Return to simpler and less efficient winding order algorithm

src/ol/geom/flat/orient.js

@@ -14,110 +14,19 @@ import {coordinates as reverseCoordinates} from './reverse.js';
  * @return {boolean} Is clockwise.
  */
 export function linearRingIsClockwise(flatCoordinates, offset, end, stride) {
-  // https://stackoverflow.com/a/1180256/2389327
-  // https://en.wikipedia.org/wiki/Curve_orientation#Orientation_of_a_simple_polygon
-  if ((end - offset) / stride >= 3) {
-    const minVertex = findCornerVertex(flatCoordinates, offset, end, stride);
-    // Orientation matrix:
-    //     [ 1  xa  ya ]
-    // O = | 1  xb  yb |
-    //     [ 1  xc  yc ]
-    const previousVertex = findUniqueVertex(
-      flatCoordinates,
-      offset,
-      end,
-      stride,
-      minVertex,
-      -1
-    );
-    const nextVertex = findUniqueVertex(
-      flatCoordinates,
-      offset,
-      end,
-      stride,
-      minVertex,
-      1
-    );
-    const aX = flatCoordinates[previousVertex];
-    const aY = flatCoordinates[previousVertex + 1];
-    const bX = flatCoordinates[minVertex];
-    const bY = flatCoordinates[minVertex + 1];
-    const cX = flatCoordinates[nextVertex];
-    const cY = flatCoordinates[nextVertex + 1];
-    const determinant =
-      bX * cY + aX * bY + aY * cX - (aY * bX + bY * cX + aX * cY);
-
-    return determinant < 0;
+  // http://tinyurl.com/clockwise-method
+  // https://github.com/OSGeo/gdal/blob/trunk/gdal/ogr/ogrlinearring.cpp
+  let edge = 0;
+  let x1 = flatCoordinates[end - stride];
+  let y1 = flatCoordinates[end - stride + 1];
+  for (; offset < end; offset += stride) {
+    const x2 = flatCoordinates[offset];
+    const y2 = flatCoordinates[offset + 1];
+    edge += (x2 - x1) * (y2 + y1);
+    x1 = x2;
+    y1 = y2;
   }
-}
-
-/**
- * Finds the next unique vertex in forward or backward direction of a ring.
- * @param {Array<number>} flatCoordinates Flat coordinates.
- * @param {number} offset Offset.
- * @param {number} end End.
- * @param {number} stride Stride.
- * @param {number} start Start vertex.
- * @param {number} direction 1 for forward, -1 for backward.
- * @return {number} vertex Index of the found vertex.
- */
-function findUniqueVertex(
-  flatCoordinates,
-  offset,
-  end,
-  stride,
-  start,
-  direction
-) {
-  let previousX, previousY, x, y;
-  let i = start;
-  while (x === previousX && y === previousY) {
-    previousX = flatCoordinates[i];
-    previousY = flatCoordinates[i + 1];
-    i += direction * stride;
-    if (i >= end) {
-      i = offset;
-    } else if (i < offset) {
-      i = end - stride;
-    }
-    x = flatCoordinates[i];
-    y = flatCoordinates[i + 1];
-  }
-  return i;
-}
-
-/**
- * Find vertex along one edge of bounding box.
- * In this case, we find smallest y; in case of tie also smallest x.
- * @param {Array<number>} flatCoordinates Flat coordinates.
- * @param {number} offset Offset.
- * @param {number} end End.
- * @param {number} stride Stride.
- * @return {number} Corner vertex.
- */
-function findCornerVertex(flatCoordinates, offset, end, stride) {
-  let iMinVertex = -1;
-  let minY = Infinity;
-  let minXAtMinY = Infinity;
-  for (let i = offset; i < end; i += stride) {
-    const x = flatCoordinates[i];
-    const y = flatCoordinates[i + 1];
-    if (y > minY) {
-      continue;
-    }
-    if (y == minY) {
-      if (x >= minXAtMinY) {
-        continue;
-      }
-    }
-
-    // Minimum so far.
-    iMinVertex = i;
-    minY = y;
-    minXAtMinY = x;
-  }
-
-  return iMinVertex;
+  return edge === 0 ? undefined : edge > 0;
 }
 
 /**