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Issue #1231 Use polygon centroid for 'point' symbol placements
uses a copy of @mourner mapbox/polylabel to find it
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,127 @@ | ||
| 'use strict'; | ||
| var Queue = require('tinyqueue'); | ||
| var Point = require('point-geometry'); | ||
| var distToSegmentSquared = require('./intersection_tests').distToSegmentSquared; | ||
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| /** | ||
| * Finds an approximation of a polygon's Pole Of Inaccessibiliy https://en.wikipedia.org/wiki/Pole_of_inaccessibility | ||
| * This is a copy of http://github.com/mapbox/polylabel adapted to use Points | ||
| * | ||
| * @param {Array<Array<Point>>} List of polygon rings first item in array is the outer ring followed optionally by the list of holes, should be an element of the result of util/classify_rings | ||
| * @param {number} [precision=1] Specified in input coordinate units. If 0 returns after first run, if > 0 repeatedly narrows the search space until the radius of the area searched for the best pole is less than precision | ||
| * @param {bool} [debug=false] Print some statistics to the console during execution | ||
| * | ||
| * @returns {Point} Pole of Inaccessibiliy. | ||
| */ | ||
| module.exports = function (polygonRings, precision, debug) { | ||
| precision = precision || 1.0; | ||
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| // find the bounding box of the outer ring | ||
| var minX, minY, maxX, maxY; | ||
| var outerRing = polygonRings[0]; | ||
| for (var i = 0; i < outerRing.length; i++) { | ||
| var p = outerRing[i]; | ||
| if (!i || p.x < minX) minX = p.x; | ||
| if (!i || p.y < minY) minY = p.y; | ||
| if (!i || p.x > maxX) maxX = p.x; | ||
| if (!i || p.y > maxY) maxY = p.y; | ||
| } | ||
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| var width = maxX - minX; | ||
| var height = maxY - minY; | ||
| var cellSize = Math.min(width, height); | ||
| var h = cellSize / 2; | ||
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| // a priority queue of cells in order of their "potential" (max distance to polygon) | ||
| var cellQueue = new Queue(null, compareMax); | ||
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| // cover polygon with initial cells | ||
| for (var x = minX; x < maxX; x += cellSize) { | ||
| for (var y = minY; y < maxY; y += cellSize) { | ||
| cellQueue.push(new Cell(x + h, y + h, h, polygonRings)); | ||
| } | ||
| } | ||
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| // take centroid as the first best guess | ||
| var bestCell = getCentroidCell(polygonRings); | ||
| var numProbes = cellQueue.length; | ||
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| while (cellQueue.length) { | ||
| // pick the most promising cell from the queue | ||
| var cell = cellQueue.pop(); | ||
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| // update the best cell if we found a better one | ||
| if (cell.d > bestCell.d) { | ||
| bestCell = cell; | ||
| if (debug) console.log('found best %d after %d probes', Math.round(1e4 * cell.d) / 1e4, numProbes); | ||
| } | ||
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| // do not drill down further if there's no chance of a better solution | ||
| if (cell.max - bestCell.d <= precision) continue; | ||
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| // split the cell into four cells | ||
| h = cell.h / 2; | ||
| cellQueue.push(new Cell(cell.p.x - h, cell.p.y - h, h, polygonRings)); | ||
| cellQueue.push(new Cell(cell.p.x + h, cell.p.y - h, h, polygonRings)); | ||
| cellQueue.push(new Cell(cell.p.x - h, cell.p.y + h, h, polygonRings)); | ||
| cellQueue.push(new Cell(cell.p.x + h, cell.p.y + h, h, polygonRings)); | ||
| numProbes += 4; | ||
| } | ||
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| if (debug) { | ||
| console.log('num probes: ' + numProbes); | ||
| console.log('best distance: ' + bestCell.d); | ||
| } | ||
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| return bestCell.p; | ||
| }; | ||
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| function compareMax(a, b) { | ||
| return b.max - a.max; | ||
| } | ||
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| function Cell(x, y, h, polygon) { | ||
| this.p = new Point(x, y); | ||
| this.h = h; // half the cell size | ||
| this.d = pointToPolygonDist(this.p, polygon); // distance from cell center to polygon | ||
| this.max = this.d + this.h * Math.SQRT2; // max distance to polygon within a cell | ||
| } | ||
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| // signed distance from point to polygon outline (negative if point is outside) | ||
| function pointToPolygonDist(p, polygon) { | ||
| var inside = false; | ||
| var minDistSq = Infinity; | ||
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| for (var k = 0; k < polygon.length; k++) { | ||
| var ring = polygon[k]; | ||
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| for (var i = 0, len = ring.length, j = len - 1; i < len; j = i++) { | ||
| var a = ring[i]; | ||
| var b = ring[j]; | ||
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| if ((a.y > p.y !== b.y > p.y) && | ||
| (p.x < (b.x - a.x) * (p.y - a.y) / (b.y - a.y) + a.x)) inside = !inside; | ||
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| minDistSq = Math.min(minDistSq, distToSegmentSquared(p, a, b)); | ||
| } | ||
| } | ||
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| return (inside ? 1 : -1) * Math.sqrt(minDistSq); | ||
| } | ||
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| // get polygon centroid | ||
| function getCentroidCell(polygon) { | ||
| var area = 0; | ||
| var x = 0; | ||
| var y = 0; | ||
| var points = polygon[0]; | ||
| for (var i = 0, len = points.length, j = len - 1; i < len; j = i++) { | ||
| var a = points[i]; | ||
| var b = points[j]; | ||
| var f = a.x * b.y - b.x * a.y; | ||
| x += (a.x + b.x) * f; | ||
| y += (a.y + b.y) * f; | ||
| area += f * 3; | ||
| } | ||
| return new Cell(x / area, y / area, 0, polygon); | ||
| } |
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,25 @@ | ||
| 'use strict'; | ||
|
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| var test = require('tap').test; | ||
| var Point = require('point-geometry'); | ||
| var poleOfInaccessibility = require('../../../js/util/polygon_poi'); | ||
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| test('polygon_poi', function(t) { | ||
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| var closedRing = [ | ||
| new Point(0, 0), | ||
| new Point(10, 10), | ||
| new Point(10, 0), | ||
| new Point(0, 0) | ||
| ]; | ||
| var closedRingHole = [ | ||
| new Point(2, 1), | ||
| new Point(6, 6), | ||
| new Point(6, 1), | ||
| new Point(2, 1) | ||
| ]; | ||
| t.deepEqual(poleOfInaccessibility([closedRing], 0.1), new Point(7.0703125, 2.9296875)); | ||
| t.deepEqual(poleOfInaccessibility([closedRing, closedRingHole], 0.1), new Point(7.96875, 2.03125)); | ||
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| t.end(); | ||
| }); |