Ejemplo n.º 1
0
// RectBound returns a bounding latitude-longitude rectangle.
// The bounds are not guaranteed to be tight.
func (c Cap) RectBound() Rect {
	if c.IsEmpty() {
		return EmptyRect()
	}

	capAngle := c.Radius().Radians()
	allLongitudes := false
	lat := r1.Interval{
		Lo: latitude(c.center).Radians() - capAngle,
		Hi: latitude(c.center).Radians() + capAngle,
	}
	lng := s1.FullInterval()

	// Check whether cap includes the south pole.
	if lat.Lo <= -math.Pi/2 {
		lat.Lo = -math.Pi / 2
		allLongitudes = true
	}

	// Check whether cap includes the north pole.
	if lat.Hi >= math.Pi/2 {
		lat.Hi = math.Pi / 2
		allLongitudes = true
	}

	if !allLongitudes {
		// Compute the range of longitudes covered by the cap. We use the law
		// of sines for spherical triangles. Consider the triangle ABC where
		// A is the north pole, B is the center of the cap, and C is the point
		// of tangency between the cap boundary and a line of longitude. Then
		// C is a right angle, and letting a,b,c denote the sides opposite A,B,C,
		// we have sin(a)/sin(A) = sin(c)/sin(C), or sin(A) = sin(a)/sin(c).
		// Here "a" is the cap angle, and "c" is the colatitude (90 degrees
		// minus the latitude). This formula also works for negative latitudes.
		//
		// The formula for sin(a) follows from the relationship h = 1 - cos(a).
		sinA := math.Sqrt(c.height * (2 - c.height))
		sinC := math.Cos(latitude(c.center).Radians())
		if sinA <= sinC {
			angleA := math.Asin(sinA / sinC)
			lng.Lo = math.Remainder(longitude(c.center).Radians()-angleA, math.Pi*2)
			lng.Hi = math.Remainder(longitude(c.center).Radians()+angleA, math.Pi*2)
		}
	}
	return Rect{lat, lng}
}
Ejemplo n.º 2
0
// initBound sets up the approximate bounding Rects for this loop.
func (l *Loop) initBound() {
	// Check for the special "empty" and "full" loops.
	if l.isEmptyOrFull() {
		if l.IsEmpty() {
			l.bound = EmptyRect()
		} else {
			l.bound = FullRect()
		}
		l.subregionBound = l.bound
		return
	}

	// The bounding rectangle of a loop is not necessarily the same as the
	// bounding rectangle of its vertices. First, the maximal latitude may be
	// attained along the interior of an edge. Second, the loop may wrap
	// entirely around the sphere (e.g. a loop that defines two revolutions of a
	// candy-cane stripe). Third, the loop may include one or both poles.
	// Note that a small clockwise loop near the equator contains both poles.
	bounder := NewRectBounder()
	for _, p := range l.vertices {
		bounder.AddPoint(p)
	}
	bounder.AddPoint(l.vertices[0])
	b := bounder.RectBound()

	if l.ContainsPoint(Point{r3.Vector{0, 0, 1}}) {
		b = Rect{r1.Interval{b.Lat.Lo, math.Pi / 2}, s1.FullInterval()}
	}
	// If a loop contains the south pole, then either it wraps entirely
	// around the sphere (full longitude range), or it also contains the
	// north pole in which case b.Lng.IsFull() due to the test above.
	// Either way, we only need to do the south pole containment test if
	// b.Lng.IsFull().
	if b.Lng.IsFull() && l.ContainsPoint(Point{r3.Vector{0, 0, -1}}) {
		b.Lat.Lo = -math.Pi / 2
	}
	l.bound = b
	l.subregionBound = ExpandForSubregions(l.bound)
}
Ejemplo n.º 3
0
func TestExpandForSubregions(t *testing.T) {
	// Test the full and empty bounds.
	if !ExpandForSubregions(FullRect()).IsFull() {
		t.Errorf("Subregion Bound of full rect should be full")
	}
	if !ExpandForSubregions(EmptyRect()).IsEmpty() {
		t.Errorf("Subregion Bound of empty rect should be empty")
	}

