Reading pixel data from xvfb

Question

So I'm having a strange problem with xvfb. Basically I have an application running through xvfb like so:

Xvfb :1 -screen 0 1920x1080x24+32 -fbdir /var/tmp &
export DISPLAY=:1
gimp &

And then I read the pixel data from the file like this:

#include <string>
#include <string.h>
#include <thread>
#include <math.h>
#include <fcntl.h>
#include <linux/fb.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <iostream>

using namespace std;

char *fbp1 = 0;
int fbfd1;
long int screensize1;

int main() {
        fbfd1 = 0;
        screensize1 = 0;
        fbfd1 = open("/var/tmp/Xvfb_screen0", O_RDWR);

        screensize1 = 1920 * 1080 * 4;
        fbp1 = (char*)mmap(0,
              screensize1,
              PROT_READ | PROT_WRITE,
              MAP_SHARED,
              fbfd1,
              0);

        for (int i = 0; i < 1000; i++) {
                cout << ((int*)fbp1)[i] << endl;
        }
        return 0;
}

For some reason when I print out the first 1000 elements or so of the buffer it prints out a bunch of random data before it starts printing the actual visual on the screen.

Any help would be greatly appreciated!

Answer 1

As stated here, your file isn't just a pixel array, but in xwd format.

You can learn more about this format in your platform-specific xwdfile.h header and use e.g. xwud utility source to learn how it's been done before you.