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Add point_on_feature examples with visualizations
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‎test/examples/point_on_feature/README.md

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# Point on Feature Examples
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This directory contains examples demonstrating the `pointOnFeature` function in the turf_dart library.
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## Function Overview
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The `pointOnFeature` function returns a point that is guaranteed to be on or inside a feature. This is useful for placing labels, icons, or other markers on geographic features.
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The function behavior varies by geometry type:
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- **Point**: Returns the original point unchanged
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- **LineString**: Returns the midpoint of the first segment
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- **Polygon**: Returns a point inside the polygon (preferably the centroid)
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- **MultiPolygon**: Uses the first polygon to compute a point
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- **FeatureCollection**: Returns a point on the largest feature
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## Directory Structure
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- **`/in`**: Input GeoJSON files with different geometry types
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- **`/out`**: Output files showing the points generated by `pointOnFeature`
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- **`visualization.geojson`**: Combined visualization of all inputs and their resulting points
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## Example Files
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1. **Point Example**: Shows that `pointOnFeature` returns the original point
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2. **LineString Example**: Shows how `pointOnFeature` finds the midpoint of the first line segment
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3. **Polygon Examples**: Show how `pointOnFeature` returns a point inside the polygon
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4. **MultiPolygon Example**: Shows how `pointOnFeature` uses the first polygon
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5. **FeatureCollection Example**: Shows how `pointOnFeature` finds a point on the largest feature
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## Visualization
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The `visualization.geojson` file combines all examples into one visualization. When viewed in a GeoJSON viewer, it shows:
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- Original geometries in blue
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- Points generated by `pointOnFeature` in red with different markers:
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- Stars for points
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- Circles for linestrings
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- Triangles for polygons
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- Squares for multipolygons
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- Circle-stroked for feature collections
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Each point includes a description explaining how it was generated.
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## Running the Examples
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To regenerate the examples and visualization:
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1. Run the output generator:
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```
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dart test/examples/point_on_feature/generate_outputs.dart
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```
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2. Run the visualization generator:
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```
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dart test/examples/point_on_feature/generate_visualization.dart
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```
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## Use Cases
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The `pointOnFeature` function is commonly used for:
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- Placing labels on geographic features
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- Positioning icons or markers on features
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- Finding representative points for complex geometries
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- Generating points for clustering or other spatial operations