	tests := []struct {
		xLat, xLng, yLat, yLng float64
		wantFull               bool
	}{
		// Cases where the bound does not straddle the equator (but almost does),
		// and spans nearly 180 degrees in longitude.
		{3e-16, 0, 1e-14, math.Pi, true},
		{9e-16, 0, 1e-14, math.Pi, false},
		{1e-16, 7e-16, 1e-14, math.Pi, true},
		{3e-16, 14e-16, 1e-14, math.Pi, false},
		{1e-100, 14e-16, 1e-14, math.Pi, true},
		{1e-100, 22e-16, 1e-14, math.Pi, false},
		// Cases where the bound spans at most 90 degrees in longitude, and almost
		// 180 degrees in latitude.  Note that DBL_EPSILON is about 2.22e-16, which
		// implies that the double-precision value just below Pi/2 can be written as
		// (math.Pi/2 - 2e-16).
		{-math.Pi / 2, -1e-15, math.Pi/2 - 7e-16, 0, true},
		{-math.Pi / 2, -1e-15, math.Pi/2 - 30e-16, 0, false},
		{-math.Pi/2 + 4e-16, 0, math.Pi/2 - 2e-16, 1e-7, true},
		{-math.Pi/2 + 30e-16, 0, math.Pi / 2, 1e-7, false},
		{-math.Pi/2 + 4e-16, 0, math.Pi/2 - 4e-16, math.Pi / 2, true},
		{-math.Pi / 2, 0, math.Pi/2 - 30e-16, math.Pi / 2, false},
		// Cases where the bound straddles the equator and spans more than 90
		// degrees in longitude.  These are the cases where the critical distance is
		// between a corner of the bound and the opposite longitudinal edge.  Unlike
		// the cases above, here the bound may contain nearly-antipodal points (to
		// within 3.055 * DBL_EPSILON) even though the latitude and longitude ranges
		// are both significantly less than (math.Pi - 3.055 * DBL_EPSILON).
		{-math.Pi / 2, 0, math.Pi/2 - 1e-8, math.Pi - 1e-7, true},
		{-math.Pi / 2, 0, math.Pi/2 - 1e-7, math.Pi - 1e-7, false},
		{-math.Pi/2 + 1e-12, -math.Pi + 1e-4, math.Pi / 2, 0, true},
		{-math.Pi/2 + 1e-11, -math.Pi + 1e-4, math.Pi / 2, 0, true},
	}

	for _, tc := range tests {
		in := RectFromLatLng(LatLng{s1.Angle(tc.xLat), s1.Angle(tc.xLng)})
		in = in.AddPoint(LatLng{s1.Angle(tc.yLat), s1.Angle(tc.yLng)})
		got := ExpandForSubregions(in)

		// Test that the bound is actually expanded.
		if !got.Contains(in) {
			t.Errorf("Subregion bound of (%f, %f, %f, %f) should contain original rect", tc.xLat, tc.xLng, tc.yLat, tc.yLng)
		}
		if in.Lat == validRectLatRange && in.Lat.ContainsInterval(got.Lat) {
			t.Errorf("Subregion bound of (%f, %f, %f, %f) shouldn't be contained by original rect", tc.xLat, tc.xLng, tc.yLat, tc.yLng)
		}

		// We check the various situations where the bound contains nearly-antipodal points. The tests are organized into pairs
		// where the two bounds are similar except that the first bound meets the nearly-antipodal criteria while the second does not.
		if got.IsFull() != tc.wantFull {
			t.Errorf("Subregion Bound of (%f, %f, %f, %f).IsFull should be %t", tc.xLat, tc.xLng, tc.yLat, tc.yLng, tc.wantFull)
		}
	}

	rectTests := []struct {
		xLat, xLng, yLat, yLng float64
		wantRect               Rect
	}{
		{1.5, -math.Pi / 2, 1.5, math.Pi/2 - 2e-16, Rect{r1.Interval{1.5, 1.5}, s1.FullInterval()}},
		{1.5, -math.Pi / 2, 1.5, math.Pi/2 - 7e-16, Rect{r1.Interval{1.5, 1.5}, s1.Interval{-math.Pi / 2, math.Pi/2 - 7e-16}}},
		// Check for cases where the bound is expanded to include one of the poles
		{-math.Pi/2 + 1e-15, 0, -math.Pi/2 + 1e-15, 0, Rect{r1.Interval{-math.Pi / 2, -math.Pi/2 + 1e-15}, s1.FullInterval()}},
		{math.Pi/2 - 1e-15, 0, math.Pi/2 - 1e-15, 0, Rect{r1.Interval{math.Pi/2 - 1e-15, math.Pi / 2}, s1.FullInterval()}},
	}

