Question

I am writing a simple program to convert grayscale binary (P5) to grayscale ascii (P2) but am having trouble reading in the binary and converting it to int.

#include <iostream>
#include <fstream>
#include <sstream>
using namespace::std;

int usage(char* arg) {
// exit program
    cout << arg << ": Error" << endl;
    return -1;
}

int main(int argc, char* argv[]) {
    int rows, cols, size, greylevels;
    string filetype;

    // open stream in binary mode
    ifstream istr(argv[1], ios::in | ios::binary);
    if(istr.fail()) return usage(argv[1]);

    // parse header
    istr >> filetype >> rows >> cols >> greylevels;
    size = rows * cols;

    // check data
    cout << "filetype: " << filetype << endl;
    cout << "rows: " << rows << endl;
    cout << "cols: " << cols << endl;
    cout << "greylevels: " << greylevels << endl;
    cout << "size: " << size << endl;

    // parse data values
    int* data = new int[size];
    int fail_tracker = 0; // find which pixel failing on
    for(int* ptr = data; ptr < data+size; ptr++) {
        char t_ch;
        // read in binary char
        istr.read(&t_ch, sizeof(char));
        // convert to integer
        int t_data = static_cast<int>(t_ch);
        // check if legal pixel
        if(t_data < 0 || t_data > greylevels) { 
            cout << "Failed on pixel: " << fail_tracker << endl;
            cout << "Pixel value: " << t_data << endl;
            return usage(argv[1]);
        }
        // if passes add value to data array
        *ptr = t_data;
        fail_tracker++;
    }
    // close the stream
    istr.close();

    // write a new P2 binary ascii image
    ofstream ostr("greyscale_ascii_version.pgm");
    // write header
    ostr << "P2 " << rows << cols << greylevels << endl;
    // write data
    int line_ctr = 0;
    for(int* ptr = data; ptr < data+size; ptr++) {
        // print pixel value
        ostr << *ptr << " ";
        // endl every ~20 pixels for some readability
        if(++line_ctr % 20 == 0) ostr << endl;
    }
    ostr.close();

    // clean up
    delete [] data;
    return 0;
}

sample image – Pulled this from an old post. Removed the comment within the image file as I am not worried about this functionality now.

When compiled with g++ I get output:

$> ./a.out a.pgm
filetype: P5
rows: 1024
cols: 768
greylevels: 255
size: 786432
Failed on pixel: 1
Pixel value: -110
a.pgm: Error

The image is a little duck and there’s no way the pixel value can be -110…where am I going wrong?
Thanks.

Answer 1

greylevels: 255

-110 is 146 as an unsigned char. It appears you are on a platform where char is a signed type, try using unsigned char.