[OpenLayers-Users] Check if the lines of an polygon are intersecting

[Fernando]

Hello,

I am sharing bellow with you a function that I made for a problem there I
step on it.
Using the DrawPolygon feature of OpenLayers, I had a restriction to the
polygon never be twisted. Twisted, we call when a line of a polygon cross
another line at the same polygon, creating something that appears to be
like 2 or more polygons with ends adjacents, but they are only one.

There is the .intersects() function in OpenLayers to check if two
Geometries are intersecting each other, but not apply to this case. So I
had to deconstruct the polygon in lines, to check line by line.

The function code:


    /**
     * Author: Fernando Norte (http://fnorte.org)
     * description: check if the lines of a polygon are intersecting
     * Returns: TRUE if intersects | FALSE don't
     */

    // Variables are globals to debug at firebug.console
    var fauxpoint, point, line, vi, vj;

    function checkPolygonIntersect(){
        // Get the polygon (in my case there is only one, but u can put the
'led' as parameter)
        led = map.layers[1].features[0];
        strng = led.geometry.toString(); // Convert to string
        coord = strng.split(','); // split the coordinates
        coord[0] = coord[0].substr(9); // remove the 'Polygon((' from the
1st coord
        coord[coord.length-1] =
coord[coord.length-1].substr(0,coord[coord.length-1].length-2); // Remove
the '))' from the last coord

        //convert to lines
        for (i=0;i<coord.length;i++) {
            lonlat = coord[i].split(' ');  // split coord in lonlat.array
            fauxpoint[i] = new OpenLayers.Geometry.Point(lonlat[0],
lonlat[1]); // convert to a point
            if (i>0) {
                // create an array with the 2 last points
                point[0] = fauxpoint[i-1];
                point[1] = fauxpoint[i];

                line[i] = new OpenLayers.Geometry.LineString(point); //
create the line

            }
        }
        var bool = false; // boolean variable

        // Check every line against every line
        for (var i=1;i<line.length;i++) {
            for(var j=1;j<line.length;j++){
                var vi = line[i].getVertices(); // get points of the I line
in an array
                var vj = line[j].getVertices(); // get points of the J line
in an array

                /*
                 * / the lines must be differents and not adjacents.
                 * // The end or start point of an adjacent line will be
intersect,
                 * // and adjacent lines never intersect in other point
than the ends.
                 */
                if (i!=j &&
                    vi[1].toString() != vj[0].toString() &&
                    vi[0].toString() != vj[1].toString()){
                    // the intersect check
                    if (line[i].intersects(line[j])) bool = true;
                }

            }
        }

        return bool;
    }

Enjoy,

Fernando G. Norte
BHte - MG - Brasil
cel: +55 31 9119 8814
-------------------------
MSN e Gtalk # fnorte at gmail.com
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