gotosocial/vendor/github.com/golang/geo/s2/stuv.go
Tobi Smethurst 98263a7de6
Grand test fixup (#138)
* start fixing up tests

* fix up tests + automate with drone

* fiddle with linting

* messing about with drone.yml

* some more fiddling

* hmmm

* add cache

* add vendor directory

* verbose

* ci updates

* update some little things

* update sig
2021-08-12 21:03:24 +02:00

428 lines
15 KiB
Go

// Copyright 2014 Google Inc. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package s2
import (
"math"
"github.com/golang/geo/r3"
)
//
// This file contains documentation of the various coordinate systems used
// throughout the library. Most importantly, S2 defines a framework for
// decomposing the unit sphere into a hierarchy of "cells". Each cell is a
// quadrilateral bounded by four geodesics. The top level of the hierarchy is
// obtained by projecting the six faces of a cube onto the unit sphere, and
// lower levels are obtained by subdividing each cell into four children
// recursively. Cells are numbered such that sequentially increasing cells
// follow a continuous space-filling curve over the entire sphere. The
// transformation is designed to make the cells at each level fairly uniform
// in size.
//
////////////////////////// S2 Cell Decomposition /////////////////////////
//
// The following methods define the cube-to-sphere projection used by
// the Cell decomposition.
//
// In the process of converting a latitude-longitude pair to a 64-bit cell
// id, the following coordinate systems are used:
//
// (id)
// An CellID is a 64-bit encoding of a face and a Hilbert curve position
// on that face. The Hilbert curve position implicitly encodes both the
// position of a cell and its subdivision level (see s2cellid.go).
//
// (face, i, j)
// Leaf-cell coordinates. "i" and "j" are integers in the range
// [0,(2**30)-1] that identify a particular leaf cell on the given face.
// The (i, j) coordinate system is right-handed on each face, and the
// faces are oriented such that Hilbert curves connect continuously from
// one face to the next.
//
// (face, s, t)
// Cell-space coordinates. "s" and "t" are real numbers in the range
// [0,1] that identify a point on the given face. For example, the point
// (s, t) = (0.5, 0.5) corresponds to the center of the top-level face
// cell. This point is also a vertex of exactly four cells at each
// subdivision level greater than zero.
//
// (face, si, ti)
// Discrete cell-space coordinates. These are obtained by multiplying
// "s" and "t" by 2**31 and rounding to the nearest unsigned integer.
// Discrete coordinates lie in the range [0,2**31]. This coordinate
// system can represent the edge and center positions of all cells with
// no loss of precision (including non-leaf cells). In binary, each
// coordinate of a level-k cell center ends with a 1 followed by
// (30 - k) 0s. The coordinates of its edges end with (at least)
// (31 - k) 0s.
//
// (face, u, v)
// Cube-space coordinates in the range [-1,1]. To make the cells at each
// level more uniform in size after they are projected onto the sphere,
// we apply a nonlinear transformation of the form u=f(s), v=f(t).
// The (u, v) coordinates after this transformation give the actual
// coordinates on the cube face (modulo some 90 degree rotations) before
// it is projected onto the unit sphere.
//
// (face, u, v, w)
// Per-face coordinate frame. This is an extension of the (face, u, v)
// cube-space coordinates that adds a third axis "w" in the direction of
// the face normal. It is always a right-handed 3D coordinate system.
