code:

QImage image = originalPixmap.toImage();
QRgb colour = image.pixel( x, y);

code:
  g++ Hello.c -L/usr/include/X11 

code:
/home/user9/tmp/ccc80kao.o: In function `main':
Hello.c:(.text+0x43): undefined reference to `XAllocSizeHints'
Hello.c:(.text+0x53): undefined reference to `XAllocWMHints'
Hello.c:(.text+0x63): undefined reference to `XAllocClassHint'
Hello.c:(.text+0xb4): undefined reference to `XOpenDisplay'
Hello.c:(.text+0x232): undefined reference to `XCreateSimpleWindow'
Hello.c:(.text+0x24e): undefined reference to `XStringListToTextProperty'
Hello.c:(.text+0x299): undefined reference to `XStringListToTextProperty'
Hello.c:(.text+0x37a): undefined reference to `XSetWMProperties'
Hello.c:(.text+0x395): undefined reference to `XSelectInput'
Hello.c:(.text+0x3a9): undefined reference to `XLoadQueryFont'
Hello.c:(.text+0x408): undefined reference to `XCreateGC'
Hello.c:(.text+0x42a): undefined reference to `XSetFont'
Hello.c:(.text+0x462): undefined reference to `XSetForeground'
Hello.c:(.text+0x478): undefined reference to `XMapWindow'
Hello.c:(.text+0x491): undefined reference to `XNextEvent'
Hello.c:(.text+0x4f6): undefined reference to `XTextWidth'
Hello.c:(.text+0x582): undefined reference to `XDrawString'
Hello.c:(.text+0x5b2): undefined reference to `XUnloadFont'
Hello.c:(.text+0x5c8): undefined reference to `XFreeGC'
Hello.c:(.text+0x5d7): undefined reference to `XCloseDisplay'
collect2: выполнение ld завершилось с кодом возврата 1
  

code:


#include <QApplication>
#include <QPixmap>
#include <QDesktopWidget>
#include <iostream>

using std::cin;
using std::cout;
using std::endl;

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QPixmap pixmap = QPixmap::grabWindow(QApplication::desktop()->winId());
    QImage image = pixmap.toImage();
    
    int x, y;
    cout << "x = ";
    cin >> x;
    cout << "y = ";
    cin >> y;
    QColor color( image.pixel( x, y));
    cout << "(" << color.red() <<   ", " 
                << color.green() << ", "
                << color.blue()  << " )" << endl;
    return 0;
}