‎test/examples/point_on_feature/generate_outputs.dart

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import 'dart:convert';
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import 'dart:io';
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import 'package:turf/turf.dart';
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void main() async {
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// Process each input file
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await processFile('polygon_feature.geojson');
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await processFile('polygon.geojson');
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await processFile('point.geojson');
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await processFile('linestring.geojson');
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await processFile('multipolygon.geojson');
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await processFile('featurecollection.geojson');
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print('All files processed successfully!');
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}
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Future<void> processFile(String filename) async {
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try {
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final inputPath = 'test/examples/point_on_feature/in/$filename';
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final outputPath = 'test/examples/point_on_feature/out/$filename';
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print('Processing $inputPath');
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// Read the input file
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final file = File(inputPath);
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final jsonString = await file.readAsString();
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final geojson = jsonDecode(jsonString);
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// Parse the GeoJSON and create appropriate object based on type
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dynamic featureInput;
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if (geojson['type'] == 'Feature') {
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featureInput = Feature.fromJson(geojson);
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} else if (geojson['type'] == 'FeatureCollection') {
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featureInput = FeatureCollection.fromJson(geojson);
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} else {
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// For raw geometry objects, create a Feature with the geometry
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final geometry = parseGeometry(geojson);
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if (geometry != null) {
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featureInput = Feature(geometry: geometry);
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} else {
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print(' Unsupported geometry type: ${geojson['type']}');
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return;
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}
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}
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// Apply point_on_feature function
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final pointResult = pointOnFeature(featureInput);
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// Generate output - wrap in a FeatureCollection for better compatibility
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Map<String, dynamic> outputJson;
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if (pointResult != null) {
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final features = <Feature>[];
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// Create a new feature based on the input geometry
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GeometryObject? inputGeometry;
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if (featureInput is Feature) {
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inputGeometry = featureInput.geometry;
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} else if (featureInput is FeatureCollection && featureInput.features.isNotEmpty) {
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inputGeometry = featureInput.features[0].geometry;
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} else {
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inputGeometry = parseGeometry(geojson);
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}
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if (inputGeometry != null) {
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// Create a new feature with the input geometry
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final styledInputFeature = Feature(
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geometry: inputGeometry,
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properties: {
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'name': 'Input Geometry',
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'description': 'Original geometry from $filename'
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}
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);
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// Add styling based on geometry type
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if (inputGeometry is Polygon || inputGeometry is MultiPolygon) {
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styledInputFeature.properties!['stroke'] = '#0000FF';
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styledInputFeature.properties!['stroke-width'] = 2;
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styledInputFeature.properties!['fill'] = '#0000FF';
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styledInputFeature.properties!['fill-opacity'] = 0.2;
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} else if (inputGeometry is LineString || inputGeometry is MultiLineString) {
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styledInputFeature.properties!['stroke'] = '#0000FF';
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styledInputFeature.properties!['stroke-width'] = 2;
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} else if (inputGeometry is Point) {
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styledInputFeature.properties!['marker-color'] = '#0000FF';
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}
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features.add(styledInputFeature);
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}
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// Create a new feature for the point result to avoid modifying unmodifiable maps
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final styledPointResult = Feature(
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geometry: pointResult.geometry,
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properties: {
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'marker-color': '#FF0000',
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'marker-size': 'medium',
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'marker-symbol': 'star',
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'name': 'Point on Feature Result',
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'description': 'Point generated by pointOnFeature function'
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}
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);
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features.add(styledPointResult);
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outputJson = FeatureCollection(features: features).toJson();
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print(' Found point at coordinates: ${pointResult.geometry?.coordinates}');
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} else {
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// Create an empty FeatureCollection with error info in properties
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outputJson = {
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'type': 'FeatureCollection',
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'features': [
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{
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'type': 'Feature',
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'properties': {
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'error': 'Could not generate point for this input',
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'name': 'Error',
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'description': 'pointOnFeature function could not generate a point'
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},
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'geometry': null
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}
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]
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};
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print(' Could not generate point for this input');
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}
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// Write to output file with pretty formatting
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final outputFile = File(outputPath);
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await outputFile.writeAsString(JsonEncoder.withIndent(' ').convert(outputJson));
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print(' Saved result to $outputPath');
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} catch (e) {
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print('Error processing $filename: $e');
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}
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}
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GeometryObject? parseGeometry(Map<String, dynamic> json) {
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final type = json['type'];
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switch (type) {
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case 'Point':
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return Point.fromJson(json);
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case 'LineString':
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return LineString.fromJson(json);
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case 'Polygon':
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return Polygon.fromJson(json);
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case 'MultiPoint':
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return MultiPoint.fromJson(json);
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case 'MultiLineString':
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return MultiLineString.fromJson(json);
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case 'MultiPolygon':
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return MultiPolygon.fromJson(json);
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case 'GeometryCollection':
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return GeometryCollection.fromJson(json);
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default:
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return null;
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}
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}

