images.h/images/bmp-format.c

#include <stdio.h>
#include <stdlib.h>
#include "images.h"

// write an int to a byte array
void bmp__write_int(char* bytes, int num) {
    for (int i = 0; i < 4; i++) {
        bytes[i] = num >> (i * 8);
    }
}

void bmp__write_byte(char* bytes, int num) {
    bytes[0] = (char) num;
}

void bmp__write_16bit(char* bytes, int num) {
    for (int i = 0; i < 2; i++) {
        bytes[i] = num >> (i * 8);
    }
}

int bmp__align_to_four_bytes(int size) {
    int os = size % 4;
    return os == 0 ? size : size + (4 - os);
}

int bmp__image_byte_size(Image image) {
    // calculate size:
    // bitmap file header = 14
    // BITMAPCOREHEADER = 12 bytes
    int size = 14 + 12;
    // width must be padded to align to 4 bytes
    int width_bytes = bmp__align_to_four_bytes(image.width * 3); // 3 bytes per pixel
    // add width_bytes * height to total size
    size += width_bytes * image.height;

    return size;
}

// header format: 
// BMP header (14 bytes)
// 0x42 0x4D            "BM"
// 0xAA 0xBB 0xCC 0xDD  bmp file size (int)
// 0x00 0x00 0x00 0x00  nulls
// 0x1A 0x00 0x00 0x00  pixel array offset (int) (= 14 + 12);
// DIB  Header (BITMAPCOREHEADER) (12 bytes)
// 0x0C 0x00 0x00 0x00  header size (int) (= 12)
// 0xAA 0xBB            width (unsigned 16 bit int)
// 0xCC 0xDD            height (unsigned 16 bit int)
// 0x01 0x00            number of color planes (must be 1)
// 0x18 0x00            number of bits per pixel (3 * 8 = 24)

int image_save_bmp(Image image, char* path) {
    int size = bmp__image_byte_size(image);
    char* bytes = calloc(sizeof(char), size);
    // magic bytes, signature "BM"
    bytes[0] = 0x42;
    bytes[1] = 0x4d;
    // write size to header
    bmp__write_int(bytes + 2, size);
    // the next 4 byte are zeros
    // piyel array offset
    bmp__write_int(bytes + 10, 14 + 12);
    bmp__write_int(bytes + 14, 12);
    bmp__write_16bit(bytes + 18, image.width);  // width
    bmp__write_16bit(bytes + 20, image.height); // height
    bmp__write_16bit(bytes + 22, 1);             // color planes
    bmp__write_16bit(bytes + 24, 3 * 8);         // bits per pixel

    int byte_offset = 26;
    int byte_pos = 0;
    int line;
    for (line = 0; line < image.height; line++) {
        byte_pos = 0;
        for (int px = 0; px < image.width * 3; px += 3) {
            bytes[byte_offset + byte_pos++] = image.bitmap[line][px + 2];
            bytes[byte_offset + byte_pos++] = image.bitmap[line][px + 1];
            bytes[byte_offset + byte_pos++] = image.bitmap[line][px + 0];
        }
        byte_offset += bmp__align_to_four_bytes(byte_pos);
    }

    printf("generated %i bytes out of %i calculated ones\n", byte_offset, size);

    FILE *fp = fopen(path, "w");
    int written = fwrite(bytes, sizeof(char), size, fp);
    printf("wrote %i bytes...\n", written);
    fclose(fp);
    free(bytes);
    return 1;
}