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Fix direction of Polygon ring coordinates #76

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Fix direction of Polygon ring coordinates #76

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24 changes: 24 additions & 0 deletions src/poly.js
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,20 @@ module.exports.write = function writePoints(geometries, extent, shpView, shxView
shxI = 0,
shxOffset = 100;

//Ensure polygon rings are ordered in correct direction
if(TYPE === types.geometries.POLYGON){
geometries = geometries.reduce(function(geom_acc, poly){
var flipped = poly.reduce(function(poly_acc, ring, idx){
//External rings must be clockwise, Internal must be counter-clockwise
idx == 0
? isClockwise(ring) ? poly_acc.push(ring) : poly_acc.push(ring.reverse())
: isClockwise(ring) ? poly_acc.push(ring.reverse()) : poly_acc.push(ring);
return poly_acc;
}, []);
geom_acc.push(flipped);
return geom_acc;
}, []);
}
geometries.forEach(writePolyLine);

function writePolyLine(coordinates, i) {
Expand Down Expand Up @@ -62,6 +76,16 @@ module.exports.write = function writePoints(geometries, extent, shpView, shxView

shpI += contentLength + 8;
}

//Takes a ring of coordinates. Returns true if they're defined in clockwise order
function isClockwise(ring){
var sum = 0;
for(var i = 0; i < ring.length; i++){
var prevIdx = i == 0 ? ring.length - 1 : i - 1;
sum += (ring[i][0] - ring[prevIdx][0])*(ring[i][1] + ring[prevIdx][1]);
}
return sum > 0 ? true : false;
}
};

module.exports.shpLength = function(geometries) {
Expand Down