diff --git a/racket/syndicate-gl/2d.rkt b/racket/syndicate-gl/2d.rkt
index f53b4bb..bd5a9f1 100644
--- a/racket/syndicate-gl/2d.rkt
+++ b/racket/syndicate-gl/2d.rkt
@@ -11,6 +11,8 @@
          (except-out (struct-out sprite) sprite)
          (rename-out [sprite <sprite>] [make-sprite sprite])
          (struct-out request-gc)
+         in-unit-circle?
+         in-unit-square?
          simple-sprite
          update-scene
          update-sprites
@@ -86,15 +88,20 @@
 (define (make-sprite z instructions)
   (sprite z (seal instructions)))
 
+(define (in-unit-circle? x y)
+  (<= (+ (sqr (- x 0.5)) (sqr (- y 0.5))) (sqr 0.5)))
+
 (define (in-unit-square? x y)
   (and (<= 0 x 1)
        (<= 0 y 1)))
 
-(define (simple-sprite z x y w h i #:touchable-id [touchable-id #f])
+(define (simple-sprite z x y w h i
+                       #:touchable-id [touchable-id #f]
+                       #:touchable-predicate [touchable-predicate in-unit-square?])
   (make-sprite z `((translate ,x ,y)
                    (scale ,w ,h)
                    ,@(if touchable-id
-                         `((touchable ,touchable-id ,in-unit-square?))
+                         `((touchable ,touchable-id ,touchable-predicate))
                          `())
                    (texture ,i))))
 
diff --git a/racket/syndicate-gl/examples/basic.rkt b/racket/syndicate-gl/examples/basic.rkt
index 284ff0e..73c4a82 100644
--- a/racket/syndicate-gl/examples/basic.rkt
+++ b/racket/syndicate-gl/examples/basic.rkt
@@ -59,11 +59,13 @@
 
 (actor (define/query-value touching-orange? #f (inbound (touching 'orange _ _ _)) #t)
        (assert (outbound (simple-sprite #:touchable-id 'orange
+                                        #:touchable-predicate in-unit-circle?
                                         0 50 50 50 50 (circle 50 "solid"
                                                               (if (touching-orange?)
                                                                   "red"
                                                                   "orange")))))
        (assert (outbound (simple-sprite #:touchable-id 'green
+                                        #:touchable-predicate in-unit-circle?
                                         -1 60 60 50 50 (circle 50 "solid" "green")))))
 (actor* (until (message (inbound (key-event #\q #t _))))
         (assert! (outbound 'stop)))