Read bmp file header size

Question

I am trying to find file size, file header size width, and height of a bmp file. I have studied the format of bmp file and the arrangement of bytes in file. When I try this code it shows wrong width and height for different files. I have tried this for three images so far. This one image results the right measurement.

This one did not:

I don't understand where I went wrong, but the bit depth showed the right value for all three images.

Here is my code:

#include<iostream>
#include<fstream>
#include<math.h>

using namespace std;


int main() {
    ifstream inputfile("bmp.bmp",ios::binary);
    char c; int imageheader[1024]; 

    double filesize=0; int width=0; int height=0;int bitCount = 0;

    for(int i=0; i<1024; i++) {
        inputfile.get(c); imageheader[i]=int(c);
    }

    filesize=filesize+(imageheader[2])*pow(2,0)+(imageheader[3])*pow(2,8)+(imageheader[4])*pow(2,16)+(imageheader[5])*pow(2,24);

    cout<<endl<<endl<<"File Size:  "<<(filesize/1024)<<" Kilo Bytes"<<endl;

    width=width+(imageheader[18])*pow(2,0)+(imageheader[19])*pow(2,8)+(imageheader[20])*pow(2,16)+(imageheader[21])*pow(2,24);

    cout<<endl<<"Width: "<<endl<<(width)<<endl;

    height=height+(imageheader[22])*pow(2,0)+(imageheader[23])*pow(2,8)+(imageheader[24])*pow(2,16)+(imageheader[25])*pow(2,24);
    cout<<endl<<"Height: "<<endl<<(height)<<endl;

    bitCount=bitCount+(imageheader[28])*pow(2,0)+(imageheader[29])*pow(2,8);
    cout<<endl<<"Bit Depth: "<<endl<<(bitCount)<<endl;
}

Answer 1

Let's start by reading the BMP header in as a series of bytes, not integers. To make this code truly portable, we'll use <stdint> types.

#include <fstream>
#include <stdint.h>

int main()
{

    ifstream inputfile("D:/test.bmp", ios::binary);
    uint8_t headerbytes[54] = {};

    inputfile.read((char*)headerbytes, sizeof(headerbytes));

Now that we've got the header in memory as an array of bytes, we can simply cast the memory address of each header field back into a integer. Referencing the wikipedia page for bmp and the layout diagram.

    uint32_t filesize = *(uint32_t*)(headerbytes+2);
    uint32_t dibheadersize = *(uint32_t*)(headerbytes + 14);
    uint32_t width = *(uint32_t*)(headerbytes + 18);
    uint32_t height = *(uint32_t*)(headerbytes + 22);
    uint16_t planes = *(uint16_t*)(headerbytes + 26);
    uint16_t bitcount = *(uint16_t*)(headerbytes + 28);

Now an astute reader of the code will recognize that the individual fieds of a a BMP headers are stored in little endian format. And that the code above relies on you to have an x86 processor or any other architecture in which the byte layout is Little Endian. On a big endian machine, you'll have to apply a workaround to convert from LE to BE for each of the variables above.

Answer 2

The bug is reading into signed char. This should fix it:

for(int i = 0; i < 1024; i++)
{
    //inputfile.get(c); imageheader[i] = int(c);

    // This version of get returns int, where -1 means EOF.  Should be checking for errors...
    imageheader[i] = inputfile.get();
}

Others have commented on improvements to the code so I won't bother.