Reading RGB pixels from bmp file

Question

I have a small bmp file and I want to get the RGB values of each pixel and output those values into a txt file if R, G, and B aren't all zero. I wrote the following program; it reads the header data correctly, but the RGB values aren't coming up. I assume I did something wrong in the for loop.

#include <iostream>
#include <fstream>
#include <cstdlib>

using namespace std;

int main()
{
ifstream ifs;
ofstream ofs;
char input[80];
char output[80];

cout<<"Input file name"<<endl;
cin>>input;
ifs.open(input, ios::binary);

if(!ifs)
{
    cout<<"Error in opening file"<<endl;
    system("pause");
    return 0;
}

cout<<"Output file name"<<endl;
cin>>output;
ofs.open(output, ios::binary);

ifs.seekg(2);

int file_size;
ifs.read((char*)&file_size, sizeof(int));

ofs<<"Bitmap size: "<<file_size<<"\r\n";

ifs.seekg(10);
int beg;
ifs.read((char*)&beg, sizeof(int));

ofs<<"Beggining of image: "<<beg<<"\r\n";

ifs.seekg(18);
int columns;
ifs.read((char*)&columns, sizeof(int));

ofs<<"Column number: "<<columns<<"\r\n";

ifs.seekg(22);
int rows;
ifs.read((char*)&rows, sizeof(int));

ofs<<"Row number: "<<rows<<"\r\n";

int image_size=0;
columns+=(3*columns)%4;
image_size=3*columns*rows;

ofs<<"Size of image"<<image_size<<"\r\n";

ifs.seekg(beg);

unsigned char R,G,B;
for(int i=0; i<image_size; i+=3)
{
    ifs.read((char*)&B, sizeof(unsigned char));
    ifs.read((char*)&G, sizeof(unsigned char));
    ifs.read((char*)&R, sizeof(unsigned char));

    if(R!=0 || G!=0 || B!=0)
    ofs<<"R: "<<R<<" G: "<<G<<" B: "<<B<<"  position in file: "<<ifs.tellg()<<"\r\n";
}


system("pause");
    return 0;


}

Answer 1

I ran the code and it works fine, I presume you mean by 'RGB values aren't coming up' you are not seeing the integer values, in which case this will fix it:

ofs<<"R: "<<int(R)<<" G: "<<int(G)<<" B: "<<int(B)<<"  position in file: "<<ifs.tellg()<<"\r\n";

Update: I posted earlier that you could replace ifs.read() with ifs >> R >> G >> B; As @Benjamin Lindley points out, this is incorrect as the >> operator is for formatted text, not binary. This means if the file contains eg a space/newline/etc character, the operator will skip it and take the next char. Better to use ifs.get(char) in this simple case.