	for _, tc := range rectTests {
		// Now we test cases where the bound does not contain nearly-antipodal
		// points, but it does contain points that are approximately 180 degrees
		// apart in latitude.
		in := RectFromLatLng(LatLng{s1.Angle(tc.xLat), s1.Angle(tc.xLng)})
		in = in.AddPoint(LatLng{s1.Angle(tc.yLat), s1.Angle(tc.yLng)})
		got := ExpandForSubregions(in)
		if !rectsApproxEqual(got, tc.wantRect, rectErrorLat, rectErrorLng) {
			t.Errorf("Subregion Bound of (%f, %f, %f, %f) = (%v) should be %v", tc.xLat, tc.xLng, tc.yLat, tc.yLng, got, tc.wantRect)
		}
	}
}
Ejemplo n.º 4
0
// RectBound returns the bounding rectangle of this cell.
func (c Cell) RectBound() Rect {
	if c.level > 0 {
		// Except for cells at level 0, the latitude and longitude extremes are
		// attained at the vertices.  Furthermore, the latitude range is
		// determined by one pair of diagonally opposite vertices and the
		// longitude range is determined by the other pair.
		//
		// We first determine which corner (i,j) of the cell has the largest
		// absolute latitude.  To maximize latitude, we want to find the point in
		// the cell that has the largest absolute z-coordinate and the smallest
		// absolute x- and y-coordinates.  To do this we look at each coordinate
		// (u and v), and determine whether we want to minimize or maximize that
		// coordinate based on the axis direction and the cell's (u,v) quadrant.
		u := c.uv.X.Lo + c.uv.X.Hi
		v := c.uv.Y.Lo + c.uv.Y.Hi
		var i, j int
		if uAxis(int(c.face)).Z == 0 {
			if u < 0 {
				i = 1
			}
		} else if u > 0 {
			i = 1
		}
		if vAxis(int(c.face)).Z == 0 {
			if v < 0 {
				j = 1
			}
		} else if v > 0 {
			j = 1
		}
		lat := r1.IntervalFromPoint(c.latitude(i, j)).AddPoint(c.latitude(1-i, 1-j))
		lng := s1.EmptyInterval().AddPoint(c.longitude(i, 1-j)).AddPoint(c.longitude(1-i, j))

		// We grow the bounds slightly to make sure that the bounding rectangle
		// contains LatLngFromPoint(P) for any point P inside the loop L defined by the
		// four *normalized* vertices.  Note that normalization of a vector can
		// change its direction by up to 0.5 * dblEpsilon radians, and it is not
		// enough just to add Normalize calls to the code above because the
		// latitude/longitude ranges are not necessarily determined by diagonally
		// opposite vertex pairs after normalization.
		//
		// We would like to bound the amount by which the latitude/longitude of a
		// contained point P can exceed the bounds computed above.  In the case of
		// longitude, the normalization error can change the direction of rounding
		// leading to a maximum difference in longitude of 2 * dblEpsilon.  In
		// the case of latitude, the normalization error can shift the latitude by
		// up to 0.5 * dblEpsilon and the other sources of error can cause the
		// two latitudes to differ by up to another 1.5 * dblEpsilon, which also
		// leads to a maximum difference of 2 * dblEpsilon.
		return Rect{lat, lng}.expanded(LatLng{s1.Angle(2 * dblEpsilon), s1.Angle(2 * dblEpsilon)}).PolarClosure()
	}

	// The 4 cells around the equator extend to +/-45 degrees latitude at the
	// midpoints of their top and bottom edges.  The two cells covering the
	// poles extend down to +/-35.26 degrees at their vertices.  The maximum
	// error in this calculation is 0.5 * dblEpsilon.
	var bound Rect
	switch c.face {
	case 0:
		bound = Rect{r1.Interval{-math.Pi / 4, math.Pi / 4}, s1.Interval{-math.Pi / 4, math.Pi / 4}}
	case 1:
		bound = Rect{r1.Interval{-math.Pi / 4, math.Pi / 4}, s1.Interval{math.Pi / 4, 3 * math.Pi / 4}}
	case 2:
		bound = Rect{r1.Interval{poleMinLat, math.Pi / 2}, s1.FullInterval()}
	case 3:
		bound = Rect{r1.Interval{-math.Pi / 4, math.Pi / 4}, s1.Interval{3 * math.Pi / 4, -3 * math.Pi / 4}}
	case 4:
		bound = Rect{r1.Interval{-math.Pi / 4, math.Pi / 4}, s1.Interval{-3 * math.Pi / 4, -math.Pi / 4}}
	default:
		bound = Rect{r1.Interval{-math.Pi / 2, -poleMinLat}, s1.FullInterval()}
	}

