TL;DR: JavaScript code for a proof of concept included at the end of this answer.
The heading and pitch parameters h0 and p0 of the panorama image corresponds to a direction. By using the focal length f of the camera to scale this direction vector, one can get the 3D coordinates (x0, y0, z0) of the viewport center at (u0, v0):
x0 = f * cos( p0 ) * sin( h0 )
y0 = f * cos( p0 ) * cos( h0 )
z0 = f * sin( p0 )
The goal is now to find the 3D coordinates of the point at to some given pixel coordinates (u, v) in the image. First, map these pixel coordinates to pixel offsets (du, dv) (to the right and to the top) from the viewport center:
du = u - u0 = u - w / 2
dv = v0 - v = h / 2 - v
Then a local orthonormal 2D basis of the viewport in 3D has to be found. The unit vector (ux, uy, uz) supports the x-axis (to the right along the direction of increasing headings) and the vector (vx, vy, vz) supports the y-axis (to the top along the direction of increasing pitches) of the image. Once these two vectors are determined, the 3D coordinates of the point on the viewport matching the (du, dv) pixel offset in the viewport are simply:
x = x0 + du * ux + dv * vx
y = y0 + du * uy + dv * vy
z = z0 + du * uz + dv * vz
And the heading and pitch parameters h and p for this point are then:
R = sqrt( x * x + y * y + z * z )
h = atan2( x, y )
p = asin( z / R )
Finally to get the two unit vectors (ux, uy, uz) and (vx, vy, vz), compute the derivatives of the spherical coordinates by the heading and pitch parameters at (p0, h0), and one should get:
vx = -sin( p0 ) * sin ( h0 )
vy = -sin( p0 ) * cos ( h0 )
vz = cos( p0 )
ux = sgn( cos ( p0 ) ) * cos( h0 )
uy = -sgn( cos ( p0 ) ) * sin( h0 )
uz = 0
where sgn( a ) is +1 if a >= 0 else -1.
Complements:
The focal length is derived from the horizontal field of view and the width of the image:
f = (w / 2) / Math.tan(fov / 2)
The reverse mapping from heading and pitch parameters to pixel coordinates can be done similarly:
- Find the 3D coordinates
(x, y, z) of the direction of the ray corresponding to the specified heading and pitch parameters,
- Find the 3D coordinates
(x0, y0, z0) of the direction of the ray corresponding to the viewport center (an associated image plane is located at (x0, y0, z0) with an (x0, y0, z0) normal),
- Intersect the ray for the specified heading and pitch parameters with the image plane, this gives the 3D offset from the viewport center,
- Project this 3D offset on the local basis, getting the 2D offsets
du and dv
- Map
du and dv to absolute pixel coordinates.
In practice, this approach seems to work similarly well on both square and rectangular viewports.
Proof of concept code (call the onLoad() function on a web page containing a sized canvas element with a "panorama" id)
'use strict';
var viewer;
function onClick(e) {
viewer.click(e);
}
function onLoad() {
var element = document.getElementById("panorama");
viewer = new PanoramaViewer(element);
viewer.update();
}
function PanoramaViewer(element) {
this.element = element;
this.width = element.width;
this.height = element.height;
this.pitch = 0;
this.heading = 0;
element.addEventListener("click", onClick, false);
}
PanoramaViewer.FOV = 90;
PanoramaViewer.prototype.makeUrl = function() {
var fov = PanoramaViewer.FOV;
return "https://maps.googleapis.com/maps/api/streetview?location=40.457375,-80.009353&size=" + this.width + "x" + this.height + "&fov=" + fov + "&heading=" + this.heading + "&pitch=" + this.pitch;
