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 th

Answer 1

sleep(50)! that's too much, it's 50 seconds. I used 5ms delay which works well.

Your problem seems with the runtime environment. You should have a composite display manager running already. (Not all display managers work as expected, better to try on different ones)

I confirm that screen below updated without any problem and I could interact with it.

This was run on:

Ubuntu 15.10
Kernel 4.2.0-18-generic
X.Org X Server 1.17.2
Compiz 0.9.12.2

Here the full code with just delay modification:

#include 
#include 
#include 
#include 

#include 
#include 
#include 

#include 
#include 

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() {
    struct timespec ts = {0, 5000000};

    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) {
      overlay = XCompositeGetOverlayWindow (d, root);
      draw(cr);
      XCompositeReleaseOverlayWindow (d, root);
      nanosleep(&ts, NULL);
    }

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