nknize commented on a change in pull request #709: LUCENE-8850: Calculate the
area of a polygon and throw error when values are invalid
URL: https://github.com/apache/lucene-solr/pull/709#discussion_r298261656
##########
File path: lucene/core/src/java/org/apache/lucene/geo/Polygon.java
##########
@@ -96,34 +99,50 @@ public Polygon(double[] polyLats, double[] polyLons,
Polygon... holes) {
this.holes = holes.clone();
// compute bounding box
- double minLat = polyLats[0];
- double maxLat = polyLats[0];
- double minLon = polyLons[0];
- double maxLon = polyLons[0];
+ double minLat = Double.POSITIVE_INFINITY;
+ double maxLat = Double.NEGATIVE_INFINITY;
+ double minLon = Double.POSITIVE_INFINITY;
+ double maxLon = Double.NEGATIVE_INFINITY;
double windingSum = 0d;
final int numPts = polyLats.length - 1;
- for (int i = 1, j = 0; i < numPts; j = i++) {
+ for (int i = 0; i < numPts; i++) {
minLat = Math.min(polyLats[i], minLat);
maxLat = Math.max(polyLats[i], maxLat);
minLon = Math.min(polyLons[i], minLon);
maxLon = Math.max(polyLons[i], maxLon);
// compute signed area
- windingSum += (polyLons[j] - polyLons[numPts])*(polyLats[i] -
polyLats[numPts])
- - (polyLats[j] - polyLats[numPts])*(polyLons[i] - polyLons[numPts]);
+ windingSum += polyLons[i] * polyLats[i + 1] - polyLats[i] * polyLons[i +
1];
+ }
+ if (windingSum == 0) {
+ throw new IllegalArgumentException("Cannot compute the polygon / hole
orientation.");
}
this.minLat = minLat;
this.maxLat = maxLat;
this.minLon = minLon;
this.maxLon = maxLon;
this.windingOrder = (windingSum < 0) ? GeoUtils.WindingOrder.CCW :
GeoUtils.WindingOrder.CW;
+ double area = Math.abs(windingSum / 2d);
+ for (Polygon hole : holes) {
+ area -= hole.area();
+ }
+ if (area <= 0) {
+ throw new IllegalArgumentException("Polygon has an invalid area (area =
" + area + ").");
+ }
+ this.area = area;
+
}
/** returns the number of vertex points */
public int numPoints() {
return polyLats.length;
}
+ /** returns the area of the polygon */
+ public double area() {
Review comment:
I agree w/ @dsmiley re: units. The calculation is using the [shoelace
method](https://en.wikipedia.org/wiki/Shoelace_formula) on vertices in decimal
degrees. So the units are deg^2. I remember exploring this a little over a year
ago in [LUCENE-8364](https://issues.apache.org/jira/browse/LUCENE-8364) and
originally having area because it was "easy". I ended up removing it until we
had support for indexing in other projections (e.g., [equal
area](https://en.wikipedia.org/wiki/Map_projection#Equal-area)). I think we
should wait until
[LUCENE-8632](https://issues.apache.org/jira/browse/LUCENE-8632) lands and only
provide this area calculation in
[XYPolygon](lucene/sandbox/src/java/org/apache/lucene/geo/XYPolygon.java) for
regular cartesian geometries in non decimal degrees.
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