display remaining sleep time for next frame in window

@@ -35,6 +35,7 @@ pub trait Canvas {
 
     fn clear(&mut self);
     fn draw(&mut self, c :&dyn Drawable, ofs :Coordinate, color :u32);
+    fn put_text(&self, ofs :Coordinate, s :&str);
     fn show(&self);
 }
 

@@ -344,12 +344,12 @@ fn main() {
     // use floating point values.
     // https://rechneronline.de/pi/tetrahedron.php
     // yi = a / 12 * √6
-    // yc = a / 4 * √6
-    // zi = √3 / 6 * a
-    // zc = √3 / 3 * a
     let yi = 60.0 / 12.0 * 6.0.sqrt().unwrap();
+    // yc = a / 4 * √6
     let yc = 60.0 /  4.0 * 6.0.sqrt().unwrap();
+    // zi = √3 / 6 * a
     let zi = 3.0.sqrt().unwrap() / 6.0 * 60.0;
+    // zc = √3 / 3 * a
     let zc = 3.0.sqrt().unwrap() / 3.0 * 60.0;
 
     let i = Vector(  0.0,  yc, 0.0);
@@ -471,7 +471,6 @@ fn main() {
             canvas.draw( &co, Coordinate(0,0), 0x0000FF);
             canvas.draw( &cl, Coordinate(0,0), 0x0000FF);
             canvas.draw( &cr, Coordinate(0,0), 0x0000FF);
-            canvas.show();
 
             let passed = Instant::now() - last;
             let f      = (passed.as_nanos() / step.as_nanos()) as u32;
@@ -481,7 +480,10 @@ fn main() {
             }
 
             last = last + step*(f + 1);
-            println!("Sleep for: {:?}", last - Instant::now());
+            canvas.put_text( Coordinate(10, 15)
+                           , &format!( "sleep: {:?}"
+                                     , last - Instant::now() ));
+            canvas.show();
             thread::sleep(last - Instant::now());
         }
     });

@@ -74,10 +74,10 @@ impl XcbEasel {
                           , screen.root(), 0, 0, width, width, 0
                           , xcb::WINDOW_CLASS_INPUT_OUTPUT as u16
                           , screen.root_visual()
-                          , &[(xcb::CW_BACK_PIXEL, screen.white_pixel())] );
+                          , &[(xcb::CW_BACK_PIXEL, screen.black_pixel())] );
 
         xcb::create_gc( &conn, gc, screen.root()
-                      , &[ (xcb::GC_FOREGROUND, screen.black_pixel())
+                      , &[ (xcb::GC_FOREGROUND, screen.white_pixel())
                       , (xcb::GC_GRAPHICS_EXPOSURES, 0) ] );
 
         let (shmid, shm) = getshm((width * height) as usize);
@@ -120,8 +120,8 @@ fn getshm<'a>(size :usize) -> (i32, &'a mut [u32]) {
 
     unsafe {
         let id = libc::shmget( libc::IPC_PRIVATE
-                               , size * 4
-                               , libc::IPC_CREAT | 0o744 );
+                             , size * 4
+                             , libc::IPC_CREAT | 0o744 );
         let ptr = libc::shmat(id, ptr::null(), 0);
         (id as i32, from_raw_parts_mut(ptr as *mut u32, size))
     }
@@ -233,6 +233,13 @@ impl<'a> Canvas for XcbCanvas<'a> {
         }
     }
 
+    fn put_text(&self, ofs :Coordinate, s :&str) {
+        let Coordinate(xofs, yofs) = ofs;
+        xcb::xproto::image_text_8( &self.conn, self.pixmap, self.gc
+                                 , xofs as i16, yofs as i16, s );
+        self.conn.flush();
+    }
+
     fn show(&self) {
         xcb::copy_area( &self.conn, self.pixmap, self.window, self.gc
                         , 0, 0, 0, 0