‎test/examples/point_on_feature/generate_visualization.dart

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import 'dart:convert';
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import 'dart:io';
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import 'package:turf/turf.dart';
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void main() async {
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// Create a single visualization file showing all inputs and their corresponding points
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await generateVisualization();
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print('Visualization file generated successfully!');
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}
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Future<void> generateVisualization() async {
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try {
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final files = [
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'polygon_feature.geojson',
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'polygon.geojson',
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'linestring.geojson',
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'multipolygon.geojson',
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'point.geojson',
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'featurecollection.geojson'
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];
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// List to store all features
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final allFeatures = <Feature>[];
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// Process each output file only (since they now contain both input and result)
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for (final filename in files) {
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final outputPath = 'test/examples/point_on_feature/out/$filename';
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print('Processing $filename for visualization');
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// Read the output file
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final outputFile = File(outputPath);
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if (!outputFile.existsSync()) {
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print(' Missing output file for $filename, skipping');
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continue;
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}
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final outputJson = jsonDecode(await outputFile.readAsString());
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// The output files are already FeatureCollections with styled features
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if (outputJson['type'] == 'FeatureCollection' && outputJson['features'] is List) {
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final outFeatures = outputJson['features'] as List;
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// Add custom markers based on the geometry type for the result point
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for (final feature in outFeatures) {
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if (feature['properties'] != null &&
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feature['properties']['name'] == 'Point on Feature Result') {
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// Update description based on geometry type
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if (filename == 'point.geojson') {
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feature['properties']['description'] = 'Point on a Point: Returns the original point unchanged';
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feature['properties']['marker-symbol'] = 'star';
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} else if (filename == 'linestring.geojson') {
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feature['properties']['description'] = 'Point on a LineString: Returns the midpoint of the first segment';
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feature['properties']['marker-symbol'] = 'circle';
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} else if (filename.contains('polygon') && !filename.contains('multi')) {
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feature['properties']['description'] = 'Point on a Polygon: Returns a point inside the polygon (prefers centroid)';
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feature['properties']['marker-symbol'] = 'triangle';
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} else if (filename == 'multipolygon.geojson') {
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feature['properties']['description'] = 'Point on a MultiPolygon: Returns a point from the first polygon';
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feature['properties']['marker-symbol'] = 'square';
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} else if (filename == 'featurecollection.geojson') {
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feature['properties']['description'] = 'Point on a FeatureCollection: Returns a point on the largest feature';
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feature['properties']['marker-symbol'] = 'circle-stroked';
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}
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feature['properties']['name'] = 'Result: $filename';
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}
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// Add the feature to our collection
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try {
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final parsedFeature = Feature.fromJson(feature);
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allFeatures.add(parsedFeature);
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} catch (e) {
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print(' Error parsing feature: $e');
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}
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}
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}
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}
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// Create the feature collection
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final featureCollection = FeatureCollection(features: allFeatures);
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// Save the visualization file with pretty formatting
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final visualizationFile = File('test/examples/point_on_feature/visualization.geojson');
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await visualizationFile.writeAsString(JsonEncoder.withIndent(' ').convert(featureCollection.toJson()));
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print('Saved visualization to ${visualizationFile.path}');
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} catch (e) {
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print('Error generating visualization: $e');
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}
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}
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// Helper function to set style properties for features
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void setFeatureStyle(Feature feature, String color, int width, double opacity) {
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feature.properties = feature.properties ?? {};
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// Different styling based on geometry type
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if (feature.geometry is Polygon || feature.geometry is MultiPolygon) {
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feature.properties!['stroke'] = color;
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feature.properties!['stroke-width'] = width;
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feature.properties!['stroke-opacity'] = 1;
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feature.properties!['fill'] = color;
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feature.properties!['fill-opacity'] = opacity;
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} else if (feature.geometry is LineString || feature.geometry is MultiLineString) {
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feature.properties!['stroke'] = color;
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feature.properties!['stroke-width'] = width;
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feature.properties!['stroke-opacity'] = 1;
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} else if (feature.geometry is Point || feature.geometry is MultiPoint) {
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feature.properties!['marker-color'] = color;
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feature.properties!['marker-size'] = 'small';
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}
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}
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// Helper function to parse geometries from JSON
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GeometryObject? parseGeometry(Map<String, dynamic> json) {
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final type = json['type'];
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switch (type) {
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case 'Point':
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return Point.fromJson(json);
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case 'LineString':
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return LineString.fromJson(json);
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case 'Polygon':
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return Polygon.fromJson(json);
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case 'MultiPoint':
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return MultiPoint.fromJson(json);
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case 'MultiLineString':
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return MultiLineString.fromJson(json);
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case 'MultiPolygon':
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return MultiPolygon.fromJson(json);
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case 'GeometryCollection':
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return GeometryCollection.fromJson(json);
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default:
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return null;
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}
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}

‎test/examples/point_on_feature/in/featurecollection.geojson

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{
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"type": "FeatureCollection",
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"features": [
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{
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"type": "Feature",
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"properties": {
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"name": "Point Feature"
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},
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"geometry": {
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"type": "Point",
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"coordinates": [5.0, 10.0]
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}
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},
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{
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"type": "Feature",
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"properties": {
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"name": "Polygon Feature",
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"stroke": "#F00",
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"stroke-width": 2,
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"fill": "#F00",
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"fill-opacity": 0.3
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},
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"geometry": {
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"type": "Polygon",
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"coordinates": [
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[
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[-10.0, -10.0],
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[10.0, -10.0],
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[10.0, 10.0],
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[-10.0, 10.0],
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[-10.0, -10.0]
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]
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]
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}
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}
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]
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}

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