gcc -o vpets vpets.c -lX11 -lXext

vpet.c

#include <X11/Xatom.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/extensions/shape.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main() {
  Display *display;
  Window window;
  XVisualInfo vinfo;
  XSetWindowAttributes attr;
  GC gc;
  XColor color;
  Colormap colormap;
  int screen;

  // circle parameters
  int circle_x = 0;
  int circle_y = 0;
  int circle_dx = 2;
  int circle_dy = 2;
  int circle_radius = 40;
  int circle_diameter = circle_radius * 2;
  int window_size = circle_diameter + 20; // the circle's bounding box is offset

  // color parameters
  unsigned long red = 65535;
  unsigned long green = 0;
  unsigned long blue = 0;
  unsigned long alpha = 32767; // 50% opacity

  // animation parameters
  int fps = 120;
  int sleep_time = 1000000 / fps;

  // open a connection to the x server
  display = XOpenDisplay(NULL);
  if (!display) {
    fprintf(stderr, "Cannot open display\n");
    return 1;
  }

  screen = DefaultScreen(display);

  // get argb visual
  if (!XMatchVisualInfo(display, screen, 32, TrueColor, &vinfo)) {
    fprintf(stderr, "No matching visual found\n");
    return 1;
  }

  // create a colormap
  colormap = XCreateColormap(display, RootWindow(display, screen), vinfo.visual,
                             AllocNone);

  // set window attributes
  attr.colormap = colormap;
  attr.background_pixel = 0; // transparent background
  attr.border_pixel = 0;
  attr.event_mask = StructureNotifyMask;

  // create a window
  window = XCreateWindow(
      display, RootWindow(display, screen), 0, 0, window_size, window_size, 0,
      vinfo.depth, InputOutput, vinfo.visual,
      CWColormap | CWBackPixel | CWBorderPixel | CWEventMask, &attr);

  // set window properties to keep it on top and ignore window manager
  Atom window_type = XInternAtom(display, "_NET_WM_WINDOW_TYPE", False);
  Atom window_type_dock =
      XInternAtom(display, "_NET_WM_WINDOW_TYPE_DOCK", False);
  XChangeProperty(display, window, window_type, XA_ATOM, 32, PropModeReplace,
                  (unsigned char *)&window_type_dock, 1);

  XSetWindowAttributes swa;
  swa.override_redirect = True;
  XChangeWindowAttributes(display, window, CWOverrideRedirect, &swa);

  // create a gc
  gc = XCreateGC(display, window, 0, NULL);

  // set a color with transparency (red, 50% opacity) in the pixel value
  // directly
  color.pixel = (alpha << 16) | (red << 8) | green;
  color.red = red;
  color.green = green;
  color.blue = blue;
  color.flags = DoRed | DoGreen | DoBlue;
  XAllocColor(display, colormap, &color);

  // create a non-rectangular shape for the window
  Pixmap mask = XCreatePixmap(display, window, window_size, window_size, 1);
  GC shape_gc = XCreateGC(display, mask, 0, 0);

  XSetForeground(display, shape_gc, 0);
  XFillRectangle(display, mask, shape_gc, 0, 0, window_size, window_size);

  XSetForeground(display, shape_gc, 1);
  XFillArc(display, mask, shape_gc, window_size - circle_diameter,
           window_size - circle_diameter, circle_diameter, circle_diameter, 0,
           360 * 64);

  XShapeCombineMask(display, window, ShapeBounding, 0, 0, mask, ShapeSet);

  XFreeGC(display, shape_gc);
  XFreePixmap(display, mask);

  // map the window
  XMapWindow(display, window);

  // animation loop
  while (1) {
    // move the circle
    circle_x += circle_dx;
    circle_y += circle_dy;

    // bounce off the edges of the screen
    if (circle_x + window_size > DisplayWidth(display, screen) ||
        circle_x < 0) {
      circle_dx = -circle_dx;
    }
    if (circle_y + window_size > DisplayHeight(display, screen) ||
        circle_y < 0) {
      circle_dy = -circle_dy;
    }

    // clear the window by making a transparent shape
    XSetForeground(display, gc, 0);
    XFillRectangle(display, window, gc, 0, 0, window_size, window_size);

    // set the color and draw the circle
    XSetForeground(display, gc, color.pixel);
    XFillArc(display, window, gc, window_size - circle_diameter,
             window_size - circle_diameter, circle_diameter, circle_diameter, 0,
             360 * 64);

    // move the window
    XMoveWindow(display, window, circle_x, circle_y);

    // flush the output buffer
    XFlush(display);

    // sleep for a short time
    usleep(sleep_time);
  }

  // clean up
  XFreeGC(display, gc);
  XDestroyWindow(display, window);
  XCloseDisplay(display);

  return 0;
}