// cube — generated from https://github.com/gl-transitions/gl-transitions/blob/master/transitions/cube.glsl
// Author: gre
// License: MIT
// Description: The two images are faces of a rotating 3D cube, with a floor reflection

uniform shader _from;
uniform shader _to;
uniform float2 _resolution;
uniform float progress;
uniform float ratio;

vec4 getFromColor(vec2 uv) { return vec4(unpremul(_from.eval(vec2(uv.x, 1.0 - uv.y) * _resolution))); }
vec4 getToColor(vec2 uv) { return vec4(unpremul(_to.eval(vec2(uv.x, 1.0 - uv.y) * _resolution))); }

uniform float persp; 
uniform float unzoom; 

uniform float reflection; 
uniform float floating; 

vec2 project (vec2 p) {
  return p * vec2(1.0, -1.2) + vec2(0.0, -floating/100.);
}

bool inBounds (vec2 p) {
  return all(lessThan(vec2(0.0), p)) && all(lessThan(p, vec2(1.0)));
}

vec4 bgColor (vec2 p, vec2 pfr, vec2 pto) {
  vec4 c = vec4(0.0, 0.0, 0.0, 1.0);
  pfr = project(pfr);
  
  if (inBounds(pfr)) {
    c += mix(vec4(0.0), getFromColor(pfr), reflection * mix(1.0, 0.0, pfr.y));
  }
  pto = project(pto);
  if (inBounds(pto)) {
    c += mix(vec4(0.0), getToColor(pto), reflection * mix(1.0, 0.0, pto.y));
  }
  return c;
}

vec2 xskew (vec2 p, float persp, float center) {
  float x = mix(p.x, 1.0-p.x, center);
  return (
    (
      vec2( x, (p.y - 0.5*(1.0-persp) * x) / (1.0+(persp-1.0)*x) )
      - vec2(0.5-distance(center, 0.5), 0.0)
    )
    * vec2(0.5 / distance(center, 0.5) * (center<0.5 ? 1.0 : -1.0), 1.0)
    + vec2(center<0.5 ? 0.0 : 1.0, 0.0)
  );
}

vec4 transition(vec2 op) {
  float uz = unzoom * 2.0*(0.5-distance(0.5, progress));
  vec2 p = -uz*0.5+(1.0+uz) * op;
  vec2 fromP = xskew(
    (p - vec2(progress, 0.0)) / vec2(1.0-progress, 1.0),
    1.0-mix(progress, 0.0, persp),
    0.0
  );
  vec2 toP = xskew(
    p / vec2(progress, 1.0),
    mix(pow(progress, 2.0), 1.0, persp),
    1.0
  );
  
  if (inBounds(fromP)) {
    return getFromColor(fromP);
  }
  else if (inBounds(toP)) {
    return getToColor(toP);
  }
  return bgColor(op, fromP, toP);
}

half4 main(float2 _coord) {
  vec2 _uv = _coord / _resolution;
  vec4 _c = clamp(transition(vec2(_uv.x, 1.0 - _uv.y)), 0.0, 1.0);
  return half4(_c.rgb * _c.a, _c.a);
}