clean up drawing with 0-width pens (hairline mode)

collects/racket/draw/private/dc.rkt

@@ -122,6 +122,11 @@
     ;; Used to keep smoothing disabled for b&w contexts
     dc-adjust-smoothing
 
+    ;; get-hairline-width
+    ;;
+    ;; Gets the pen width to use in place of 0 in 'smoothed mode
+    get-hairline-width
+
     ;; install-color : cairo_t color<%> alpha boolean? -> void
     ;; 
     ;; Installs a color, which a monochrome context might reduce
@@ -186,6 +191,7 @@
     (define/public (ok?) #t)
 
     (define/public (dc-adjust-smoothing s) s)
+    (define/public (get-hairline-width sx) (/ 1 sx))
 
     (define/public (install-color cr c a bg?)
       (let ([norm (lambda (v) (/ v 255.0))])
@@ -236,7 +242,7 @@
     (super-new)
 
     (inherit flush-cr get-cr release-cr end-cr init-cr-matrix get-pango
-             install-color dc-adjust-smoothing reset-clip
+             install-color dc-adjust-smoothing get-hairline-width reset-clip
              collapse-bitmap-b&w?
              ok? can-combine-text? can-mask-bitmap? get-clear-operator)
 
@@ -873,11 +879,14 @@
                                  alpha
                                  #f)))
             (cairo_set_line_width cr (let* ([v (send pen get-width)]
-                                            [v (if (aligned? smoothing)
+                                            [align? (aligned? smoothing)]
+                                            [v (if align?
                                                    (/ (floor (* effective-scale-x v)) effective-scale-x)
                                                    v)])
                                        (if (zero? v)
-                                           1
+                                           (if align?
+                                               (/ 1 effective-scale-x)
+                                               (get-hairline-width effective-scale-x))
                                            v)))
             (unless (or (eq? s 'solid)
                         (eq? s 'xor))

collects/racket/draw/private/local.rkt

@@ -44,6 +44,7 @@
   get-font-metrics-key
   install-color
   dc-adjust-smoothing
+  get-hairline-width
   can-combine-text?
   can-mask-bitmap?
   reset-clip

collects/racket/draw/private/post-script-dc.rkt

@@ -182,6 +182,8 @@
     (define/override (can-mask-bitmap?)
       #f)
 
+    (define/override (get-hairline-width cx) (/ 1.0 (* cx 4)))
+
     (define is-eps? (and as-eps #t))
     (define/public (multiple-pages-ok?) (not is-eps?))
 

collects/scribblings/draw/pen-class.scrbl

@@ -49,10 +49,15 @@ To avoid creating multiple pens with the same characteristics, use the
  global @scheme[pen-list%] object @indexed-scheme[the-pen-list], or
  provide a color, width, and style to @xmethod[dc<%> set-pen].
 
-A pen of size @scheme[0] uses the minimum line size for the
- destination drawing context.  In (unscaled) canvases and bitmaps, 
- a zero-width pen behaves the nearly same as a pen of
- size @scheme[1].
+When drawing in @racket['smoothed] or @racket['aligned] mode, a pen's
+ size is truncated after scaling to an integral size. A pen of size
+ @scheme[0] (after truncation, if applicable) uses a non-zero,
+ scale-insensitive line size for the destination drawing context:
+ @racket[1/4] unit (after scaling) for @racket[post-script-dc%] or
+ @racket[pdf-dc%] contexts in @racket['smoothed] mode, or @racket[1]
+ unit (after scaling) for any other context.  For example, in unscaled 
+ canvas and bitmap contexts, a zero-width pen behaves the same as a
+ pen of size @scheme[1].
 
 
 @defconstructor[([color (or/c string? (is-a?/c color%)) "black"]

doc/release-notes/racket/Draw_and_GUI_5_1.txt

@@ -108,6 +108,13 @@ generates PDF output.
 The new `svg-dc%' drawing context is similar to `post-script-dc%',
 but it generates SVG output.
 
+All drawing contexts, including `post-script-dc%' support smoothing
+modes, and the default is 'unsmoothed even for `post-script-dc%' ---
+which means that PostScript drawing is aligned and pen widths are
+truncated to integer sizes by default. Drawing at pen size 0 no longer
+triggers ``hairline'' mode, but instead just uses a 1/4-width pen
+when in 'smoothed mode.
+
 
 Other Drawing-Context Changes
 -----------------------------