	// Finally, we expand the bound to account for the error when a point P is
	// converted to an LatLng to test for containment. (The bound should be
	// large enough so that it contains the computed LatLng of any contained
	// point, not just the infinite-precision version.) We don't need to expand
	// longitude because longitude is calculated via a single call to math.Atan2,
	// which is guaranteed to be semi-monotonic.
	return bound.expanded(LatLng{s1.Angle(dblEpsilon), s1.Angle(0)})
}
Ejemplo n.º 5
0
// PolarClosure returns the rectangle unmodified if it does not include either pole.
// If it includes either pole, PolarClosure returns an expansion of the rectangle along
// the longitudinal range to include all possible representations of the contained poles.
func (r Rect) PolarClosure() Rect {
	if r.Lat.Lo == -math.Pi/2 || r.Lat.Hi == math.Pi/2 {
		return Rect{r.Lat, s1.FullInterval()}
	}
	return r
}
Ejemplo n.º 6
0
	"fmt"
	"math"

	"github.com/golang/geo/r1"
	"github.com/golang/geo/s1"
)

// Rect represents a closed latitude-longitude rectangle.
type Rect struct {
	Lat r1.Interval
	Lng s1.Interval
}

var (
	validRectLatRange = r1.Interval{-math.Pi / 2, math.Pi / 2}
	validRectLngRange = s1.FullInterval()
)

// EmptyRect returns the empty rectangle.
func EmptyRect() Rect { return Rect{r1.EmptyInterval(), s1.EmptyInterval()} }

// FullRect returns the full rectangle.
func FullRect() Rect { return Rect{validRectLatRange, validRectLngRange} }

// RectFromLatLng constructs a rectangle containing a single point p.
func RectFromLatLng(p LatLng) Rect {
	return Rect{
		Lat: r1.Interval{p.Lat.Radians(), p.Lat.Radians()},
		Lng: s1.Interval{p.Lng.Radians(), p.Lng.Radians()},
	}
}
Ejemplo n.º 7
0
// AddPoint adds the given point to the chain. The Point must be unit length.
func (r *RectBounder) AddPoint(b Point) {
	bLL := LatLngFromPoint(b)

	if r.bound.IsEmpty() {
		r.a = b
		r.aLL = bLL
		r.bound = r.bound.AddPoint(bLL)
		return
	}

	// First compute the cross product N = A x B robustly. This is the normal
	// to the great circle through A and B. We don't use RobustSign
	// since that method returns an arbitrary vector orthogonal to A if the two
	// vectors are proportional, and we want the zero vector in that case.
	n := r.a.Sub(b.Vector).Cross(r.a.Add(b.Vector)) // N = 2 * (A x B)

	// The relative error in N gets large as its norm gets very small (i.e.,
	// when the two points are nearly identical or antipodal). We handle this
	// by choosing a maximum allowable error, and if the error is greater than
	// this we fall back to a different technique. Since it turns out that
	// the other sources of error in converting the normal to a maximum
	// latitude add up to at most 1.16 * dblEpsilon, and it is desirable to
	// have the total error be a multiple of dblEpsilon, we have chosen to
	// limit the maximum error in the normal to be 3.84 * dblEpsilon.
	// It is possible to show that the error is less than this when
	//
	// n.Norm() >= 8 * sqrt(3) / (3.84 - 0.5 - sqrt(3)) * dblEpsilon
	//          = 1.91346e-15 (about 8.618 * dblEpsilon)
	nNorm := n.Norm()
	if nNorm < 1.91346e-15 {
		// A and B are either nearly identical or nearly antipodal (to within
		// 4.309 * dblEpsilon, or about 6 nanometers on the earth's surface).
		if r.a.Dot(b.Vector) < 0 {
			// The two points are nearly antipodal. The easiest solution is to
			// assume that the edge between A and B could go in any direction
			// around the sphere.
			r.bound = FullRect()
		} else {
			// The two points are nearly identical (to within 4.309 * dblEpsilon).
			// In this case we can just use the bounding rectangle of the points,
			// since after the expansion done by GetBound this Rect is
			// guaranteed to include the (lat,lng) values of all points along AB.
			r.bound = r.bound.Union(RectFromLatLng(r.aLL).AddPoint(bLL))
		}
		r.a = b
		r.aLL = bLL
		return
	}

