Implement text rendering along paths

This commit also changes the TextReplay.drawText() signature, and moves
geometry calculation into drawText(). This improves performance where no
text needs to be rendered (TextStyle.getText() == ''), which is used often
in applications.

src/ol/geom/flat/textpath.js

@@ -0,0 +1,75 @@
+goog.provide('ol.geom.flat.textpath');
+
+goog.require('ol.math');
+
+
+/**
+ * @param {Array.<number>} flatCoordinates Path to put text on.
+ * @param {number} offset Start offset of the `flatCoordinates`.
+ * @param {number} end End offset of the `flatCoordinates`.
+ * @param {number} stride Stride.
+ * @param {string} text Text to place on the path.
+ * @param {function(string):number} measure Measure function returning the
+ * width of the character passed as 1st argument.
+ * @param {number} startM m along the path where the text starts.
+ * @param {number} maxAngle Max angle between adjacent chars in radians.
+ * @param {Array.<Array.<number>>=} opt_result Array that will be populated with the
+ * result. Each entry consists of an array of x, y and z of the char to draw.
+ * If provided, this array will not be truncated to the number of characters of
+ * the `text`.
+ * @return {Array.<Array.<number>>} The result array of null if `maxAngle` was
+ * exceeded.
+ */
+ol.geom.flat.textpath.lineString = function(
+    flatCoordinates, offset, end, stride, text, measure, startM, maxAngle, opt_result) {
+  var result = opt_result ? opt_result : [];
+
+  // Keep text upright
+  var reverse = flatCoordinates[offset] > flatCoordinates[end - stride];
+
+  var numChars = text.length;
+
+  var x1 = flatCoordinates[offset];
+  var y1 = flatCoordinates[offset + 1];
+  offset += stride;
+  var x2 = flatCoordinates[offset];
+  var y2 = flatCoordinates[offset + 1];
+  var segmentM = 0;
+  var segmentLength = Math.sqrt(Math.pow(x2 - x1, 2) + Math.pow(y2 - y1, 2));
+
+  var index, previousAngle;
+  for (var i = 0; i < numChars; ++i) {
+    index = reverse ? numChars - i - 1 : i;
+    var char = text[index];
+    var charLength = measure(char);
+    var charM = startM + charLength / 2;
+    while (offset < end - stride && segmentM + segmentLength < charM) {
+      x1 = x2;
+      y1 = y2;
+      offset += stride;
+      x2 = flatCoordinates[offset];
+      y2 = flatCoordinates[offset + 1];
+      segmentM += segmentLength;
+      segmentLength = Math.sqrt(Math.pow(x2 - x1, 2) + Math.pow(y2 - y1, 2));
+    }
+    var segmentPos = charM - segmentM;
+    var angle = Math.atan2(y2 - y1, x2 - x1);
+    if (reverse) {
+      angle += angle > 0 ? -Math.PI : Math.PI;
+    }
+    if (previousAngle !== undefined) {
+      var delta = angle - previousAngle;
+      delta += (delta > Math.PI) ? -2 * Math.PI : (delta < -Math.PI) ? 2 * Math.PI : 0;
+      if (Math.abs(delta) > maxAngle) {
+        return null;
+      }
+    }
+    previousAngle = angle;
+    var interpolate = segmentPos / segmentLength;
+    var x = ol.math.lerp(x1, x2, interpolate);
+    var y = ol.math.lerp(y1, y2, interpolate);
+    result[index] = [x, y, angle];
+    startM += charLength;
+  }
+  return result;
+};