turf-tessellate: Pass original coordinate elevations through the tessellate function. (#2855)

* fix tessellate support for elevation

---------

Co-authored-by: James Beard <[email protected]>

Author: prozvora

packages/turf-tesselate/index.ts

@@ -50,15 +50,28 @@ function tesselate(
 
 function processPolygon(coordinates: Position[][]) {
   const data = flattenCoords(coordinates);
-  const dim = 2;
+  // coordinates are normalized to 3 dimensions by passing through original elevation value, or padding with undefined
+  const dim = 3;
   const result = earcut(data.vertices, data.holes, dim);
 
   const features: Feature<Polygon>[] = [];
   const vertices: Position[] = [];
 
   result.forEach(function (vert: any, i: number) {
     const index = result[i];
-    vertices.push([data.vertices[index * dim], data.vertices[index * dim + 1]]);
+    // if elevation component is included in the original coordinate, include it in the output coordinate
+    if (data.vertices[index * dim + 2] !== undefined) {
+      vertices.push([
+        data.vertices[index * dim],
+        data.vertices[index * dim + 1],
+        data.vertices[index * dim + 2],
+      ]);
+    } else {
+      vertices.push([
+        data.vertices[index * dim],
+        data.vertices[index * dim + 1],
+      ]);
+    }
   });
 
   for (var i = 0; i < vertices.length; i += 3) {
@@ -71,7 +84,8 @@ function processPolygon(coordinates: Position[][]) {
 }
 
 function flattenCoords(data: Position[][]) {
-  const dim: number = data[0][0].length,
+  // coordinates are normalized to 3 dimensions by passing through original elevation value, or padding with undefined
+  const dim = 3,
     result: { vertices: number[]; holes: number[]; dimensions: number } = {
       vertices: [],
       holes: [],
@@ -81,6 +95,7 @@ function flattenCoords(data: Position[][]) {
 
   for (let i = 0; i < data.length; i++) {
     for (let j = 0; j < data[i].length; j++) {
+      // elevation member is either included, or undefined is put in its place
       for (let d = 0; d < dim; d++) result.vertices.push(data[i][j][d]);
     }
     if (i > 0) {

packages/turf-tesselate/package.json

@@ -7,7 +7,8 @@
     "Abel Vázquez <@AbelVM>",
     "Morgan Herlocker <@morganherlocker>",
     "Tom MacWright <@tmcw>",
-    "Vladimir Agafonkin <@mourner>"
+    "Vladimir Agafonkin <@mourner>",
+    "Pavel Rozvora <@prozvora>"
   ],
   "license": "MIT",
   "bugs": {

packages/turf-tesselate/test.ts

@@ -3499,5 +3499,98 @@ test("tesselate", function (t) {
     tesselate(featurecollection([]));
   }, /input must be a Polygon or MultiPolygon/);
 
+  var simplePolygonWithElevation = {
+    type: "Feature",
+    id: "CoordsWithElevation",
+    properties: { name: "CoordsWithElevation" },
+    geometry: {
+      type: "Polygon",
+      coordinates: [
+        [
+          [-123.233256, 42.006186, 130],
+          [-114.634459, 35.00118, 130],
+          [-118.183517, 33.763391, 130],
+          [-124.213628, 42.000709, 130],
+          [-123.233256, 42.006186, 130],
+        ],
+      ],
+    },
+  };
+
+  var simpleTrianglesWithElevation = tesselate(simplePolygonWithElevation);
+  t.equal(
+    simpleTrianglesWithElevation.type,
+    "FeatureCollection",
+    "Polygon returns a FeatureCollection"
+  );
+  t.equal(
+    simpleTrianglesWithElevation.features[0].geometry.type,
+    "Polygon",
+    "contains at least 1 triangle"
+  );
+  t.equal(
+    simpleTrianglesWithElevation.features[0].geometry.coordinates[0].length,
+    4,
+    "triangle is valid"
+  );
+  t.equal(
+    simpleTrianglesWithElevation.features[0].geometry.coordinates[0][0][2],
+    130,
+    "triangle coordinates contain elevation"
+  );
+
+  var simpleSquareWithVariableElevation = {
+    type: "Feature",
+    id: "SquareWithVariableElevation",
+    properties: { name: "SquareWithVariableElevation" },
+    geometry: {
+      type: "Polygon",
+      coordinates: [
+        [
+          [1, 1],
+          [1, 2, 50],
+          [2, 2, 75],
+          [2, 1],
+          [1, 1],
+        ],
+      ],
+    },
+  };
+
+  var simpleVariableElevationTriangles = tesselate(
+    simpleSquareWithVariableElevation
+  );
+
+  t.equal(
+    simpleVariableElevationTriangles.type,
+    "FeatureCollection",
+    "Polygon returns a FeatureCollection"
+  );
+
+  t.deepEqual(
+    simpleVariableElevationTriangles.features[0].geometry.coordinates,
+    [
+      [
+        [1, 2, 50],
+        [1, 1],
+        [2, 1],
+        [1, 2, 50],
+      ],
+    ],
+    "first triangle coordinates contain original elevations"
+  );
+  t.deepEqual(
+    simpleVariableElevationTriangles.features[1].geometry.coordinates,
+    [
+      [
+        [2, 1],
+        [2, 2, 75],
+        [1, 2, 50],
+        [2, 1],
+      ],
+    ],
+    "second triangle coordinates contain original elevations"
+  );
+
   t.end();
 });