Add support for paths to svg.el

* doc/lispref/display.texi (SVG Images): Document svg-path,
svg-clip-path and svg-node (bug#32359).

* doc/lispref/display.texi (SVG Path Commands): New node.

* lisp/svg.el (svg--plist-delete, svg--path-command-symbol)
(svg--elliptical-arc-coordinates, svg--elliptical-arc-command)
(svg--moveto-command, svg--closepath-command)
(svg--lineto-command, svg--horizontal-lineto-command)
(svg--vertical-lineto-command, svg--curveto-command)
(svg--smooth-curveto-command)
(svg--quadratic-bezier-curveto-command)
(svg--smooth-quadratic-bezier-curveto-command)
(svg--eval-path-command, svg-path, svg-clip-path, svg-node): New
functions.

1 files changed, 148 insertions, 0 deletions

diff --git a/lisp/svg.el b/lisp/svg.el
index 86b56a03d56..2ab56d3960d 100644
--- a/lisp/svg.el
+++ b/lisp/svg.el
@@ -3,6 +3,7 @@
 ;; Copyright (C) 2014-2019 Free Software Foundation, Inc.
 
 ;; Author: Lars Magne Ingebrigtsen <larsi@gnus.org>
+;;         Felix E. Klee <felix.klee@inka.de>
 ;; Keywords: image
 ;; Version: 1.0
 ;; Package-Requires: ((emacs "25"))
@@ -324,6 +325,153 @@ If the SVG is later changed, the image will also be updated."
                              "\\'")))))
     (when node (dom-remove-node svg node))))
 
+;; Function body copied from `org-plist-delete' in Emacs 26.1.
+(defun svg--plist-delete (plist property)
+  "Delete PROPERTY from PLIST.
+This is in contrast to merely setting it to 0."
+  (let (p)
+    (while plist
+      (if (not (eq property (car plist)))
+          (setq p (plist-put p (car plist) (nth 1 plist))))
+      (setq plist (cddr plist)))
+    p))
+
+(defun svg--path-command-symbol (command-symbol command-args)
+  (let ((char (symbol-name command-symbol))
+        (relative (if (plist-member command-args :relative)
+                      (plist-get command-args :relative)
+                    (plist-get command-args :default-relative))))
+    (intern (if relative (downcase char) (upcase char)))))
+
+(defun svg--elliptical-arc-coordinates
+    (rx ry x y &rest args)
+  (list
+   rx ry
+   (or (plist-get args :x-axis-rotation) 0)
+   (if (plist-get args :large-arc) 1 0)
+   (if (plist-get args :sweep) 1 0)
+   x y))
+
+(defun svg--elliptical-arc-command (coordinates-list &rest args)
+  (cons
+   (svg--path-command-symbol 'a args)
+   (apply 'append
+          (mapcar
+           (lambda (coordinates)
+             (apply 'svg--elliptical-arc-coordinates
+                    coordinates))
+           coordinates-list))))
+
+(defun svg--moveto-command (coordinates-list &rest args)
+  (cons
+   (svg--path-command-symbol 'm args)
+   (apply 'append
+          (mapcar
+           (lambda (coordinates)
+             (list (car coordinates) (cdr coordinates)))
+           coordinates-list))))
+
+(defun svg--closepath-command (&rest args)
+  (list (svg--path-command-symbol 'z args)))
+
+(defun svg--lineto-command (coordinates-list &rest args)
+  (cons
+   (svg--path-command-symbol 'l args)
+   (apply 'append
+          (mapcar
+           (lambda (coordinates)
+             (list (car coordinates) (cdr coordinates)))
+           coordinates-list))))
+
+(defun svg--horizontal-lineto-command (coordinate-list &rest args)
+  (cons
+   (svg--path-command-symbol 'h args)
+   coordinate-list))
+
+(defun svg--vertical-lineto-command (coordinate-list &rest args)
+  (cons
+   (svg--path-command-symbol 'v args)
+   coordinate-list))
+
+(defun svg--curveto-command (coordinates-list &rest args)
+  (cons
+   (svg--path-command-symbol 'c args)
+   (apply 'append coordinates-list)))
+
+(defun svg--smooth-curveto-command (coordinates-list &rest args)
+  (cons
+   (svg--path-command-symbol 's args)
+   (apply 'append coordinates-list)))
+
+(defun svg--quadratic-bezier-curveto-command (coordinates-list
+                                              &rest args)
+  (cons
+   (svg--path-command-symbol 'q args)
+   (apply 'append coordinates-list)))
+
+(defun svg--smooth-quadratic-bezier-curveto-command (coordinates-list
+                                                     &rest args)
+  (cons
+   (svg--path-command-symbol 't args)
+   (apply 'append coordinates-list)))
+
+(defun svg--eval-path-command (command default-relative)
+  (cl-letf
+      (((symbol-function 'moveto) #'svg--moveto-command)
+       ((symbol-function 'closepath) #'svg--closepath-command)
+       ((symbol-function 'lineto) #'svg--lineto-command)
+       ((symbol-function 'horizontal-lineto)
+        #'svg--horizontal-lineto-command)
+       ((symbol-function 'vertical-lineto)
+        #'svg--vertical-lineto-command)
+       ((symbol-function 'curveto) #'svg--curveto-command)
+       ((symbol-function 'smooth-curveto)
+        #'svg--smooth-curveto-command)
+       ((symbol-function 'quadratic-bezier-curveto)
+        #'svg--quadratic-bezier-curveto-command)
+       ((symbol-function 'smooth-quadratic-bezier-curveto)
+        #'svg--smooth-quadratic-bezier-curveto-command)
+       ((symbol-function 'elliptical-arc)
+        #'svg--elliptical-arc-command)
+       (extended-command (append command (list :default-relative
+                                               default-relative))))
+    (mapconcat 'prin1-to-string (apply extended-command) " ")))
+
+(defun svg-path (svg commands &rest args)
+  "Add the outline of a shape to SVG according to COMMANDS.
+Coordinates by default are absolute.  ARGS is a plist of
+modifiers.  If :relative is t, then coordinates are relative to
+the last position, or -- initially -- to the origin."
+  (let* ((default-relative (plist-get args :relative))
+         (stripped-args (svg--plist-delete args :relative))
+         (d (mapconcat 'identity
+                       (mapcar
+                        (lambda (command)
+                          (svg--eval-path-command command
+                                                  default-relative))
+                        commands) " ")))
+    (svg--append
+     svg
+     (dom-node 'path
+               `((d . ,d)
+                 ,@(svg--arguments svg stripped-args))))))
+
+(defun svg-clip-path (svg &rest args)
+  "Add a clipping path to SVG, where ARGS is a plist of modifiers.
+If applied to a shape via the :clip-path property, parts of that
+shape which lie outside of the clipping path are not drawn."
+  (let ((new-dom-node (dom-node 'clipPath
+                                `(,@(svg--arguments svg args)))))
+    (svg--append svg new-dom-node)
+    new-dom-node))
+
+(defun svg-node (svg tag &rest args)
+  "Add the custom node TAG to SVG."
+  (let ((new-dom-node (dom-node tag
+                                `(,@(svg--arguments svg args)))))
+    (svg--append svg new-dom-node)
+    new-dom-node))
+
 (provide 'svg)
 
 ;;; svg.el ends here