// rotate_scale_fade — generated from https://github.com/gl-transitions/gl-transitions/blob/master/transitions/rotate_scale_fade.glsl
// Author: Fernando Kuteken
// License: MIT
// Description: The outgoing image spins and zooms while fading to the incoming one, over a background color

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 vec2 center; 

uniform float rotations; 

uniform float scale; 

uniform vec4 backColor; 

vec4 transition (vec2 uv) {
  
  vec2 difference = uv - center;
  vec2 dir = normalize(difference);
  float dist = length(difference);
  
  float angle = 2.0 * 3.14159265359 * rotations * progress;
  
  float c = cos(angle);
  float s = sin(angle);
  
  float currentScale = mix(scale, 1.0, 2.0 * abs(progress - 0.5));
  
  vec2 rotatedDir = vec2(dir.x  * c - dir.y * s, dir.x * s + dir.y * c);
  vec2 rotatedUv = center + rotatedDir * dist / currentScale;
  
  if (rotatedUv.x < 0.0 || rotatedUv.x > 1.0 ||
      rotatedUv.y < 0.0 || rotatedUv.y > 1.0)
    return backColor;
    
  return mix(getFromColor(rotatedUv), getToColor(rotatedUv), progress);
}

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);
}