expose the area of the geometry as *geometry*.area

It can be used, for example, to filter out "frames", i.e. the whole rectangle returned for a value < min(values), by testing geometry.area > n * m - 3/4
https://observablehq.com/d/0cef46faf6b9c53c

See #14

README.md

@@ -87,7 +87,7 @@ The returned geometry objects are typically passed to [d3.geoPath](https://githu
 
 <a name="contours_contour" href="#contours_contour">#</a> <i>contours</i>.<b>contour</b>(<i>values</i>, <i>threshold</i>) [<>](https://github.com/d3/d3-contour/blob/master/src/contours.js "Source")
 
-Computes a single contour, returning a [GeoJSON](http://geojson.org/geojson-spec.html) [MultiPolygon](http://geojson.org/geojson-spec.html#multipolygon) [geometry object](http://geojson.org/geojson-spec.html#geometry-objects) representing the area where the input <i>values</i> are greater than or equal to the given [*threshold* value](#contours_thresholds); the threshold value for each geometry object is exposed as <i>geometry</i>.value.
+Computes a single contour, returning a [GeoJSON](http://geojson.org/geojson-spec.html) [MultiPolygon](http://geojson.org/geojson-spec.html#multipolygon) [geometry object](http://geojson.org/geojson-spec.html#geometry-objects) representing the area where the input <i>values</i> are greater than or equal to the given [*threshold* value](#contours_thresholds); the threshold value for each geometry object is exposed as <i>geometry</i>.value, and its area exposed as <i>geometry</i>.area.
 
 The input *values* must be an array of length <i>n</i>×<i>m</i> where [<i>n</i>, <i>m</i>] is the contour generator’s [size](#contours_size); furthermore, each <i>values</i>[<i>i</i> + <i>jn</i>] must represent the value at the position ⟨<i>i</i>, <i>j</i>⟩. See [*contours*](#_contours) for an example.
 

src/contours.js

@@ -52,12 +52,15 @@ export default function() {
   // Based on https://github.com/mbostock/shapefile/blob/v0.6.2/shp/polygon.js
   function contour(values, value) {
     var polygons = [],
-        holes = [];
+        holes = [],
+        a = 0;
 
     isorings(values, value, function(ring) {
       smooth(ring, values, value);
-      if (area(ring) > 0) polygons.push([ring]);
+      var ar = area(ring);
+      if (ar > 0) polygons.push([ring]);
       else holes.push(ring);
+      a += ar / 2;
     });
 
     holes.forEach(function(hole) {
@@ -72,6 +75,7 @@ export default function() {
     return {
       type: "MultiPolygon",
       value: value,
+      area: a,
       coordinates: polygons
     };
   }

test/contours-test.js

@@ -18,6 +18,7 @@ tape("contours(values) returns the expected result for an empty polygon", functi
     {
       "type": "MultiPolygon",
       "value": 0.5,
+      "area": 0,
       "coordinates": []
     }
   ]);
@@ -41,6 +42,7 @@ tape("contours(values) returns the expected result for a simple polygon", functi
     {
       "type": "MultiPolygon",
       "value": 0.5,
+      "area": 14.5,
       "coordinates": [
         [
           [[6, 7.5], [6, 6.5], [6, 5.5], [6, 4.5], [6, 3.5], [5.5, 3], [4.5, 3],
@@ -69,6 +71,7 @@ tape("contours(values).contour(value) returns the expected result for a simple p
   ], 0.5), {
     "type": "MultiPolygon",
     "value": 0.5,
+    "area": 14.5,
     "coordinates": [
       [
         [[6, 7.5], [6, 6.5], [6, 5.5], [6, 4.5], [6, 3.5], [5.5, 3], [4.5, 3],
@@ -97,6 +100,7 @@ tape("contours.smooth(false)(values) returns the expected result for a simple po
     {
       "type": "MultiPolygon",
       "value": 0.5,
+      "area": 14.5,
       "coordinates": [
         [
           [[6, 7.5], [6, 6.5], [6, 5.5], [6, 4.5], [6, 3.5], [5.5, 3], [4.5, 3],
@@ -126,6 +130,7 @@ tape("contours(values) returns the expected result for a polygon with a hole", f
     {
       "type": "MultiPolygon",
       "value": 0.5,
+      "area": 12,
       "coordinates": [
         [
           [[6, 7.5], [6, 6.5], [6, 5.5], [6, 4.5], [6, 3.5], [5.5, 3], [4.5, 3],
@@ -157,6 +162,7 @@ tape("contours(values) returns the expected result for a multipolygon", function
     {
       "type": "MultiPolygon",
       "value": 0.5,
+      "area": 14,
       "coordinates": [
         [
           [[5, 7.5], [5, 6.5], [5, 5.5], [5, 4.5], [5, 3.5], [4.5, 3], [3.5, 3],
@@ -190,6 +196,7 @@ tape("contours(values) returns the expected result for a multipolygon with holes
     {
       "type": "MultiPolygon",
       "value": 0.5,
+      "area": 16,
       "coordinates": [
         [
           [[4, 5.5], [4, 4.5], [4, 3.5], [3.5, 3], [2.5, 3], [1.5, 3], [1, 3.5],