Get Pixmap out of Xlib

Question

I've been trying to find a way of grabbing video directly from an X window and got pointed to Xlib and the compositing extension.

So far, I've been able to listen to change events and grab a Pixmap with this code:

#include <X11/Intrinsic.h>
#include <X11/extensions/Xcomposite.h>
#include <X11/extensions/Xdamage.h>
#include <iostream>
#include <fstream>

using namespace std;

int main(int argc, char ** argv) {
    int br, base;
    Window win = 0x3400003;
    Display *dsp = XOpenDisplay(0);
    XCompositeQueryExtension(dsp, &base, &br);
    XDamageQueryExtension(dsp,&base, &br);


    cout << base << endl;

    Damage damage = XDamageCreate(dsp, win, XDamageReportRawRectangles);
    XCompositeRedirectWindow(dsp, win, CompositeRedirectAutomatic);

    XEvent ev;
    while(true) {
        XNextEvent(dsp, &ev);
        if(ev.type == base + XDamageNotify) {
            Pixmap pm = XCompositeNameWindowPixmap(dsp, win);
            XWindowAttributes attr;
            XGetWindowAttributes(dsp, win, &attr);
            XWriteBitmapFile(dsp, "test.bmp", pm, attr.width, attr.height, -1, -1);
            return 0;
        }
    }

    XCompositeUnredirectWindow(dsp, win, CompositeRedirectAutomatic);
    XDamageDestroy(dsp, damage);
    return 0;
}

The problem here is that the resulting bmp (created by XWriteBitmapFile) is black&white horribleness. Disregarding the fact that I don't want to write the data to file anyway, I am apparently doing something wrong in reading it. I would love to convert the Pixmap to either a framebuffer in opengl or a binary blob in ram.

Thanks in advance for any help you can give me on this.

Best regards.

Answer 1

"Bitmaps" in X world refer to two-colored images. I guess XWriteBitmapFile does internally GetImage request and transforms server pixmap to a bitmap. Note that file format is it's own 'X11 bitmap', not windows bitmap.

Use XGetImage function If you actually need image's binary blob.