path docs: Add another illustration

docs/reference/gsk/gsk4.toml.in

@@ -43,6 +43,8 @@ content_images = [
   "images/caps-light.png",
   "images/cubic-dark.png",
   "images/cubic-light.png",
+  "images/curvature-dark.png",
+  "images/curvature-light.png",
   "images/directions-dark.png",
   "images/directions-light.png",
   "images/fill-even-odd.png",

docs/reference/gsk/images/curvature.svg

@@ -0,0 +1,65 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!-- Created with Inkscape (http://www.inkscape.org/) -->
+
+<svg
+   width="210mm"
+   height="297mm"
+   viewBox="0 0 210 297"
+   version="1.1"
+   id="svg1"
+   inkscape:version="1.3 (0e150ed6c4, 2023-07-21)"
+   sodipodi:docname="curvature.svg"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg">
+  <sodipodi:namedview
+     id="namedview1"
+     pagecolor="#ffffff"
+     bordercolor="#000000"
+     borderopacity="0.25"
+     inkscape:showpageshadow="2"
+     inkscape:pageopacity="0.0"
+     inkscape:pagecheckerboard="0"
+     inkscape:deskcolor="#d1d1d1"
+     inkscape:document-units="mm"
+     inkscape:zoom="1.0746575"
+     inkscape:cx="396.87064"
+     inkscape:cy="561.57428"
+     inkscape:window-width="1920"
+     inkscape:window-height="1123"
+     inkscape:window-x="0"
+     inkscape:window-y="0"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="layer1" />
+  <defs
+     id="defs1" />
+  <g
+     inkscape:label="Layer 1"
+     inkscape:groupmode="layer"
+     id="layer1">
+    <path
+       style="fill:none;stroke:#000000;stroke-width:0.2;stroke-linecap:square;stroke-linejoin:bevel;stroke-miterlimit:10;stroke-dasharray:none"
+       d="m 61.499887,117.85302 0.05055,-14.89554"
+       id="path3"
+       sodipodi:nodetypes="cc" />
+    <path
+       style="fill:none;stroke:#000000;stroke-width:1;stroke-linecap:square;stroke-linejoin:bevel;stroke-miterlimit:10;stroke-dasharray:none"
+       d="m 47.704374,176.11887 c 0,0 -17.048386,-67.06278 9.674156,-72.76426 29.478854,-6.289566 30.433539,55.72526 58.46942,50.81836"
+       id="path1"
+       sodipodi:nodetypes="csc" />
+    <ellipse
+       style="fill:none;stroke:#000000;stroke-width:0.2;stroke-linecap:square;stroke-linejoin:bevel;stroke-miterlimit:10;stroke-dasharray:none"
+       id="path2"
+       cy="117.87344"
+       cx="61.445835"
+       rx="14.692688"
+       ry="14.525347" />
+    <circle
+       style="fill:#818181;fill-opacity:1;stroke:#000000;stroke-width:0.2;stroke-linecap:square;stroke-linejoin:bevel;stroke-miterlimit:10;stroke-dasharray:none;stroke-opacity:1"
+       id="path2-8-9"
+       cx="61.42588"
+       cy="117.79034"
+       r="1.5" />
+  </g>
+</svg>

gsk/gskpathpoint.c

@@ -253,10 +253,16 @@ gsk_path_point_get_rotation (const GskPathPoint *point,
  * Calculates the curvature of the path at the point.
  *
  * Optionally, returns the center of the osculating circle as well.
+ * The curvature is the inverse of the radius of the osculating circle.
  *
  * If the curvature is infinite (at line segments), zero is returned,
  * and @center is not modified.
  *
+ * <picture>
+ *   <source srcset="curvature-dark.png" media="(prefers-color-scheme: dark)">
+ *   <img alt="Osculating circle" src="curvature-light.png">
+ * </picture>
+
  * Returns: The curvature of the path at the given point
  *
  * Since: 4.14