dither.c

// depends on stb_image and stb_image_write
// build using cc dither.c -o dither -lm

#include <stdio.h>

#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"

#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"

int main(int argc, char **argv) {
    if (argc != 4) {
        printf("[!] Invalid usage of arguments\n[!] Tool should be used like this\n");
        printf("[!] %s /path/to/image.ext /path/to/output.png <mono/color>\n", argv[0]);
        return -1;
    }

    char *file_path = argv[1];
    int32_t x, y, n;
    uint32_t *data = (uint32_t*)stbi_load(file_path, &x, &y, &n, 4);
    uint32_t *out = (uint32_t*)malloc(x * y * 4);

    if(!data) {
        printf("[!] Failed to load image\n");
        return -1;
    }

    const uint8_t map[16][16] =   {
                                    {     0, 191,  48, 239,  12, 203,  60, 251,   3, 194,  51, 242,  15, 206,  63, 254  },
                                    {   127,  64, 175, 112, 139,  76, 187, 124, 130,  67, 178, 115, 142,  79, 190, 127  },
                                    {    32, 223,  16, 207,  44, 235,  28, 219,  35, 226,  19, 210,  47, 238,  31, 222  },
                                    {   159,  96, 143,  80, 171, 108, 155,  92, 162,  99, 146,  83, 174, 111, 158,  95  },
                                    {     8, 199,  56, 247,   4, 195,  52, 243,  11, 202,  59, 250,   7, 198,  55, 246  },
                                    {   135,  72, 183, 120, 131,  68, 179, 116, 138,  75, 186, 123, 134,  71, 182, 119  },
                                    {    40, 231,  24, 215,  36, 227,  20, 211,  43, 234,  27, 218,  39, 230,  23, 214  },
                                    {   167, 104, 151,  88, 163, 100, 147,  84, 170, 107, 154,  91, 166, 103, 150,  87  },
                                    {     2, 193,  50, 241,  14, 205,  62, 253,   1, 192,  49, 240,  13, 204,  61, 252  },
                                    {   129,  66, 177, 114, 141,  78, 189, 126, 128,  65, 176, 113, 140,  77, 188, 125  },
                                    {    34, 225,  18, 209,  46, 237,  30, 221,  33, 224,  17, 208,  45, 236,  29, 220  },
                                    {   161,  98, 145,  82, 173, 110, 157,  94, 160,  97, 144,  81, 172, 109, 156,  93  },
                                    {    10, 201,  58, 249,   6, 197,  54, 245,   9, 200,  57, 248,   5, 196,  53, 244  },
                                    {   137,  74, 185, 122, 133,  70, 181, 118, 136,  73, 184, 121, 132,  69, 180, 117  },
                                    {    42, 233,  26, 217,  38, 229,  22, 213,  41, 232,  25, 216,  37, 228,  21, 212  },
                                    {   169, 106, 153,  90, 165, 102, 149,  86, 168, 105, 152,  89, 164, 101, 148,  85  }
                                 };

    int32_t row, col;
    for (size_t i = 0; i < x; i++) {
        col = i % 16;
        for (size_t j = 0; j < y; j++) {
            row = j % 16;
            uint8_t *rgba = (uint8_t *)&data[j * x + i];
            uint8_t new[4];
            new[3] = 0xff;
            new[0] = rgba[0] > map[col][row] ? 0xff : 0x00;
            if(strcmp(argv[3], "mono")) { // if you know how strcmp works
                new[1] = rgba[1] > map[col][row] ? 0xff : 0x00;
                new[2] = rgba[2] > map[col][row] ? 0xff : 0x00;
            }
            else {
                new[1] = new[0];
                new[2] = new[0];
            }
            uint32_t new_color = *(uint32_t*)&new;
            out[j * x + i] = new_color;
        }
    }

    stbi_write_png(argv[2], x, y, 4, out, x * 4);

    return 0;
}