// Cube-space coordinates can be converted to this frame by setting w=1,
// while (u,v,w) coordinates can be projected onto the cube face by
// dividing by w, i.e. (face, u/w, v/w).
//
// (x, y, z)
// Direction vector (Point). Direction vectors are not necessarily unit
// length, and are often chosen to be points on the biunit cube
// [-1,+1]x[-1,+1]x[-1,+1]. They can be be normalized to obtain the
// corresponding point on the unit sphere.
//
// (lat, lng)
// Latitude and longitude (LatLng). Latitudes must be between -90 and
// 90 degrees inclusive, and longitudes must be between -180 and 180
// degrees inclusive.
//
// Note that the (i, j), (s, t), (si, ti), and (u, v) coordinate systems are
// right-handed on all six faces.
//
//
// There are a number of different projections from cell-space (s,t) to
// cube-space (u,v): linear, quadratic, and tangent. They have the following
// tradeoffs:
//
// Linear - This is the fastest transformation, but also produces the least
// uniform cell sizes. Cell areas vary by a factor of about 5.2, with the
// largest cells at the center of each face and the smallest cells in
// the corners.
//
// Tangent - Transforming the coordinates via Atan makes the cell sizes
// more uniform. The areas vary by a maximum ratio of 1.4 as opposed to a
// maximum ratio of 5.2. However, each call to Atan is about as expensive
// as all of the other calculations combined when converting from points to
// cell ids, i.e. it reduces performance by a factor of 3.
//
// Quadratic - This is an approximation of the tangent projection that
// is much faster and produces cells that are almost as uniform in size.
// It is about 3 times faster than the tangent projection for converting
// cell ids to points or vice versa. Cell areas vary by a maximum ratio of
// about 2.1.
//
// Here is a table comparing the cell uniformity using each projection. Area
// Ratio is the maximum ratio over all subdivision levels of the largest cell
// area to the smallest cell area at that level, Edge Ratio is the maximum
// ratio of the longest edge of any cell to the shortest edge of any cell at
// the same level, and Diag Ratio is the ratio of the longest diagonal of
// any cell to the shortest diagonal of any cell at the same level.
//
// Area Edge Diag
// Ratio Ratio Ratio
// -----------------------------------
// Linear: 5.200 2.117 2.959
// Tangent: 1.414 1.414 1.704
// Quadratic: 2.082 1.802 1.932
//
// The worst-case cell aspect ratios are about the same with all three
// projections. The maximum ratio of the longest edge to the shortest edge
// within the same cell is about 1.4 and the maximum ratio of the diagonals
// within the same cell is about 1.7.
//
// For Go we have chosen to use only the Quadratic approach. Other language
// implementations may offer other choices.
const (
// maxSiTi is the maximum value of an si- or ti-coordinate.
// It is one shift more than maxSize. The range of valid (si,ti)
// values is [0..maxSiTi].
maxSiTi = maxSize << 1
)
// siTiToST converts an si- or ti-value to the corresponding s- or t-value.
// Value is capped at 1.0 because there is no DCHECK in Go.
func siTiToST(si uint32) float64 {
if si > maxSiTi {
return 1.0
}
return float64(si) / float64(maxSiTi)
}
// stToSiTi converts the s- or t-value to the nearest si- or ti-coordinate.
// The result may be outside the range of valid (si,ti)-values. Value of
// 0.49999999999999994 (math.NextAfter(0.5, -1)), will be incorrectly rounded up.
func stToSiTi(s float64) uint32 {
if s < 0 {
return uint32(s*maxSiTi - 0.5)
}
