WebGL: stereoscopic projection transformation (Fragment Shader)

Equirectangular-to-Stereographic-Projection-Shader.frag

// credits to: https://github.com/notlion/streetview-stereographic

precision mediump float;

uniform sampler2D texture;
uniform float scale, aspect, time;
uniform mat3 transform;

varying vec2 v_texcoord;

#define PI 3.141592653589793

void main(){
  vec2 rads = vec2(PI * 2., PI);

  vec2 pnt = (v_texcoord - .5) * vec2(scale, scale * aspect);

  // Project to Sphere
  float x2y2 = pnt.x * pnt.x + pnt.y * pnt.y;
  vec3 sphere_pnt = vec3(2. * pnt, x2y2 - 1.) / (x2y2 + 1.);
  sphere_pnt *= transform;

  // Convert to Spherical Coordinates
  float r = length(sphere_pnt);
  float lon = atan(sphere_pnt.y, sphere_pnt.x);
  float lat = acos(sphere_pnt.z / r);

  gl_FragColor = texture2D(texture, vec2(lon, lat) / rads);
}