X11 - Draw on Overlay Window

Question

I want to draw simple primitives at specific pixels on the screen (similar to this question). In order to do that I draw on top of all windows using the Overlay Window of the Window Manager. I can see the shape I am drawing and mouse events pass through but I don't see for example Window movements that are below the Overlay Window (unless I kill my application). I am new to Xlib programming, sry for asking a maybe simple question.

#include <assert.h>
#include <stdio.h>
#include <X11/Xlib.h>

#include <X11/extensions/Xcomposite.h>
#include <X11/extensions/Xfixes.h>
#include <X11/extensions/shape.h>

#include <cairo.h>
#include <cairo-xlib.h>

Display *d;
Window overlay;
Window root;
int width, height;

void
allow_input_passthrough (Window w)
{
    XserverRegion region = XFixesCreateRegion (d, NULL, 0);

    XFixesSetWindowShapeRegion (d, w, ShapeBounding, 0, 0, 0);
    XFixesSetWindowShapeRegion (d, w, ShapeInput, 0, 0, region);

    XFixesDestroyRegion (d, region);
}

void
prep_overlay (void)
{
    overlay = XCompositeGetOverlayWindow (d, root);
    allow_input_passthrough (overlay);
}

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() {
    d = XOpenDisplay(NULL);

    int s = DefaultScreen(d);
    root = RootWindow(d, s);

    XCompositeRedirectSubwindows (d, root, CompositeRedirectAutomatic);
    XSelectInput (d, root, SubstructureNotifyMask);

    width = DisplayWidth(d, s);
    height = DisplayHeight(d, s);

    prep_overlay();

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

    XSelectInput(d, overlay, ExposureMask);

    draw(cr);

    XEvent ev;
    while (1) {
    XNextEvent(d, &ev);
        if (ev.type == Expose) {
            draw(cr);
        }
    }

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

How can I draw pixels on the Overlay Window and still see the windows below?

Answer 1

May I suggest a simpler, pure X11 solution that does not have the flickering problem I experienced and also mentioned here. It uses the override_redirect functionality in Xlib:

#include <assert.h>
#include <stdio.h>
#include <X11/Xlib.h>
#include <X11/X.h>
#include <X11/Xutil.h>

#include <cairo.h>
#include <cairo-xlib.h>

#include <chrono>
#include <thread>

void draw(cairo_t *cr) {
    cairo_set_source_rgba(cr, 1.0, 0.0, 0.0, 0.5);
    cairo_rectangle(cr, 0, 0, 200, 200);
    cairo_fill(cr);
}

int main() {
    Display *d = XOpenDisplay(NULL);
    Window root = DefaultRootWindow(d);
    int default_screen = XDefaultScreen(d);

    // these two lines are really all you need
    XSetWindowAttributes attrs;
    attrs.override_redirect = true;

    XVisualInfo vinfo;
    if (!XMatchVisualInfo(d, DefaultScreen(d), 32, TrueColor, &vinfo)) {
        printf("No visual found supporting 32 bit color, terminating\n");
        exit(EXIT_FAILURE);
    }
    // these next three lines add 32 bit depth, remove if you dont need and change the flags below
    attrs.colormap = XCreateColormap(d, root, vinfo.visual, AllocNone);
    attrs.background_pixel = 0;
    attrs.border_pixel = 0;

    // Window XCreateWindow(
    //     Display *display, Window parent,
    //     int x, int y, unsigned int width, unsigned int height, unsigned int border_width,
    //     int depth, unsigned int class, 
    //     Visual *visual,
    //     unsigned long valuemask, XSetWindowAttributes *attributes
    // );
    Window overlay = XCreateWindow(
        d, root,
        0, 0, 200, 200, 0,
        vinfo.depth, InputOutput, 
        vinfo.visual,
        CWOverrideRedirect | CWColormap | CWBackPixel | CWBorderPixel, &attrs
    );

    XMapWindow(d, overlay);

    cairo_surface_t* surf = cairo_xlib_surface_create(d, overlay,
                                  vinfo.visual,
                                  200, 200);
    cairo_t* cr = cairo_create(surf);

    draw(cr);
    XFlush(d);

    std::this_thread::sleep_for(std::chrono::milliseconds(10000));

    cairo_destroy(cr);
    cairo_surface_destroy(surf);

    XUnmapWindow(d, overlay);
    XCloseDisplay(d);
    return 0;
}

I went ahead and added 32 bit depth, but you get the picture. You can remove it if you desire.