}
PanoramaViewer.prototype.update = function() {
var element = this.element;
element.style.backgroundImage = "url(" + this.makeUrl() + ")";
var width = this.width;
var height = this.height;
var context = element.getContext('2d');
context.strokeStyle = '#FFFF00';
context.beginPath();
context.moveTo(0, height / 2);
context.lineTo(width, height / 2);
context.stroke();
context.beginPath();
context.moveTo(width / 2, 0);
context.lineTo(width / 2, height);
context.stroke();
}
function sgn(x) {
return x >= 0 ? 1 : -1;
}
PanoramaViewer.prototype.unmap = function(heading, pitch) {
var PI = Math.PI
var cos = Math.cos;
var sin = Math.sin;
var tan = Math.tan;
var fov = PanoramaViewer.FOV * PI / 180.0;
var width = this.width;
var height = this.height;
var f = 0.5 * width / tan(0.5 * fov);
var h = heading * PI / 180.0;
var p = pitch * PI / 180.0;
var x = f * cos(p) * sin(h);
var y = f * cos(p) * cos(h);
var z = f * sin(p);
var h0 = this.heading * PI / 180.0;
var p0 = this.pitch * PI / 180.0;
var x0 = f * cos(p0) * sin(h0);
var y0 = f * cos(p0) * cos(h0);
var z0 = f * sin(p0);
//
// Intersect the ray O, v = (x, y, z)
// with the plane at M0 of normal n = (x0, y0, z0)
//
// n . (O + t v - M0) = 0
// t n . v = n . M0 = f^2
//
var t = f * f / (x0 * x + y0 * y + z0 * z);
var ux = sgn(cos(p0)) * cos(h0);
var uy = -sgn(cos(p0)) * sin(h0);
var uz = 0;
var vx = -sin(p0) * sin(h0);
var vy = -sin(p0) * cos(h0);
var vz = cos(p0);
var x1 = t * x;
var y1 = t * y;
var z1 = t * z;
var dx10 = x1 - x0;
var dy10 = y1 - y0;
var dz10 = z1 - z0;
// Project on the local basis (u, v) at M0
var du = ux * dx10 + uy * dy10 + uz * dz10;
var dv = vx * dx10 + vy * dy10 + vz * dz10;
return {
u: du + width / 2,
v: height / 2 - dv,
};
}
PanoramaViewer.prototype.map = function(u, v) {
var PI = Math.PI;
var cos = Math.cos;
var sin = Math.sin;
var tan = Math.tan;
var sqrt = Math.sqrt;
var atan2 = Math.atan2;
var asin = Math.asin;
var fov = PanoramaViewer.FOV * PI / 180.0;
var width = this.width;
var height = this.height;
var h0 = this.heading * PI / 180.0;
var p0 = this.pitch * PI / 180.0;
var f = 0.5 * width / tan(0.5 * fov);
var x0 = f * cos(p0) * sin(h0);
var y0 = f * cos(p0) * cos(h0);
var z0 = f * sin(p0);
var du = u - width / 2;
var dv = height / 2 - v;
var ux = sgn(cos(p0)) * cos(h0);
var uy = -sgn(cos(p0)) * sin(h0);
var uz = 0;
var vx = -sin(p0) * sin(h0);
var vy = -sin(p0) * cos(h0);
var vz = cos(p0);
var x = x0 + du * ux + dv * vx;
var y = y0 + du * uy + dv * vy;
var z = z0 + du * uz + dv * vz;
var R = sqrt(x * x + y * y + z * z);
var h = atan2(x, y);
var p = asin(z / R);
return {
heading: h * 180.0 / PI,
pitch: p * 180.0 / PI
};
}
PanoramaViewer.prototype.click = function(e) {
var rect = e.target.getBoundingClientRect();
var u = e.clientX - rect.left;
var v = e.clientY - rect.top;
var uvCoords = this.unmap(this.heading, this.pitch);
console.log("current viewport center");
console.log(" heading: " + this.heading);
console.log(" pitch: " + this.pitch);
console.log(" u: " + uvCoords.u)
console.log(" v: " + uvCoords.v);
var hpCoords = this.map(u, v);
uvCoords = this.unmap(hpCoords.heading, hpCoords.pitch);
console.log("click at (" + u + "," + v + ")");
console.log(" heading: " + hpCoords.heading);
console.log(" pitch: " + hpCoords.pitch);
console.log(" u: " + uvCoords.u);
console.log(" v: " + uvCoords.v);
this.heading = hpCoords.heading;
this.pitch = hpCoords.pitch;
this.update();
}
lengthOfChord == diameter(i.e. a 180° angle). – Satori