Converting 1-bit bmp file to array in C/C++

Question

I'm looking to turn a 1-bit bmp file of variable height/width into a simple two-dimensional array with values of either 0 or 1. I don't have any experience with image editing in code and most libraries that I've found involve higher bit-depth than what I need. Any help regarding this would be great.

Answer 1

Here's the code to read a monochrome .bmp file

(See dmb's answer below for a small fix for odd-sized .bmps)

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

unsigned char *read_bmp(char *fname,int* _w, int* _h)
{
    unsigned char head[54];
    FILE *f = fopen(fname,"rb");

    // BMP header is 54 bytes
    fread(head, 1, 54, f);

    int w = head[18] + ( ((int)head[19]) << 8) + ( ((int)head[20]) << 16) + ( ((int)head[21]) << 24);
    int h = head[22] + ( ((int)head[23]) << 8) + ( ((int)head[24]) << 16) + ( ((int)head[25]) << 24);

    // lines are aligned on 4-byte boundary
    int lineSize = (w / 8 + (w / 8) % 4);
    int fileSize = lineSize * h;

    unsigned char *img = malloc(w * h), *data = malloc(fileSize);

    // skip the header
    fseek(f,54,SEEK_SET);

    // skip palette - two rgb quads, 8 bytes
    fseek(f, 8, SEEK_CUR);

    // read data
    fread(data,1,fileSize,f);

    // decode bits
    int i, j, k, rev_j;
    for(j = 0, rev_j = h - 1; j < h ; j++, rev_j--) {
        for(i = 0 ; i < w / 8; i++) {
            int fpos = j * lineSize + i, pos = rev_j * w + i * 8;
            for(k = 0 ; k < 8 ; k++)
                img[pos + (7 - k)] = (data[fpos] >> k ) & 1;
        }
    }

    free(data);
    *_w = w; *_h = h;
    return img;
}

int main()
{
    int w, h, i, j;
    unsigned char* img = read_bmp("test1.bmp", &w, &h);

    for(j = 0 ; j < h ; j++)
    {
        for(i = 0 ; i < w ; i++)
            printf("%c ", img[j * w + i] ? '0' : '1' );

        printf("\n");
    }

    return 0;
}

It is plain C, so no pointer casting - beware while using it in C++.

The biggest problem is that the lines in .bmp files are 4-byte aligned which matters a lot with single-bit images. So we calculate the line size as "width / 8 + (width / 8) % 4". Each byte contains 8 pixels, not one, so we use the k-based loop.

I hope the other code is obvious - much has been told about .bmp header and pallete data (8 bytes which we skip).

Expected output:

0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 
0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 
0 0 0 0 0 0 1 1 1 1 0 0 1 1 0 0 
0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 
0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 
0 0 0 1 0 0 1 1 1 1 0 0 0 0 0 0 
0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 
0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 
0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 
0 0 0 0 0 0 1 1 1 1 0 0 1 0 0 0 
0 0 0 0 0 0 1 1 1 1 0 0 0 0 0 0 
0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 
0 0 0 0 0 1 1 1 1 1 0 0 0 0 1 0 
0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 
0 0 0 1 0 1 1 1 1 1 0 0 0 0 0 0 
0 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0