, NORMALISE, INVERT };
    static struct option long_options[] = {
        {"width",       1, NULL, WIDTH},
        {"height",      1, NULL, HEIGHT},
        {"mode",        1, NULL, MODE},
        {"normalise",   0, NULL, NORMALISE},
        {"invert",      0, NULL, INVERT},
        {0, 0, 0, NURUPO}
    };
    int c = 0;
    extern int optind;

    while ((c = getopt_long(argc, argv, "w:h:m:ni", long_options, NULL)) != -1) {
        switch (c) {
        case WIDTH:
        case 'w':
            if (strcmp(optarg, "auto") == 0) {
                struct winsize w;
                ioctl(STDOUT_FILENO, TIOCGWINSZ, &w);
                width = w.ws_col;
            } else
                width = atoi(optarg);
            if (width < 0)
                usage(argv[0]);
            break;

        case HEIGHT:
        case 'h':
            if (strcmp(optarg, "auto") == 0) {
                struct winsize w;
                ioctl(STDOUT_FILENO, TIOCGWINSZ, &w);
                height = w.ws_row;
            } else
                height = atoi(optarg);
            if (height < 0)
                usage(argv[0]);
            break;

        case MODE:
        case 'm':
            if (strcmp(optarg, "no-white") == 0)
                mode = 1;
            else if (strncmp(optarg, "mono", 4) == 0)
                mode = 2;
            else 
                usage(argv[0]);
            break;

        case NORMALISE:
        case 'n':
            normalise = 1;
            break;

        case INVERT:
        case 'i':
            invert = 1;
            break;

        default:
            usage(argv[0]);
        }
    }

    shades = sh[mode];

    if (optind == argc)
        usage(argv[0]);


    /* Try to load image. */

    MagickWand *m_wand;

    MagickWandGenesis();
    m_wand = NewMagickWand();

    if (MagickReadImage(m_wand, argv[optind]) == MagickFalse) {
        perror("Can't into file");
        return 1;
    }


    /* Resize and optionally normalise and invert image. */

    size_t w = MagickGetImageWidth(m_wand),
           h = MagickGetImageHeight(m_wand);

    if (!height) {
        if (!width)
            width = 80;
        height = h * width / w / 2;
    }
    if (!width)
        width = w * height * 2 / h;

    MagickResizeImage(m_wand, width, height, LanczosFilter, 0.5);

    if (normalise) {
        MagickSetImageType(m_wand, GrayscaleType);
        MagickNormalizeImage(m_wand);
    }

    if (invert)
        MagickNegateImage(m_wand, MagickFalse);


    /* Output. */

    if (mode == 0)
        mode = 5;
    else if (mode == 1)
        mode = 4;
    else
        mode = 2;

    for (int i = 0; i < height; ++i) {
        for (int j = 0; j < width; ++j) {
            PixelWand *p_wand = NewPixelWand();
            double h, s, l;
            int level;

            MagickGetImagePixelColor(m_wand, j, i, p_wand);
            PixelGetHSL(p_wand, &h, &s, &l);

            level = (int)(l * mode);
            if (level == mode)
                --level;

            printf("%s", shades[level]);

            DestroyPixelWand(p_wand);
        }
        puts("");
    }

    return 0;
}

Makefile

MW = `MagickWand-config --cflags --ldflags --libs`
WARNS = -Wall -Wextra -pedantic -std=c99

.PHONY: all
all: imgcat

imgcat: imgcat.c
	$(CC) -o $@ $(MW) $(WARNS) $(CFLAGS) $^

.PHONY: clean
clean:
	rm -rf imgcat