pict: octagon: Consider the border-width in the total dimensions.

* pict.scm (octagon): Do not cut off the edges by considering the line
width when computing the outer dimensions; add docstring.

1 files changed, 15 insertions, 9 deletions

diff --git a/pict.scm b/pict.scm
index 1d4777e..e1a6f82 100644
--- a/pict.scm
+++ b/pict.scm
@@ -649,15 +649,21 @@ override the line color and line thickness."
                   #:key
                   (border-color "black")
                   (border-width 1))
-  (let ((third (exact->inexact (/ size 3))))
-    (polygon `((0 . ,(* 2 third))
-               (0 . ,third)
-               (,third . 0)
-               (,(* 2 third) . 0)
-               (,size . ,third)
-               (,size . ,(* 2 third))
-               (,(* 2 third) . ,size)
-               (,third . ,size))
+  "Return an octagon (more accurately a p8 symmetric isogonal octagon)
+with a maximum width of SIZE.  The keys BORDER-COLOR (a string) and
+BORDER-WIDTH (a number) are accepted to override the default line
+color and line thickness."
+  (let* ((half-line-width (exact->inexact (/ border-width 2)))
+         (size* (- size half-line-width))
+         (third (exact->inexact (/ size 3))))
+    (polygon `((,half-line-width . ,(* 2 third))
+               (,half-line-width . ,third)
+               (,third . ,half-line-width)
+               (,(* 2 third) . ,half-line-width)
+               (,size* . ,third)
+               (,size* . ,(* 2 third))
+               (,(* 2 third) . ,size*)
+               (,third . ,size*))
              #:border-color border-color
              #:border-width border-width)))