improve the area math

modules/math/area.js

@@ -3,19 +3,60 @@
  * @Date: 2020-04-24 14:49:37
  */
 
+import { Cesium } from '@dc-modules/namespace'
 import { Transform } from '@dc-modules/transform'
 
+/**
+ *
+ * @param p0
+ * @param p1
+ * @param p2
+ * @returns {number}
+ * @private
+ */
+function triangleArea(p0, p1, p2) {
+  let v0 = Cesium.Cartesian3.subtract(p0, p1, new Cesium.Cartesian3())
+  let v1 = Cesium.Cartesian3.subtract(p2, p1, new Cesium.Cartesian3())
+  let cross = Cesium.Cartesian3.cross(v0, v1, v0)
+  return Cesium.Cartesian3.magnitude(cross) * 0.5
+}
+
 export default function area(positions) {
   let result = 0
-  if (positions && Array.isArray(positions)) {
-    let h = 0
-    let pos = positions.concat(positions[0])
-    for (let i = 1; i < pos.length; i++) {
-      let oel = Transform.transformWGS84ToCartesian(pos[i - 1])
-      let el = Transform.transformWGS84ToCartesian(pos[i])
-      h += oel.x * el.y - el.x * oel.y
-    }
-    result = Math.abs(h).toFixed(2)
+  if (!Array.isArray(positions)) {
+    return result
+  }
+  if (!(positions[0] instanceof Cesium.Cartesian3)) {
+    positions = Transform.transformWGS84ArrayToCartesianArray(positions)
+  }
+  let geometry = Cesium.CoplanarPolygonGeometry.createGeometry(
+    Cesium.CoplanarPolygonGeometry.fromPositions({
+      positions: positions,
+      vertexFormat: Cesium.VertexFormat.POSITION_ONLY
+    })
+  )
+  if (!geometry) {
+    return result
+  }
+  let flatPositions = geometry.attributes.position.values
+  let indices = geometry.indices
+  for (let i = 0; i < indices.length; i += 3) {
+    let p0 = Cesium.Cartesian3.unpack(
+      flatPositions,
+      indices[i] * 3,
+      new Cesium.Cartesian3()
+    )
+    let p1 = Cesium.Cartesian3.unpack(
+      flatPositions,
+      indices[i + 1] * 3,
+      new Cesium.Cartesian3()
+    )
+    let p2 = Cesium.Cartesian3.unpack(
+      flatPositions,
+      indices[i + 2] * 3,
+      new Cesium.Cartesian3()
+    )
+    result += triangleArea(p0, p1, p2)
   }
   return result
 }