Implement overlay/offset and underlay/offset functions in image

whalesong/image/private/js-impl.js

@@ -474,6 +474,22 @@ EXPORTS['overlay/xy'] =
 				    jsnums.toFixnum(deltaY));
         });
 
+EXPORTS['overlay/offset'] = 
+    makePrimitiveProcedure(
+        'overlay/offset',
+        4,
+        function(MACHINE) {
+	    var img1 = checkImage(MACHINE, "overlay/offset", 0);
+	    var deltaX = checkReal(MACHINE, "overlay/offset", 1);
+	    var deltaY = checkReal(MACHINE, "overlay/offset", 2);
+	    var img2 = checkImage(MACHINE, "overlay/offset", 3);
+            var middleX = (img1.getWidth() - img2.getWidth()) / 2;
+            var middleY = (img1.getHeight() - img2.getHeight()) / 2;
+	    return makeOverlayImage(img1,
+				    img2,
+				    jsnums.toFixnum(middleX + deltaX),
+				    jsnums.toFixnum(middleY + deltaY));
+        });
 
 
  EXPORTS['overlay/align'] = 
@@ -540,6 +556,23 @@ EXPORTS['underlay/xy'] =
 				    -(jsnums.toFixnum(deltaY)));
         });
 
+EXPORTS['underlay/offset'] = 
+    makePrimitiveProcedure(
+        'underlay/offset',
+        4,
+        function(MACHINE) {
+	    var img1 = checkImage(MACHINE, "underlay/offset", 0);
+	    var deltaX = checkReal(MACHINE, "underlay/offset", 1);
+	    var deltaY = checkReal(MACHINE, "underlay/offset", 2);
+	    var img2 = checkImage(MACHINE, "underlay/offset", 3);
+            var middleX = (img1.getWidth() - img2.getWidth()) / 2;
+            var middleY = (img1.getHeight() - img2.getHeight()) / 2;
+	    return makeOverlayImage(img2,
+				    img1,
+				    -jsnums.toFixnum(middleX + deltaX),
+				    -jsnums.toFixnum(middleY + deltaY));
+        });
+
 EXPORTS['underlay/align'] = 
     makePrimitiveProcedure(
         'underlay/align',

whalesong/image/private/main.rkt

@@ -19,9 +19,11 @@
                     video/url
                     play-sound
                     overlay
+                    overlay/offset
                     overlay/xy
                     overlay/align
                     underlay
+                    underlay/offset
                     underlay/xy
                     underlay/align
                     beside

whalesong/image/private/racket-impl.rkt

@@ -5,9 +5,11 @@
 (provide text
          text/font
          overlay
+         overlay/offset
          overlay/xy
          overlay/align
          underlay
+         underlay/offset
          underlay/xy
          underlay/align
          beside
@@ -101,9 +103,11 @@
   text
   text/font
   overlay
+  overlay/offset
   overlay/xy
   overlay/align
   underlay
+  underlay/offset
   underlay/xy
   underlay/align
   beside