[php] 다각형 영역내에 포인트가 포함되어 있는지 체크하는 함수

GPS 관련 처리를 하다보면 사용자가 선택한 포인트가 영역내에 포함되어 있는지 체크해 별도의 처리를 해주어야 하는 경우가 있는데 이를 지원해주는 함수는 아래와 같습니다.

 

function ptInPolygon($polygon, $point) {
    $p0 = end($polygon);
    $ctr = 0;
    foreach ( $polygon as $p1 ) {
    	// there is a bug with this algorithm, when a point in "on" a vertex
        // in that case just add an epsilon
        if ($point[1] == $p0[1])
        	$point[1]+=0.0000000001; #epsilon

        // ignore edges of constant latitude (yes, this is correct!)
        if ( $p0[1] != $p1[1] ) {
        	// scale latitude of $test_point so that $p0 maps to 0 and $p1 to 1:
            $interp = ($point[1] - $p0[1]) / ($p1[1] - $p0[1]);

            // does the edge intersect the latitude of $test_point?
            // (note: use >= and < to avoid double-counting exact endpoint hits)
            if ( $interp >= 0 && $interp < 1 ) {
            	// longitude of the edge at the latitude of the test point:
                // (could use fancy spherical interpolation here, but for small
                // regions linear interpolation should be fine)
                $long = $interp * $p1[0] + (1 - $interp) * $p0[0];
                // is the intersection east of the test point?
                if ( $long > $point[0] ) {
                	// if so, count it:
                    $ctr++;
                }
            }
        }
        $p0 = $p1;
    }
    return ($ctr & 1);
}