How to draw directly on the desktop?

Question

I'm wondering how to draw directly on the root window in an X11 environment with Cairo (in C), in order to make widgets. I've copied some parts of the code of tint2, but it's quite enormous, and the only result I have is not satisfying. I would be pleased to have a complete working sample code, or at least some tips or little programs to study. Thank you guys !

Answer 1

The "bottom" window is the root window. The problem is that in some desktop environments we have windows on top of the root window, so if you change the root window, you won't see your changes: you need to change the window that's on the top.

This program does what you ask for: draw on the root window. To test it, I suggest you to:

• ctrl+alt+f1
• login as root
• stop your desktop environment ("/etc/init.d/gdm stop", "/etc/init.d/kdm stop" or whatever is needed in your distro)
• X -noreset -retro &
• DISPLAY=:0.0 xterm &
• DISPLAY=:0.0 metacity &

Then, go back to X (ctrl+alt+f7 or maybe f8) and run the program.

If you want to draw on Nautilus' top window, you will need to find out its window ID and then use it as the "w" variable. The "xwininfo" command might help you testing...

#include <assert.h>
#include <stdio.h>
#include <X11/Xlib.h>
#include <cairo.h>
#include <cairo-xlib.h>

int width, height;

void draw(cairo_t *cr) {
    int quarter_w = width / 4;
    int quarter_h = height / 4;
    cairo_set_source_rgb(cr, 1.0, 0.0, 0.0);
    cairo_rectangle(cr, quarter_w, quarter_h, quarter_w * 2, quarter_h * 2);
    cairo_fill(cr);
}

int main() {
    Display *d = XOpenDisplay(NULL);
    assert(d);

    int s = DefaultScreen(d);
    Window w = RootWindow(d, s);
    width = DisplayWidth(d, s);
    height = DisplayHeight(d, s);

    cairo_surface_t *surf = cairo_xlib_surface_create(d, w,
                                  DefaultVisual(d, s),
                                  width, height);
    cairo_t *cr = cairo_create(surf);

    XSelectInput(d, w, ExposureMask);

    draw(cr);

    XEvent ev;
    while (1) {
    XNextEvent(d, &ev);
        printf("Event!\n");
        if (ev.type == Expose) {
            draw(cr);
        }
    }

    cairo_destroy(cr);
    cairo_surface_destroy(surf);
    XCloseDisplay(d);
    return 0;
}