Path Generation

/**
 * Creates a path generator for rendering GeoJSON features
 * @param projection - Optional projection to transform coordinates (null for identity)
 * @param context - Optional Canvas context for drawing (null for SVG string output)
 * @returns Path generator instance
 */
function geoPath(projection?: Projection | null, context?: CanvasRenderingContext2D | null): Path;

interface Path {
  /**
   * Generates a path string or draws to canvas for the specified GeoJSON object
   * @param object - GeoJSON feature or geometry
   * @returns SVG path string (if no context) or null (if canvas context)
   */
  (object: GeoJSON.Feature | GeoJSON.Geometry): string | null;
  
  /**
   * Computes the projected planar area of the specified GeoJSON object
   * @param object - GeoJSON feature or geometry
   * @returns Area in square pixels
   */
  area(object: GeoJSON.Feature | GeoJSON.Geometry): number;
  
  /**
   * Computes the projected planar bounding box of the specified GeoJSON object
   * @param object - GeoJSON feature or geometry
   * @returns Bounding box as [[x0, y0], [x1, y1]]
   */
  bounds(object: GeoJSON.Feature | GeoJSON.Geometry): [[number, number], [number, number]];
  
  /**
   * Computes the projected planar centroid of the specified GeoJSON object
   * @param object - GeoJSON feature or geometry
   * @returns Centroid as [x, y]
   */
  centroid(object: GeoJSON.Feature | GeoJSON.Geometry): [number, number];
  
  /**
   * Computes the projected planar length of the specified GeoJSON object
   * @param object - GeoJSON feature or geometry
   * @returns Length in pixels
   */
  measure(object: GeoJSON.Feature | GeoJSON.Geometry): number;
  
  /**
   * Sets or gets the path generator's projection
   * @param projection - Projection function or null for identity
   * @returns This path generator or current projection
   */
  projection(projection?: Projection | null): this | Projection | null;
  
  /**
   * Sets or gets the path generator's rendering context
   * @param context - Canvas context or null for string output
   * @returns This path generator or current context
   */
  context(context?: CanvasRenderingContext2D | null): this | CanvasRenderingContext2D | null;
  
  /**
   * Sets or gets the radius for rendering Point and MultiPoint geometries
   * @param radius - Point radius in pixels or function that returns radius
   * @returns This path generator or current point radius
   */
  pointRadius(radius?: number | ((object: any, ...args: any[]) => number)): this | number | ((object: any, ...args: any[]) => number);
  
  /**
   * Sets or gets the number of fractional digits for string output
   * @param digits - Number of digits or null for maximum precision
   * @returns This path generator or current digits setting
   */
  digits(digits?: number | null): this | number | null;
}

import { geoPath, geoMercator } from "d3-geo";

// Create projection and path generator
const projection = geoMercator()
  .scale(150)
  .translate([480, 250]);

const path = geoPath(projection);

// Generate SVG paths for GeoJSON features
const countries = {
  type: "FeatureCollection",
  features: [/* country polygons */]
};

// Create SVG elements
const svg = d3.select("body").append("svg")
  .attr("width", 960)
  .attr("height", 500);

svg.selectAll("path")
  .data(countries.features)
  .enter().append("path")
  .attr("d", path) // path generator creates d attribute
  .attr("fill", "steelblue")
  .attr("stroke", "white");

import { geoPath, geoOrthographic } from "d3-geo";

// Setup canvas
const canvas = d3.select("body").append("canvas")
  .attr("width", 960)
  .attr("height", 500);

const context = canvas.node().getContext("2d");

// Create projection and path generator with canvas context
const projection = geoOrthographic()
  .scale(250)
  .translate([480, 250]);

const path = geoPath(projection, context);

// Draw to canvas
function render(countries) {
  context.clearRect(0, 0, 960, 500);
  
  context.beginPath();
  path({type: "Sphere"}); // Draw sphere outline
  context.strokeStyle = "#ccc";
  context.stroke();
  
  countries.features.forEach(country => {
    context.beginPath();
    path(country); // Draw country
    context.fillStyle = "steelblue";
    context.fill();
    context.strokeStyle = "white";
    context.stroke();
  });
}

import { geoPath, geoAlbers } from "d3-geo";

const projection = geoAlbers();
const path = geoPath(projection);

