Convex hull

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Sample viewer app

Create a convex hull for a given set of points. The convex hull is a polygon with shortest perimeter that encloses a set of points. As a visual analogy, consider a set of points as nails in a board. The convex hull of the points would be like a rubber band stretched around the outermost nails.

Image of convex hull

Use case

A convex hull can be useful in collision detection. For example, when charting the position of two yacht fleets (with each vessel represented by a point), if their convex hulls have been precomputed, it is efficient to first check if their convex hulls intersect before computing their proximity point-by-point.

How to use the sample

Click on the map to add points. Click the "Create Convex Hull" button to generate the convex hull of those points. Click the "Clear" button to start over.

How it works

  1. Create an input geometry such as a Multipoint object.
  2. Use GeometryEngine.convexHull(inputGeometry)to create a new Geometry object representing the convex hull of the input points. The returned geometry will either be a Point, Polyline, or Polygon based on the number of input points.

Relevant API

  • Geometry
  • GeometryEngine

Tags

convex hull, geometry, spatial analysis

Sample Code

ConvexHullSample.java
                                                                                                                                                                                                       
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/*
 * Copyright 2018 Esri.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

package com.esri.samples.convex_hull;

import java.util.ArrayList;
import java.util.List;

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.geometry.Point2D;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.input.MouseButton;
import javafx.scene.layout.Background;
import javafx.scene.layout.BackgroundFill;
import javafx.scene.layout.CornerRadii;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.scene.paint.Paint;
import javafx.stage.Stage;

import com.esri.arcgisruntime.ArcGISRuntimeEnvironment;
import com.esri.arcgisruntime.geometry.Geometry;
import com.esri.arcgisruntime.geometry.GeometryEngine;
import com.esri.arcgisruntime.geometry.Multipoint;
import com.esri.arcgisruntime.geometry.Point;
import com.esri.arcgisruntime.geometry.PointCollection;
import com.esri.arcgisruntime.mapping.ArcGISMap;
import com.esri.arcgisruntime.mapping.BasemapStyle;
import com.esri.arcgisruntime.mapping.view.Graphic;
import com.esri.arcgisruntime.mapping.view.GraphicsOverlay;
import com.esri.arcgisruntime.mapping.view.MapView;
import com.esri.arcgisruntime.symbology.SimpleFillSymbol;
import com.esri.arcgisruntime.symbology.SimpleLineSymbol;
import com.esri.arcgisruntime.symbology.SimpleMarkerSymbol;

public class ConvexHullSample extends Application {

  private MapView mapView;

  @Override
  public void start(Stage stage) {

    try {
      // create stack pane and application scene
      StackPane stackPane = new StackPane();
      Scene scene = new Scene(stackPane);

      // set title, size, and add scene to stage
      stage.setTitle("Convex Hull Sample");
      stage.setWidth(800);
      stage.setHeight(700);
      stage.setScene(scene);
      stage.show();

      // authentication with an API key or named user is required to access basemaps and other location services
      String yourAPIKey = System.getProperty("apiKey");
      ArcGISRuntimeEnvironment.setApiKey(yourAPIKey);

      // create a map with the topographic basemap style
      ArcGISMap map = new ArcGISMap(BasemapStyle.ARCGIS_TOPOGRAPHIC);

      // create a map view and set the map to it
      mapView = new MapView();
      mapView.setMap(map);

      // create a graphics overlay to show the input points and convex hull
      GraphicsOverlay graphicsOverlay = new GraphicsOverlay();
      mapView.getGraphicsOverlays().add(graphicsOverlay);

      // create a graphic to show the points
      SimpleMarkerSymbol markerSymbol = new SimpleMarkerSymbol(SimpleMarkerSymbol.Style.CIRCLE, 0xFFFF0000, 10);
      Graphic inputsGraphic = new Graphic();
      inputsGraphic.setSymbol(markerSymbol);
      graphicsOverlay.getGraphics().add(inputsGraphic);

