added the radial-star primitive

collects/2htdp/image.rkt

@@ -87,6 +87,7 @@ and they all have good sample contracts. (It is amazing what we can do with kids
          polygon
          star
          star-polygon
+         radial-star
          triangle
          isosceles-triangle
          right-triangle

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

@@ -924,6 +924,32 @@
   (make-a-polygon (adjust (regular-polygon-points side-length side-count)) 
                   mode color))
 
+(define/chk (radial-star point-count radius1 radius2 mode color)
+  (make-a-polygon (star-points radius1 radius2 point-count) mode color))
+  
+(define (star-points in-small-rad in-large-rad points)
+  (let* ([small-rad (- in-small-rad 1)]
+         [large-rad (- in-large-rad 1)]
+         [roff (floor (/ large-rad 2))])
+    (let loop ([i points])
+      (cond
+        [(zero? i) '()]
+        [else 
+         (let* ([this-p (- i 1)]
+                [theta1 (* 2 pi (/ this-p points))]
+                [theta2 (* 2 pi (/ (- this-p 1/2) points))])
+           (let-values ([(x1 y1) (find-xy small-rad theta1)]
+                        [(x2 y2) (find-xy large-rad theta2)])
+             (let ([p1 (make-point (+ large-rad x1)
+                                   (+ large-rad y1))]
+                   [p2 (make-point (+ large-rad x2)
+                                   (+ large-rad y2))])
+               (list* p1 p2 (loop (- i 1))))))]))))
+
+(define (find-xy radius theta)
+  (values (* radius (cos theta))
+          (* radius (sin theta))))
+
 (define (make-a-polygon points mode color)
   (let* ([poly (make-polygon points mode color)]
          [ltrb (simple-bb poly)]
@@ -1087,6 +1113,7 @@
          right-triangle
          star
          star-polygon
+         radial-star
          
          line
          add-line

collects/2htdp/private/img-err.rkt

@@ -114,11 +114,18 @@
      (if (string? arg)
          (string->symbol arg)
          arg)]
-    [(width height radius side-length side-length1 side-length2)
+    [(width height radius radius1 radius2 side-length side-length1 side-length2)
      (check-arg fn-name
                 (and (real? arg)
                      (not (negative? arg)))
-                'non-negative-real-number
+                'non\ negative\ real\ number
+                i arg)
+     arg]
+    [(point-count)
+     (check-arg fn-name
+                (and (integer? arg)
+                     (>= arg 2))
+                'integer\ greater\ than\ 2
                 i arg)
      arg]
     [(dx dy x1 y1 x2 y2 pull1 pull2)

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

@@ -251,6 +251,28 @@ other. The top and bottom pair of angles is @racket[angle] and the left and righ
  
 }
                 
+@defproc*[([(radial-star [point-count (and/c integer? (>=/c 2))]
+                         [inner-radius (and/c real? (not/c negative?))]
+                         [outer-radius (and/c real? (not/c negative?))]
+                         [mode mode?]
+                         [color image-color?])
+            image?]
+           [(radial-star [point-count (and/c integer? (>=/c 2))]
+                         [inner-radius (and/c real? (not/c negative?))]
+                         [outer-radius (and/c real? (not/c negative?))]
+                         [outline-mode (or/c 'outline "outline")]
+                         [pen-or-color (or/c pen? image-color?)])
+            image?])]{
+                                   
+Constructs a star-like polygon where the star is specified by two radii and a number of points. 
+The first radius determines where the points begin, the second determines where they end, and
+the @scheme[point-count] argument determines how many points the star has.
+
+@image-examples[(radial-star 8 8 64 "solid" "darkslategray")
+                (radial-star 32 30 40 "outline" "black")]
+
+}
+
 @defproc*[([(polygon [vertices (listof posn?)] 
                      [mode mode?]
                      [color image-color?])