	// Compute the longitude range spanned by AB.
	lngAB := s1.EmptyInterval().AddPoint(r.aLL.Lng.Radians()).AddPoint(bLL.Lng.Radians())
	if lngAB.Length() >= math.Pi-2*dblEpsilon {
		// The points lie on nearly opposite lines of longitude to within the
		// maximum error of the calculation. The easiest solution is to assume
		// that AB could go on either side of the pole.
		lngAB = s1.FullInterval()
	}

	// Next we compute the latitude range spanned by the edge AB. We start
	// with the range spanning the two endpoints of the edge:
	latAB := r1.IntervalFromPoint(r.aLL.Lat.Radians()).AddPoint(bLL.Lat.Radians())

	// This is the desired range unless the edge AB crosses the plane
	// through N and the Z-axis (which is where the great circle through A
	// and B attains its minimum and maximum latitudes). To test whether AB
	// crosses this plane, we compute a vector M perpendicular to this
	// plane and then project A and B onto it.
	m := n.Cross(PointFromCoords(0, 0, 1).Vector)
	mA := m.Dot(r.a.Vector)
	mB := m.Dot(b.Vector)

	// We want to test the signs of "mA" and "mB", so we need to bound
	// the error in these calculations. It is possible to show that the
	// total error is bounded by
	//
	// (1 + sqrt(3)) * dblEpsilon * nNorm + 8 * sqrt(3) * (dblEpsilon**2)
	//   = 6.06638e-16 * nNorm + 6.83174e-31

	mError := 6.06638e-16*nNorm + 6.83174e-31
	if mA*mB < 0 || math.Abs(mA) <= mError || math.Abs(mB) <= mError {
		// Minimum/maximum latitude *may* occur in the edge interior.
		//
		// The maximum latitude is 90 degrees minus the latitude of N. We
		// compute this directly using atan2 in order to get maximum accuracy
		// near the poles.
		//
		// Our goal is compute a bound that contains the computed latitudes of
		// all S2Points P that pass the point-in-polygon containment test.
		// There are three sources of error we need to consider:
		// - the directional error in N (at most 3.84 * dblEpsilon)
		// - converting N to a maximum latitude
		// - computing the latitude of the test point P
		// The latter two sources of error are at most 0.955 * dblEpsilon
		// individually, but it is possible to show by a more complex analysis
		// that together they can add up to at most 1.16 * dblEpsilon, for a
		// total error of 5 * dblEpsilon.
		//
		// We add 3 * dblEpsilon to the bound here, and GetBound() will pad
		// the bound by another 2 * dblEpsilon.
		maxLat := math.Min(
			math.Atan2(math.Sqrt(n.X*n.X+n.Y*n.Y), math.Abs(n.Z))+3*dblEpsilon,
			math.Pi/2)

		// In order to get tight bounds when the two points are close together,
		// we also bound the min/max latitude relative to the latitudes of the
		// endpoints A and B. First we compute the distance between A and B,
		// and then we compute the maximum change in latitude between any two
		// points along the great circle that are separated by this distance.
		// This gives us a latitude change "budget". Some of this budget must
		// be spent getting from A to B; the remainder bounds the round-trip
		// distance (in latitude) from A or B to the min or max latitude
		// attained along the edge AB.
		latBudget := 2 * math.Asin(0.5*(r.a.Sub(b.Vector)).Norm()*math.Sin(maxLat))
		maxDelta := 0.5*(latBudget-latAB.Length()) + dblEpsilon

		// Test whether AB passes through the point of maximum latitude or
		// minimum latitude. If the dot product(s) are small enough then the
		// result may be ambiguous.
		if mA <= mError && mB >= -mError {
			latAB.Hi = math.Min(maxLat, latAB.Hi+maxDelta)
		}
		if mB <= mError && mA >= -mError {
			latAB.Lo = math.Max(-maxLat, latAB.Lo-maxDelta)
		}
	}
	r.a = b
	r.aLL = bLL
	r.bound = r.bound.Union(Rect{latAB, lngAB})
}