// Set dynamic point radius based on feature properties
path.pointRadius((d) => {
  if (d.properties && d.properties.population) {
    return Math.sqrt(d.properties.population / 100000);
  }
  return 3; // default radius
});

// Render cities with population-based sizing
const cities = {
  type: "FeatureCollection",
  features: [
    {
      type: "Feature",
      properties: { name: "New York", population: 8000000 },
      geometry: { type: "Point", coordinates: [-74, 40.7] }
    },
    // ... more cities
  ]
};

svg.selectAll(".city")
  .data(cities.features)
  .enter().append("path")
  .attr("class", "city")
  .attr("d", path);

import { geoPath, geoMercator } from "d3-geo";

const projection = geoMercator().scale(1000);
const path = geoPath(projection);

const polygon = {
  type: "Polygon",
  coordinates: [[[0, 0], [1, 0], [1, 1], [0, 1], [0, 0]]]
};

const lineString = {
  type: "LineString",
  coordinates: [[0, 0], [1, 0], [1, 1]]
};

// Calculate projected measurements
const area = path.area(polygon);        // Area in square pixels
const bounds = path.bounds(polygon);    // Bounding box [[x0,y0], [x1,y1]]
const centroid = path.centroid(polygon); // Centroid [x, y]
const length = path.measure(lineString); // Length in pixels

console.log(`Projected area: ${area} square pixels`);
console.log(`Bounds: ${bounds}`);
console.log(`Centroid: ${centroid}`);
console.log(`Length: ${length} pixels`);

import { geoPath, geoMercator } from "d3-geo";

const projection = geoMercator();
const path = geoPath(projection);

// Control decimal precision for SVG output
path.digits(2); // Limit to 2 decimal places

const feature = {
  type: "Polygon",
  coordinates: [[[0, 0], [0.123456, 0], [0.123456, 0.123456], [0, 0.123456], [0, 0]]]
};

const pathString = path(feature);
// Output will have coordinates rounded to 2 decimal places
console.log(pathString); // "M0,250L73.23,250L73.23,236.16L0,236.16Z"

// Use maximum precision
path.digits(null);
const precisePathString = path(feature);
console.log(precisePathString); // Full precision coordinates

import { geoPath, geoMercator } from "d3-geo";

const projection = geoMercator();
const path = geoPath(projection);

// Handle different geometry types
const geometries = [
  {
    type: "Point",
    coordinates: [-74, 40.7]
  },
  {
    type: "LineString", 
    coordinates: [[-74, 40], [-73, 41]]
  },
  {
    type: "Polygon",
    coordinates: [[[0, 0], [1, 0], [1, 1], [0, 1], [0, 0]]]
  },
  {
    type: "MultiPoint",
    coordinates: [[-74, 40], [-73, 41], [-72, 42]]
  },
  {
    type: "MultiLineString",
    coordinates: [[[-74, 40], [-73, 41]], [[-72, 42], [-71, 43]]]
  },
  {
    type: "MultiPolygon",
    coordinates: [
      [[[0, 0], [1, 0], [1, 1], [0, 1], [0, 0]]],
      [[[2, 2], [3, 2], [3, 3], [2, 3], [2, 2]]]
    ]
  }
];

// Path generator handles all geometry types
geometries.forEach((geometry, i) => {
  const pathString = path(geometry);
  console.log(`Geometry ${i}: ${pathString}`);
});

import { geoPath, geoAlbers } from "d3-geo";

const projection = geoAlbers();
const path = geoPath(projection);

const featureCollection = {
  type: "FeatureCollection",
  features: [
    {
      type: "Feature",
      properties: { name: "State 1", value: 100 },
      geometry: { /* polygon geometry */ }
    },
    {
      type: "Feature", 
      properties: { name: "State 2", value: 200 },
      geometry: { /* polygon geometry */ }
    }
  ]
};

// Process entire feature collection
const collectionPath = path(featureCollection);

// Or process individual features
const individualPaths = featureCollection.features.map(feature => ({
  name: feature.properties.name,
  path: path(feature),
  area: path.area(feature),
  centroid: path.centroid(feature)
}));

// Create choropleth map
const colorScale = d3.scaleSequential(d3.interpolateBlues)
  .domain([0, d3.max(featureCollection.features, d => d.properties.value)]);

svg.selectAll("path")
  .data(featureCollection.features)
  .enter().append("path")
  .attr("d", path)
  .attr("fill", d => colorScale(d.properties.value))
  .attr("stroke", "white");