## emcconville/point-reduction-algorithms

A collection of algorithms for reducing the number of points in polyline

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Requires

- php: >=5.4

Requires (Dev)

- phpunit/phpunit: 4.4.*
- satooshi/php-coveralls: 1.*
- squizlabs/php_codesniffer: 2.1.*

This package is **not** auto-updated.

Last update: 2020-07-03 01:33:55 UTC

# README

#Point Reduction Algorithms

A collection of algorithms for reducing the number of points in polygon / polyline.

Custom shapes, polygon & polyline in web-map applications, can have **too many**
points. Often a shape will be rendered at a distant zoom-level, and wouldn't
require such high-resolution. This library's goal is to provided basic methods
to simplify & reduce shapes.

## Install

With composer.

```
$ curl -sS https://getcomposer.org/installer | php
$ cat > composer.json <<EOF
{
"require": {
"emcconville/point-reduction-algorithms" : "~1.0"
}
}
EOF
$ php composer.phar install
```

## Algorithms & Usage

Below is an original, uncompressed, polyline with 2151 points, and is delivered as SVG weighing in @ 175K.

All algorithms share a common abstraction, but each class implements a unique
*reduce* method. Most reduction methods require one argument for tolerance /
threshold; except, Visvalingam–Whyatt requires the desired final point count,
and Opheim requires two independent thresholds.

**Example Pseudo code:**

use PointReduction\Algorithms\ALGORITHM_NAME; $obj = new ALGORITHM_NAME($myPoints); $myReducedPoints = $obj->reduce($tolerance);

The **$myPoints** must be an array, or traversable object that supports removing
element at index. Each user defined point must implement
**PointReduction\Common\PointInterface** which enforces a **getCoordinates**
method. This common interface allows user objects to have any type of property
(ie. x/y, latitude/longitude, left/top).

use PointReduction\Common\PointInterface; class MyPoint implements PointInterface { // ... my stuff here ... public function getCoordinates() { return array($this->myOwnX, $this->myOwnY); } }

### Ramer–Douglas–Peucker

use PointReduction\Common\Point, PointReduction\Algorithms\RamerDouglasPeucker; $givenPoints = array( new Point(-84.158640, -39.822480), new Point(-84.159250, -39.820120), // ... and so one ); $epsilon = 0.001 $reducer = new RamerDouglasPeucker($givenPoints); $reducedPoints = $reducer->reduce($epsilon);

The original polygon of 2151 points has been reduced to 343.

### Visvalingam–Whyatt

use PointReduction\Common\Point, PointReduction\Algorithms\VisvalingamWhyatt; $givenPoints = array( new Point(-84.158640, -39.822480), new Point(-84.159250, -39.820120), // ... and so one ); $desiredPointCount = 343; $reducer = new VisvalingamWhyatt($givenPoints); $reducedPoints = $reducer->reduce($desiredPointCount);

The original polygon of 2151 points has been reduced to 343.

### Reumann–Witkam

use PointReduction\Common\Point, PointReduction\Algorithms\ReumannWitkam; $givenPoints = array( new Point(-84.158640, -39.822480), new Point(-84.159250, -39.820120), // ... and so one ); $threshold = 0.001; $reducer = new ReumannWitkam($givenPoints); $reducedPoints = $reducer->reduce($threshold);

The original polygon of 2151 points has been reduced to 872.

### Opheim

use PointReduction\Common\Point, PointReduction\Algorithms\Opheim; $givenPoints = array( new Point(-84.158640, -39.822480), new Point(-84.159250, -39.820120), // ... and so one ); $perpendicularTolerance = 0.005; $radialTolerance = 0.01; $reducer = new Opheim($givenPoints); $reducedPoints = $reducer->reduce($perpendicularTolerance, $radialTolerance);

The original polygon of 2151 points has been reduced to 684.

### Lang

use PointReduction\Common\Point, PointReduction\Algorithms\Lang; $givenPoints = array( new Point(-84.158640, -39.822480), new Point(-84.159250, -39.820120), // ... and so one ); $threshold = 0.001; $lookAhead = 7; $reducer = new Lang($givenPoints); $reducedPoints = $reducer->reduce($threshold, $lookAhead);

The original polygon of 2151 points has been reduced to 293.

### Zhao-Saalfeld

Added in version `1.2.0`

.

use PointReduction\Common\Point, PointReduction\Algorithms\ZhaoSAalfeld; $givenPoints = array( new Point(-84.158640, -39.822480), new Point(-84.159250, -39.820120), // ... and so one ); $degree = 7; $lookAhead = 7; $reducer = new ZhaoSAalfeld($givenPoints); $reducedPoints = $reducer->reduce($degree, $lookAhead);

The original polygon of 2151 points has been reduced to 1493.

### Radial Distance

Added in version `1.2.0`

.

use PointReduction\Common\Point, PointReduction\Algorithms\RadialDistance; $givenPoints = array( new Point(-84.158640, -39.822480), new Point(-84.159250, -39.820120), // ... and so one ); $tolerance = 0.0025; $reducer = new RadialDistance($givenPoints); $reducedPoints = $reducer->reduce($tolerance);

The original polygon of 2151 points has been reduced to 449.