return uint32(s*maxSiTi + 0.5)
}
// stToUV converts an s or t value to the corresponding u or v value.
// This is a non-linear transformation from [-1,1] to [-1,1] that
// attempts to make the cell sizes more uniform.
// This uses what the C++ version calls 'the quadratic transform'.
func stToUV(s float64) float64 {
if s >= 0.5 {
return (1 / 3.) * (4*s*s - 1)
}
return (1 / 3.) * (1 - 4*(1-s)*(1-s))
}
// uvToST is the inverse of the stToUV transformation. Note that it
// is not always true that uvToST(stToUV(x)) == x due to numerical
// errors.
func uvToST(u float64) float64 {
if u >= 0 {
return 0.5 * math.Sqrt(1+3*u)
}
return 1 - 0.5*math.Sqrt(1-3*u)
}
// face returns face ID from 0 to 5 containing the r. For points on the
// boundary between faces, the result is arbitrary but deterministic.
func face(r r3.Vector) int {
f := r.LargestComponent()
switch {
case f == r3.XAxis && r.X < 0:
f += 3
case f == r3.YAxis && r.Y < 0:
f += 3
case f == r3.ZAxis && r.Z < 0:
f += 3
}
return int(f)
}
// validFaceXYZToUV given a valid face for the given point r (meaning that
// dot product of r with the face normal is positive), returns
// the corresponding u and v values, which may lie outside the range [-1,1].
func validFaceXYZToUV(face int, r r3.Vector) (float64, float64) {
switch face {
case 0:
return r.Y / r.X, r.Z / r.X
case 1:
return -r.X / r.Y, r.Z / r.Y
case 2:
return -r.X / r.Z, -r.Y / r.Z
case 3:
return r.Z / r.X, r.Y / r.X
case 4:
return r.Z / r.Y, -r.X / r.Y
}
return -r.Y / r.Z, -r.X / r.Z
}
// xyzToFaceUV converts a direction vector (not necessarily unit length) to
// (face, u, v) coordinates.
func xyzToFaceUV(r r3.Vector) (f int, u, v float64) {
f = face(r)
u, v = validFaceXYZToUV(f, r)
return f, u, v
}
// faceUVToXYZ turns face and UV coordinates into an unnormalized 3 vector.
func faceUVToXYZ(face int, u, v float64) r3.Vector {
switch face {
case 0:
return r3.Vector{1, u, v}
case 1:
return r3.Vector{-u, 1, v}
case 2:
return r3.Vector{-u, -v, 1}
case 3:
return r3.Vector{-1, -v, -u}
case 4:
return r3.Vector{v, -1, -u}
default:
return r3.Vector{v, u, -1}
}
}
// faceXYZToUV returns the u and v values (which may lie outside the range
// [-1, 1]) if the dot product of the point p with the given face normal is positive.
func faceXYZToUV(face int, p Point) (u, v float64, ok bool) {
switch face {
case 0:
if p.X <= 0 {
return 0, 0, false
}
case 1:
if p.Y <= 0 {
return 0, 0, false
}
case 2:
if p.Z <= 0 {
return 0, 0, false
}
case 3:
if p.X >= 0 {
return 0, 0, false
}
case 4:
if p.Y >= 0 {
return 0, 0, false
}
default:
if p.Z >= 0 {
return 0, 0, false
}
}
u, v = validFaceXYZToUV(face, p.Vector)
return u, v, true
}
// faceXYZtoUVW transforms the given point P to the (u,v,w) coordinate frame of the given
// face where the w-axis represents the face normal.
func faceXYZtoUVW(face int, p Point) Point {
// The result coordinates are simply the dot products of P with the (u,v,w)
// axes for the given face (see faceUVWAxes).
switch face {
case 0:
return Point{r3.Vector{p.Y, p.Z, p.X}}
case 1:
return Point{r3.Vector{-p.X, p.Z, p.Y}}
case 2:
return Point{r3.Vector{-p.X, -p.Y, p.Z}}
case 3:
return Point{r3.Vector{-p.Z, -p.Y, -p.X}}
case 4:
return Point{r3.Vector{-p.Z, p.X, -p.Y}}
default:
return Point{r3.Vector{p.Y, p.X, -p.Z}}
}
}
// faceSiTiToXYZ transforms the (si, ti) coordinates to a (not necessarily
// unit length) Point on the given face.
func faceSiTiToXYZ(face int, si, ti uint32) Point {
return Point{faceUVToXYZ(face, stToUV(siTiToST(si)), stToUV(siTiToST(ti)))}
}
// xyzToFaceSiTi transforms the (not necessarily unit length) Point to
