Merge pull request #252 from twpayne/point-resolution

Point resolution

src/ol/projection.exports

@@ -2,6 +2,7 @@
 @exportProperty ol.Projection.prototype.getAxisOrientation
 @exportProperty ol.Projection.prototype.getCode
 @exportProperty ol.Projection.prototype.getExtent
+@exportProperty ol.Projection.prototype.getPointResolution
 @exportProperty ol.Projection.prototype.getUnits
 
 @exportSymbol ol.ProjectionUnits

src/ol/projection.js

@@ -8,6 +8,7 @@ goog.require('goog.object');
 goog.require('ol.Coordinate');
 goog.require('ol.Extent');
 goog.require('ol.TransformFunction');
+goog.require('ol.sphere.NORMAL');
 
 
 /**
@@ -85,6 +86,14 @@ ol.Projection.prototype.getExtent = function() {
 };
 
 
+/**
+ * @param {number} resolution Resolution.
+ * @param {ol.Coordinate} point Point.
+ * @return {number} Point resolution.
+ */
+ol.Projection.prototype.getPointResolution = goog.abstractMethod;
+
+
 /**
  * @return {ol.ProjectionUnits} Units.
  */
@@ -121,10 +130,49 @@ ol.Proj4jsProjection_ = function(code, proj4jsProj) {
    */
   this.proj4jsProj_ = proj4jsProj;
 
+  /**
+   * @private
+   * @type {?ol.TransformFunction}
+   */
+  this.toEPSG4326_ = null;
+
 };
 goog.inherits(ol.Proj4jsProjection_, ol.Projection);
 
 
+/**
+ * @inheritDoc
+ */
+ol.Proj4jsProjection_.prototype.getPointResolution =
+    function(resolution, point) {
+  if (this.getUnits() == ol.ProjectionUnits.DEGREES) {
+    return resolution;
+  } else {
+    // Estimate point resolution by transforming the center pixel to EPSG:4326,
+    // measuring its width and height on the normal sphere, and taking the
+    // average of the width and height.
+    if (goog.isNull(this.toEPSG4326_)) {
+      this.toEPSG4326_ = ol.projection.getTransform(
+          this, ol.projection.getProj4jsProjectionFromCode_('EPSG:4326'));
+    }
+    var vertices = [
+      point.x - resolution / 2, point.y,
+      point.x + resolution / 2, point.y,
+      point.x, point.y - resolution / 2,
+      point.x, point.y + resolution / 2
+    ];
+    vertices = this.toEPSG4326_(vertices, vertices, 2);
+    var width = ol.sphere.NORMAL.haversineDistance(
+        new ol.Coordinate(vertices[0], vertices[1]),
+        new ol.Coordinate(vertices[2], vertices[3]));
+    var height = ol.sphere.NORMAL.haversineDistance(
+        new ol.Coordinate(vertices[4], vertices[5]),
+        new ol.Coordinate(vertices[6], vertices[7]));
+    return (width + height) / 2;
+  }
+};
+
+
 /**
  * @return {Proj4js.Proj} Proj4js projection.
  */

src/ol/projection/epsg3857.js

@@ -4,6 +4,7 @@ goog.require('goog.array');
 goog.require('ol.Extent');
 goog.require('ol.Projection');
 goog.require('ol.ProjectionUnits');
+goog.require('ol.math');
 goog.require('ol.projection');
 
 
@@ -128,3 +129,12 @@ ol.projection.EPSG3857.toEPSG4326 = function(input, opt_output, opt_dimension) {
   }
   return output;
 };
+
+
+/**
+ * @inheritDoc
+ */
+ol.projection.EPSG3857.prototype.getPointResolution =
+    function(resolution, point) {
+  return resolution / ol.math.cosh(point.y / ol.projection.EPSG3857.RADIUS);
+};

src/ol/projection/epsg4326.js

@@ -41,3 +41,12 @@ ol.projection.EPSG4326.PROJECTIONS = [
   new ol.projection.EPSG4326('urn:ogc:def:crs:EPSG:6.6:4326', 'neu'),
   new ol.projection.EPSG4326('urn:ogc:def:crs:OGC:1.3:CRS84')
 ];
+
+
+/**
+ * @inheritDoc
+ */
+ol.projection.EPSG4326.prototype.getPointResolution =
+    function(resolution, point) {
+  return resolution;
+};

