Improve area calculation for glyph orientation

lib/svg.js

@@ -17,17 +17,40 @@ this.svg = {};
 
 // Returns twice the area contained in the path. The result is positive iff the
 // path winds in the counter-clockwise direction.
-const get2xArea = (path) => {
-  let area = 0;
+//
+// The approximation error is an upper-bound on the distance between consecutive
+// points in the polygon approximation used to compute the area. The default
+// error of 64 is chosen so that the sign will be correct with good likelihood.
+const get2xArea = (path, approximation_error) => {
+  const polygon_approximation = [path[0].start];
+  approximation_error = approximation_error || 64;
   for (let x of path) {
-    area += (x.end[0] + x.start[0])*(x.end[1] - x.start[1]);
+    let num_points = 0;
+    if (x.control !== undefined) {
+      const distance = Math.sqrt(Point.distance2(x.start, x.end));
+      num_points = Math.floor(distance/approximation_error);
+    }
+    for (let i = 0; i < num_points; i++) {
+      const t = (i + 1)/(num_points + 1);
+      const s = 1 - t;
+      polygon_approximation.push(
+          [s*s*x.start[0] + 2*s*t*x.control[0] + t*t*x.end[0],
+           s*s*x.start[1] + 2*s*t*x.control[1] + t*t*x.end[1]]);
+    }
+    polygon_approximation.push(x.end);
+  }
+  let area = 0;
+  for (var i = 0; i < polygon_approximation.length; i++) {
+    const p1 = polygon_approximation[i];
+    const p2 = polygon_approximation[(i + 1) % polygon_approximation.length];
+    area += (p2[0] + p1[0])*(p2[1] - p1[1]);
   }
   return area;
 }
 
 // Takes a list of paths and orients them so that exterior contours are oriented
 // counter-clockwise and interior contours clockwise.
-const orientPaths = (paths, reverse) => {
+const orientPaths = (paths, approximation_error) => {
   for (var i = 0; i < paths.length; i++) {
     const path = paths[i];
     let contains = 0;
@@ -43,7 +66,7 @@ const orientPaths = (paths, reverse) => {
     // other paths. It is counter-clockwise iff its area is positive. The path
     // should be reversed if (CCW && internal) || (CW && external).
     const should_reverse = (area > 0) !== (contains % 2 === 0);
-    if (should_reverse && !reverse) {
+    if (should_reverse) {
       for (let segment of path) {
         [segment.start, segment.end] = [segment.end, segment.start];
       }
@@ -163,10 +186,8 @@ svg.convertCommandsToPath = (commands) => {
 // Converts a normal-form SVG path string to a list of paths. The paths obey an
 // orientation constraint: the external paths are oriented counter-clockwise,
 // while the internal paths are oriented clockwise.
-//
-// If 'reverse' is true, the paths will all be oriented opposite this rule.
-svg.convertSVGPathToPaths = (path, reverse) => {
-  return orientPaths(splitPath(path), reverse);
+svg.convertSVGPathToPaths = (path) => {
+  return orientPaths(splitPath(path));
 }
 
 // Takes the given list of paths and returns a normal-form SVG path string.

server/migration.js

@@ -2,7 +2,23 @@
 
 const completionCallback = undefined;
 
-const perGlyphCallback = undefined; 
+const perGlyphCallback = (glyph) => {
+  assert(glyph.stages.strokes !== undefined);
+  let changed = 0;
+  for (var i = 0; i < glyph.stages.strokes.length; i++) {
+    const stroke = glyph.stages.strokes[i];
+    const paths = svg.convertSVGPathToPaths(stroke);
+    assert(paths.length === 1);
+    const path = svg.convertPathsToSVGPath(paths);
+    if (path !== stroke) {
+      glyph.stages.strokes[i] = path;
+      changed += 1;
+    }
+  }
+  if (changed) {
+    console.log(`Flipped ${changed} path(s) in glyph: ${glyph.character}`);
+  }
+}
 
 // Runs the given per-glyph callback for each glyph in the database.
 // When all the glyphs are migrated, runs the completion callback.