added empty-scene

svn: r18138

collects/2htdp/image.ss

@@ -76,6 +76,7 @@ and they all have good sample contracts. (It is amazing what we can do with kids
          circle
          ellipse
          rectangle
+         empty-scene
          square
          rhombus
          regular-polygon

collects/2htdp/private/image-more.ss

@@ -654,6 +654,10 @@
   (check-mode/color-combination 'square 3 mode color)
   (make-a-polygon (rectangle-points side-length side-length) mode color))
 
+(define/chk (empty-scene width height)
+  (overlay (rectangle width height 'outline 'black)
+           (rectangle width height 'solid 'white)))
+
 (define/chk (rhombus side-length angle mode color)
   (check-mode/color-combination 'rhombus 3 mode color)
   (let* ([left-corner (make-polar side-length (+ (* pi 1/2) (/ (degrees->radians angle) 2)))]
@@ -956,6 +960,7 @@
          circle
          ellipse
          rectangle
+         empty-scene
          square
          rhombus
          

collects/2htdp/tests/test-image.ss

@@ -630,7 +630,10 @@
        (make-bb 100 100 100)
        #f))
 
-
+(test (empty-scene 185 100)
+      =>
+      (overlay (rectangle 185 100 'outline 'black)
+               (rectangle 185 100 'solid 'white)))
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
 ;;  testing normalization

collects/teachpack/2htdp/scribblings/image-toc.ss

@@ -244,6 +244,7 @@
       (rectangle 48 48 "solid" "gray"))
     'image
     "126418b230e.png")
+   (list '(empty-scene 160 90) 'image "216addb7809.png")
    (list
     '(above/align
       "left"

collects/teachpack/2htdp/scribblings/image.scrbl

@@ -621,11 +621,21 @@ Unlike @scheme[scene+curve], if the line passes outside of @scheme[image], the i
   
   }
 
-@section{Placing Images}
+@section{Placing Images & Scenes}
 
 Placing images into scenes is particularly useful when building worlds
 and universes using @scheme[2htdp/universe].
 
+@defproc[(empty-scene [width (and/c real? (not/c negative?))]
+                      [height (and/c real? (not/c negative?))])
+         image?]{
+
+Creates an empty scene, i.e., a rectangle with a black outline.
+
+@image-examples[(empty-scene 160 90)]
+                                                                 
+}
+
 @defproc[(place-image [image image?] [x real?] [y real?] [scene image?]) image?]{
 
  Places @scheme[image] onto @scheme[scene] with its center at the coordinates