src/ol/sphere/normal.js

@@ -0,0 +1,10 @@
+goog.provide('ol.sphere.NORMAL');
+
+goog.require('ol.Sphere');
+
+
+/**
+ * The normal sphere.
+ * @const {ol.Sphere}
+ */
+ol.sphere.NORMAL = new ol.Sphere(6370997);

src/ol/sphere/wgs84.js

@@ -0,0 +1,10 @@
+goog.provide('ol.sphere.WGS84');
+
+goog.require('ol.Sphere');
+
+
+/**
+ * A sphere with radius equal to the semi-major axis of the WGS84 ellipsoid.
+ * @const {ol.Sphere}
+ */
+ol.sphere.WGS84 = new ol.Sphere(6378137);

test/spec/ol/projection.epsg3857.test.js

@@ -0,0 +1,49 @@
+goog.provide('ol.test.projection.EPSG3857');
+
+
+describe('ol.projection.EPSG3857', function() {
+
+  describe('getPointResolution', function() {
+
+    it('returns the correct point scale at the equator', function() {
+      // @see http://msdn.microsoft.com/en-us/library/aa940990.aspx
+      var epsg3857 = ol.projection.getFromCode('EPSG:3857');
+      var resolution = 19.11;
+      var point = new ol.Coordinate(0, 0);
+      expect(epsg3857.getPointResolution(resolution, point)).
+          toRoughlyEqual(19.11, 1e-1);
+    });
+
+    it('returns the correct point scale at the latitude of Toronto',
+        function() {
+      // @see http://msdn.microsoft.com/en-us/library/aa940990.aspx
+      var epsg3857 = ol.projection.getFromCode('EPSG:3857');
+      var epsg4326 = ol.projection.getFromCode('EPSG:4326');
+      var resolution = 19.11;
+      var point = ol.projection.transform(
+          new ol.Coordinate(0, 43.65), epsg4326, epsg3857);
+      expect(epsg3857.getPointResolution(resolution, point)).
+          toRoughlyEqual(19.11 * Math.cos(Math.PI * 43.65 / 180), 1e-9);
+    });
+
+    it('returns the correct point scale at various latitudes', function() {
+      // @see http://msdn.microsoft.com/en-us/library/aa940990.aspx
+      var epsg3857 = ol.projection.getFromCode('EPSG:3857');
+      var epsg4326 = ol.projection.getFromCode('EPSG:4326');
+      var resolution = 19.11;
+      var latitude;
+      for (latitude = 0; latitude < 90; ++latitude) {
+        var point = ol.projection.transform(
+            new ol.Coordinate(0, latitude), epsg4326, epsg3857);
+        expect(epsg3857.getPointResolution(resolution, point)).
+            toRoughlyEqual(19.11 * Math.cos(Math.PI * latitude / 180), 1e-9);
+      }
+    });
+
+  });
+
+});
+
+
+goog.require('ol.Coordinate');
+goog.require('ol.projection.EPSG3857');

test/spec/ol/projection.test.js

@@ -144,6 +144,36 @@ describe('ol.projection', function() {
       expect(point.y).toRoughlyEqual(200146.976, 1);
     });
 
+    it('numerically estimates point scale at the equator', function() {
+      var googleProjection = ol.projection.getFromCode('GOOGLE');
+      expect(googleProjection.getPointResolution(1, new ol.Coordinate(0, 0))).
+          toRoughlyEqual(1, 1e-1);
+    });
+
+    it('numerically estimates point scale at various latitudes', function() {
+      var epsg3857Projection = ol.projection.getFromCode('EPSG:3857');
+      var googleProjection = ol.projection.getFromCode('GOOGLE');
+      var point, y;
+      for (y = -20; y <= 20; ++y) {
+        point = new ol.Coordinate(0, 1000000 * y);
+        expect(googleProjection.getPointResolution(1, point)).toRoughlyEqual(
+            epsg3857Projection.getPointResolution(1, point), 1e-1);
+      }
+    });
+
+    it('numerically estimates point scale at various points', function() {
+      var epsg3857Projection = ol.projection.getFromCode('EPSG:3857');
+      var googleProjection = ol.projection.getFromCode('GOOGLE');
+      var point, x, y;
+      for (x = -20; x <= 20; ++x) {
+        for (y = -20; y <= 20; ++y) {
+          point = new ol.Coordinate(1000000 * x, 1000000 * y);
+          expect(googleProjection.getPointResolution(1, point)).toRoughlyEqual(
+              epsg3857Projection.getPointResolution(1, point), 1e-1);
+        }
+      }
+    });
+
   });
 
   describe('ol.projection.getTransform()', function() {