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fix link cross checking code to handle meridians (north-south) lines correctly

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possible fixes for issues seen on mobile (likely precision/rounding problems)
This commit is contained in:
Jon Atkins 2014-06-25 06:44:46 +01:00
parent d8cad49d84
commit 9f695e303f
1 changed files with 53 additions and 22 deletions
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75
plugins/cross_link.user.js
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@ -59,24 +59,27 @@ window.plugin.crossLinks.greatCircleArcIntersect = function(a0,a1,b0,b1) {
// class to hold the pre-calculated maths for a geodesic line // class to hold the pre-calculated maths for a geodesic line
// TODO: move this outside this function, so it can be pre-calculated once for each line we test // TODO: move this outside this function, so it can be pre-calculated once for each line we test
var GeodesicLine = function(start,end) { var GeodesicLine = function(start,end) {
var R = 6378137; // earth radius in meters (doesn't have to be exact)
var d2r = Math.PI/180.0; var d2r = Math.PI/180.0;
var r2d = 180.0/Math.PI; var r2d = 180.0/Math.PI;
// maths based on http://williams.best.vwh.net/avform.htm#Int // maths based on http://williams.best.vwh.net/avform.htm#Int
if (start.lng == end.lng) {
throw 'Error: cannot calculate latitude for meridians';
}
// only the variables needed to calculate a latitude for a given longitude are stored in 'this' // only the variables needed to calculate a latitude for a given longitude are stored in 'this'
var lat1 = start.lat * d2r; this.lat1 = start.lat * d2r;
var lat2 = end.lat * d2r; this.lat2 = end.lat * d2r;
this.lng1 = start.lng * d2r; this.lng1 = start.lng * d2r;
this.lng2 = end.lng * d2r; this.lng2 = end.lng * d2r;
var dLng = this.lng2-this.lng1; var dLng = this.lng1-this.lng2;
var sinLat1 = Math.sin(lat1); var sinLat1 = Math.sin(this.lat1);
var sinLat2 = Math.sin(lat2); var sinLat2 = Math.sin(this.lat2);
var cosLat1 = Math.cos(lat1); var cosLat1 = Math.cos(this.lat1);
var cosLat2 = Math.cos(lat2); var cosLat2 = Math.cos(this.lat2);
this.sinLat1CosLat2 = sinLat1*cosLat2; this.sinLat1CosLat2 = sinLat1*cosLat2;
this.sinLat2CosLat1 = sinLat2*cosLat1; this.sinLat2CosLat1 = sinLat2*cosLat1;
@ -84,12 +87,25 @@ window.plugin.crossLinks.greatCircleArcIntersect = function(a0,a1,b0,b1) {
this.cosLat1CosLat2SinDLng = cosLat1*cosLat2*Math.sin(dLng); this.cosLat1CosLat2SinDLng = cosLat1*cosLat2*Math.sin(dLng);
} }
GeodesicLine.prototype.isMeridian = function() {
return this.lng1 == this.lng2;
}
GeodesicLine.prototype.latAtLng = function(lng) { GeodesicLine.prototype.latAtLng = function(lng) {
lng = lng * Math.PI / 180; //to radians lng = lng * Math.PI / 180; //to radians
var lat = Math.atan ( (this.sinLat1CosLat2*Math.sin(this.lng2-lng) + this.sinLat2CosLat1*Math.sin(lng-this.lng1)) var lat;
/ this.cosLat1CosLat2SinDLng); // if we're testing the start/end point, return that directly rather than calculating
// 1. this may be fractionally faster, no complex maths
// 2. there's odd rounding issues that occur on some browsers (noticed on IITC MObile) for very short links - this may help
if (lng == this.lng1) {
lat = this.lat1;
} else if (lng == this.lng2) {
lat = this.lat2;
} else {
lat = Math.atan ( (this.sinLat1CosLat2*Math.sin(lng-this.lng2) - this.sinLat2CosLat1*Math.sin(lng-this.lng1))
/ this.cosLat1CosLat2SinDLng);
}
return lat * 180 / Math.PI; // return value in degrees return lat * 180 / Math.PI; // return value in degrees
} }
@ -98,20 +114,35 @@ window.plugin.crossLinks.greatCircleArcIntersect = function(a0,a1,b0,b1) {
var leftLng = Math.max( Math.min(a0.lng,a1.lng), Math.min(b0.lng,b1.lng) ); var leftLng = Math.max( Math.min(a0.lng,a1.lng), Math.min(b0.lng,b1.lng) );
var rightLng = Math.min( Math.max(a0.lng,a1.lng), Math.max(b0.lng,b1.lng) ); var rightLng = Math.min( Math.max(a0.lng,a1.lng), Math.max(b0.lng,b1.lng) );
// prepare geodesic line maths // calculate the latitudes for each line at left + right longitudes
var aGeo = new GeodesicLine(a0,a1); // NOTE: need a special case for meridians - as GeodesicLine.latAtLng method is invalid in that case
var bGeo = new GeodesicLine(b0,b1); var aLeftLat, aRightLat;
if (a0.lng == a1.lng) {
// 'left' and 'right' now become 'top' and 'bottom' (in some order) - which is fine for the below intersection code
aLeftLat = a0.lat;
aRightLat = a1.lat;
} else {
var aGeo = new GeodesicLine(a0,a1);
aLeftLat = aGeo.latAtLng(leftLng);
aRightLat = aGeo.latAtLng(rightLng);
}
// calculate the latitudes for each line at left + right points var bLeftLat, bRightLat;
var aLeftLat = aGeo.latAtLng(leftLng); if (b0.lng == b1.lng) {
var aRightLat = aGeo.latAtLng(rightLng); // 'left' and 'right' now become 'top' and 'bottom' (in some order) - which is fine for the below intersection code
bLeftLat = b0.lat;
var bLeftLat = bGeo.latAtLng(leftLng); bRightLat = b1.lat;
var bRightLat = bGeo.latAtLng(rightLng); } else {
var bGeo = new GeodesicLine(b0,b1);
bLeftLat = bGeo.latAtLng(leftLng);
bRightLat = bGeo.latAtLng(rightLng);
}
// if both a are less or greater than both b, then lines do not cross // if both a are less or greater than both b, then lines do not cross
if (aLeftLat < bLeftLat && aRightLat < bRightLat) return false;
if (aLeftLat > bLeftLat && aRightLat > bRightLat) return false; var margin = 0; //FIXME? include a small margin of error, to help ensure rounding errors don't prodice false negatives?
if (aLeftLat+margin < bLeftLat && aRightLat+margin < bRightLat) return false;
if (aLeftLat-margin > bLeftLat && aRightLat-margin > bRightLat) return false;
// latitudes cross between left and right - so geodesic lines cross // latitudes cross between left and right - so geodesic lines cross
return true; return true;