added image-baseline

svn: r16745

collects/2htdp/image.ss

@@ -90,4 +90,5 @@ and they all have good sample contracts. (It is amazing what we can do with kids
          side-count?
          
          image-width
-         image-height)
+         image-height
+         image-baseline)

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

@@ -6,6 +6,10 @@
 
 (define mapping
   (list
+   (list '(image-height (rectangle 100 100 "solid" "black")) 'val 100)
+   (list '(image-baseline (rectangle 100 100 "solid" "black")) 'val 100)
+   (list '(image-height (text "Hello" 24 "black")) 'val 24.0)
+   (list '(image-baseline (text "Hello" 24 "black")) 'val 18.0)
    (list
     '(image-height
       (overlay (circle 20 "solid" "orange") (circle 30 "solid" "purple")))

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

@@ -36,11 +36,11 @@
                                  (schemeblock #,(schemeresult #,(cadr line)))]
                                 [(image)
                                  (let ([fn (format "2htdp/scribblings/img/~a" (cadr line))])
-                                   (if (file-exists? fn)
-                                       (schemeblock #,(image fn))
-                                       (make-paragraph
-                                        error-color 
-                                        (format "missing image! ~a" (cadr line)))))]))))
+                                   (schemeblock #,(image fn)))]
+                                [(missing)
+                                 (make-paragraph
+                                  error-color 
+                                  "missing result; need to re-run image-gen.ss")]))))
                     expr-paras
                     val-list+outputs)))))
 
@@ -51,4 +51,4 @@
       [else
        (unless (getenv "PLTSHOWIMAGES")
          (fprintf (current-error-port) "exp->filename: unknown exp ~s\n" exp))
-       (list 'image "unk.png")])))
+       (list 'missing)])))

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

@@ -451,6 +451,17 @@ other. The top and bottom pair of angles is @scheme[angle] and the left and righ
                                          (circle 30 "solid" "purple")))]
   }
 
+@defproc[(image-baseline [i image?]) (and/c number? positive?)]{
+  Returns the distance from the top of the image to its baseline. 
+  Unless the image was constructed with @scheme[text] or @scheme[text/font],
+  this will be the same as its height.
+  
+  @image-examples[(image-baseline (text "Hello" 24 "black"))
+                  (image-height (text "Hello" 24 "black"))
+                  (image-baseline (rectangle 100 100 "solid" "black"))
+                  (image-height (rectangle 100 100 "solid" "black"))]
+}
+
 @section{Image Predicates}
 
 This section lists predicates for the basic structures provided by the image library.