Painting on screen

[Alex_123]

Hi. Could someone tell me if it's possible to paint on screen using Qt? I mean to paint without having a widget. I'm going to use it on Linux and MacOS. Thanks in advance.

[wysota]

There is no "screen" You probably mean the desktop, right? No, Qt doesn't allow it. You can only place a transparent widget over the desktop and paint on that. Although on KDE4 the desktop is a Qt4 widget, it's in a separate application, so you can't paint on it.

[oob2]

this is code in Qt3 to draw a rect with QColor color;
since you are drawing on the desktop, this rect can be easily erased by any other image
events from your desktop.

if X11
  x11Disp = QPaintDevice::x11AppDisplay();
  x11Screen = QPaintDevice::x11AppScreen();
  x11Rootw = QPaintDevice::x11AppRootWindow();
 
  long gcflags;
  XGCValues gcv;
  gcv.foreground = 100000;
  gcv.function = GXcopy;
  gcv.subwindow_mode = IncludeInferiors;
  gcflags = GCForeground | GCFunction | GCSubwindowMode;
 
  //X11 GC
  x11GC = XCreateGC(x11Disp, x11Rootw, gcflags, &gcv);
 
  XColor fg;
 
  fg.red   = (int)((float)color.red() * 65535.0 / 255.0);
  fg.green = (int)((float)color.green() * 65535.0 / 255.0);
  fg.blue  = (int)((float)color.blue() * 65535.0 / 255.0);
 
  //X11 Colormap
  x11Colormap = QPaintDevice::x11AppColormap(x11Screen);
 
  XAllocColor(x11Disp, x11Colormap, &fg);
  XSetForeground(x11Disp, x11GC, fg.pixel);
 
  XDrawRectangle(x11Disp, x11Rootw, x11GC, x, y, w, h);
  XFlush(x11Disp);
 
else if WIN32
  QPainter p(QApplication::desktop());
  m_currPen.setColor(color);
  p.setPen(m_currPen);
  p.drawRect(x, y, w, h);

if X11
  x11Disp = QPaintDevice::x11AppDisplay();
  x11Screen = QPaintDevice::x11AppScreen();
  x11Rootw = QPaintDevice::x11AppRootWindow();

  long gcflags;
  XGCValues gcv;
  gcv.foreground = 100000;
  gcv.function = GXcopy;
  gcv.subwindow_mode = IncludeInferiors;
  gcflags = GCForeground | GCFunction | GCSubwindowMode;

  //X11 GC
  x11GC = XCreateGC(x11Disp, x11Rootw, gcflags, &gcv);

  XColor fg;

  fg.red   = (int)((float)color.red() * 65535.0 / 255.0);
  fg.green = (int)((float)color.green() * 65535.0 / 255.0);
  fg.blue  = (int)((float)color.blue() * 65535.0 / 255.0);

  //X11 Colormap
  x11Colormap = QPaintDevice::x11AppColormap(x11Screen);

  XAllocColor(x11Disp, x11Colormap, &fg);
  XSetForeground(x11Disp, x11GC, fg.pixel);

  XDrawRectangle(x11Disp, x11Rootw, x11GC, x, y, w, h);
  XFlush(x11Disp);

else if WIN32
  QPainter p(QApplication::desktop());
  m_currPen.setColor(color);
  p.setPen(m_currPen);
  p.drawRect(x, y, w, h);

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