Fix #92: centering for rectangle shape (#93)

examples/dune

@@ -83,6 +83,11 @@
  (modules rectangle_canvas)
  (libraries joy))
 
+(executable
+ (name square)
+ (modules square)
+ (libraries joy))
+
 (executable
  (name star)
  (modules star)

examples/rectangle.ml

@@ -1,12 +1,9 @@
 open Joy
 
-let size = 100
-
 let () =
   init ();
   background (255, 255, 255, 255);
-  (* creating a rectangle from points *)
-  let rect = rectangle size size in
   set_color (0, 0, 0);
-  render rect;
+  let r = rectangle 100 200 in
+  show [r];
   write ~filename:"rectangle.png" ()

examples/square.ml

@@ -0,0 +1,10 @@
+open Joy
+
+let () =
+  init ();
+  background (255, 255, 255, 255);
+  let square = rectangle 100 100 in
+  set_color (0, 0, 0);
+  show [square];
+  write ~filename:"square.png" ()
+

lib/shape.ml

@@ -30,14 +30,15 @@ let center = { x = 0.; y = 0. }
 let circle ?(c = center) r = Circle { c; radius = float_of_int r }
 
 let rectangle ?(c = center) width height =
-  let width, height = (float_of_int width, float_of_int height) in
-  let { x; y } = c -! ((width +. height) /. 4.) in
+  let w, h = (float_of_int width, float_of_int height) in
+  let x1 = c.x -. (w /. 2.) in
+  let y1 = c.x -. (h /. 2.) in
   Polygon
     [
-      { x; y };
-      { x; y = y +. height };
-      { x = x +. width; y = y +. height };
-      { x = x +. width; y };
+      { x = x1; y = y1 };
+      { x = x1; y = y1 +. h };
+      { x = x1 +. w; y = y1 +. h };
+      { x = x1 +. w; y = y1 };
     ]
 
 let ellipse ?(c = center) rx ry =