      // create a graphic to show the convex hull as a blue outline
      SimpleLineSymbol lineSymbol = new SimpleLineSymbol(SimpleLineSymbol.Style.SOLID, 0xFF0000FF, 3);
      SimpleFillSymbol fillSymbol = new SimpleFillSymbol(SimpleFillSymbol.Style.NULL, 0x00000000, lineSymbol);
      Graphic convexHullGraphic = new Graphic();
      graphicsOverlay.getGraphics().add(convexHullGraphic);

      // keep track of the points added by the user
      List<Point> inputs = new ArrayList<>();

      // create a button to create and show the convex hull
      Button convexHullButton = new Button("Create Convex Hull");
      convexHullButton.setMaxWidth(130);
      convexHullButton.setDisable(true);
      convexHullButton.setOnAction(e -> {
        convexHullButton.setDisable(true);
        // change the symbol depending on the returned geometry type
        Geometry convexHull = GeometryEngine.convexHull(inputsGraphic.getGeometry());
        switch (convexHull.getGeometryType()) {
          case POINT:
            convexHullGraphic.setSymbol(markerSymbol);
            break;
          case POLYLINE:
            convexHullGraphic.setSymbol(lineSymbol);
            break;
          case POLYGON:
            convexHullGraphic.setSymbol(fillSymbol);
            break;
        }
        convexHullGraphic.setGeometry(convexHull);
      });

      // create a button to clear all graphics
      Button clearButton = new Button("Clear");
      clearButton.setMaxWidth(130);
      clearButton.setDisable(true);
      clearButton.setOnAction(e -> {
        inputs.clear();
        inputsGraphic.setGeometry(null);
        convexHullGraphic.setGeometry(null);
        convexHullButton.setDisable(true);
        clearButton.setDisable(true);
      });

      VBox vBox = new VBox(6);
      vBox.setPickOnBounds(false);
      vBox.setBackground(new Background(new BackgroundFill(Paint.valueOf("rgba(0,0,0,0.3)"), CornerRadii.EMPTY, Insets.EMPTY)));
      vBox.setPadding(new Insets(10.0));
      vBox.setMaxSize(150, 50);
      vBox.getStyleClass().add("panel-region");
      vBox.getChildren().addAll(convexHullButton, clearButton);

      // create a point from where the user clicked
      mapView.setOnMouseClicked(e -> {
        if (e.isStillSincePress() && e.getButton() == MouseButton.PRIMARY) {

          Point2D point = new Point2D(e.getX(), e.getY());

          // create a map point from a point
          Point mapPoint = mapView.screenToLocation(point);

          // the map point should be normalized to the central meridian when wrapping around a map, so its value stays within the coordinate system of the map view
          Point normalizedMapPoint = (Point) GeometryEngine.normalizeCentralMeridian(mapPoint);

          // add a point where the user clicks on the map
          inputs.add(normalizedMapPoint);
          // update the inputs graphic geometry
          Multipoint inputsGeometry = new Multipoint(new PointCollection(inputs));
          inputsGraphic.setGeometry(inputsGeometry);
          // if a new point is added, enable the convex hull and clear buttons
          if (inputs.size() > 0) {
            convexHullButton.setDisable(false);
            clearButton.setDisable(false);
          }
        }
      });

      // add the map view to the stack pane
      stackPane.getChildren().addAll(mapView, vBox);
      StackPane.setAlignment(vBox, Pos.TOP_LEFT);
      StackPane.setMargin(vBox, new Insets(10, 0, 0, 10));
    } catch (Exception e) {
      // on any error, display the stack trace.
      e.printStackTrace();
    }
  }

  /**
   * Stops and releases all resources used in application.
   */
  @Override
  public void stop() {

    if (mapView != null) {
      mapView.dispose();
    }
  }

  /**
   * Opens and runs application.
   *
   * @param args arguments passed to this application
   */
  public static void main(String[] args) {

    Application.launch(args);
  }

}

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