// (face, si, ti) coordinates and the level the Point is at.
func xyzToFaceSiTi(p Point) (face int, si, ti uint32, level int) {
face, u, v := xyzToFaceUV(p.Vector)
si = stToSiTi(uvToST(u))
ti = stToSiTi(uvToST(v))
// If the levels corresponding to si,ti are not equal, then p is not a cell
// center. The si,ti values of 0 and maxSiTi need to be handled specially
// because they do not correspond to cell centers at any valid level; they
// are mapped to level -1 by the code at the end.
level = maxLevel - findLSBSetNonZero64(uint64(si|maxSiTi))
if level < 0 || level != maxLevel-findLSBSetNonZero64(uint64(ti|maxSiTi)) {
return face, si, ti, -1
}
// In infinite precision, this test could be changed to ST == SiTi. However,
// due to rounding errors, uvToST(xyzToFaceUV(faceUVToXYZ(stToUV(...)))) is
// not idempotent. On the other hand, the center is computed exactly the same
// way p was originally computed (if it is indeed the center of a Cell);
// the comparison can be exact.
if p.Vector == faceSiTiToXYZ(face, si, ti).Normalize() {
return face, si, ti, level
}
return face, si, ti, -1
}
// uNorm returns the right-handed normal (not necessarily unit length) for an
// edge in the direction of the positive v-axis at the given u-value on
// the given face. (This vector is perpendicular to the plane through
// the sphere origin that contains the given edge.)
func uNorm(face int, u float64) r3.Vector {
switch face {
case 0:
return r3.Vector{u, -1, 0}
case 1:
return r3.Vector{1, u, 0}
case 2:
return r3.Vector{1, 0, u}
case 3:
return r3.Vector{-u, 0, 1}
case 4:
return r3.Vector{0, -u, 1}
default:
return r3.Vector{0, -1, -u}
}
}
// vNorm returns the right-handed normal (not necessarily unit length) for an
// edge in the direction of the positive u-axis at the given v-value on
// the given face.
func vNorm(face int, v float64) r3.Vector {
switch face {
case 0:
return r3.Vector{-v, 0, 1}
case 1:
return r3.Vector{0, -v, 1}
case 2:
return r3.Vector{0, -1, -v}
case 3:
return r3.Vector{v, -1, 0}
case 4:
return r3.Vector{1, v, 0}
default:
return r3.Vector{1, 0, v}
}
}
// faceUVWAxes are the U, V, and W axes for each face.
var faceUVWAxes = [6][3]Point{
{Point{r3.Vector{0, 1, 0}}, Point{r3.Vector{0, 0, 1}}, Point{r3.Vector{1, 0, 0}}},
{Point{r3.Vector{-1, 0, 0}}, Point{r3.Vector{0, 0, 1}}, Point{r3.Vector{0, 1, 0}}},
{Point{r3.Vector{-1, 0, 0}}, Point{r3.Vector{0, -1, 0}}, Point{r3.Vector{0, 0, 1}}},
{Point{r3.Vector{0, 0, -1}}, Point{r3.Vector{0, -1, 0}}, Point{r3.Vector{-1, 0, 0}}},
{Point{r3.Vector{0, 0, -1}}, Point{r3.Vector{1, 0, 0}}, Point{r3.Vector{0, -1, 0}}},
{Point{r3.Vector{0, 1, 0}}, Point{r3.Vector{1, 0, 0}}, Point{r3.Vector{0, 0, -1}}},
}
// faceUVWFaces are the precomputed neighbors of each face.
var faceUVWFaces = [6][3][2]int{
{{4, 1}, {5, 2}, {3, 0}},
{{0, 3}, {5, 2}, {4, 1}},
{{0, 3}, {1, 4}, {5, 2}},
{{2, 5}, {1, 4}, {0, 3}},
{{2, 5}, {3, 0}, {1, 4}},
{{4, 1}, {3, 0}, {2, 5}},
}
// uvwAxis returns the given axis of the given face.
func uvwAxis(face, axis int) Point {
return faceUVWAxes[face][axis]
}
// uvwFaces returns the face in the (u,v,w) coordinate system on the given axis
// in the given direction.
func uvwFace(face, axis, direction int) int {
return faceUVWFaces[face][axis][direction]
}
// uAxis returns the u-axis for the given face.
func uAxis(face int) Point {
return uvwAxis(face, 0)
}
// vAxis returns the v-axis for the given face.
func vAxis(face int) Point {
return uvwAxis(face, 1)
}
// Return the unit-length normal for the given face.
func unitNorm(face int) Point {
return uvwAxis(face, 2)
}