Center of gravity of a polygon

Question

I am trying to write a PHP function that will calculate the center of gravity of a polygon.

I\'ve looked at the other similar questions but I can\'t seem to find a s

Answer 1

In php:

// Find the polygon's centroid.
function getCenter($polygon)
{ 
    $NumPoints = count($polygon);

    if($polygon[$NumPoints-1] == $polygon[0]){
        $NumPoints--;
    }else{
        //Add the first point at the end of the array.
        $polygon[$NumPoints] = $polygon[0];
    }

    // Find the centroid.
    $X = 0;
    $Y = 0;
    For ($pt = 0 ;$pt<= $NumPoints-1;$pt++){
        $factor = $polygon[$pt][0] * $polygon[$pt + 1][1] - $polygon[$pt + 1][0] * $polygon[$pt][1];
        $X += ($polygon[$pt][0] + $polygon[$pt + 1][0]) * $factor;
        $Y += ($polygon[$pt][1] + $polygon[$pt + 1][1]) * $factor;
    }

    // Divide by 6 times the polygon's area.
    $polygon_area = ComputeArea($polygon);
    $X = $X / 6 / $polygon_area;
    $Y = $Y / 6 / $polygon_area;

    return array($X, $Y);
}


function ComputeArea($polygon)
{ 
    $NumPoints = count($polygon);

    if($polygon[$NumPoints-1] == $polygon[0]){
        $NumPoints--;
    }else{
        //Add the first point at the end of the array.
        $polygon[$NumPoints] = $polygon[0];
    }

    $area = 0;

    for ($i = 0; $i < $NumPoints; $i++) {
      $i1 = ($i + 1) % $NumPoints;
      $area += ($polygon[$i][1] + $polygon[$i1][1]) * ($polygon[$i1][0] - $polygon[$i][0]);
    }

    $area /= 2;
    return $area;
}

Read more at:

PHP: How to determine the center of a Polygon