// displacement — generated from https://github.com/gl-transitions/gl-transitions/blob/master/transitions/displacement.glsl
// Author: Travis Fischer
// License: MIT
// Description: A crossfade in which a displacement map (an extra grayscale texture) warps both images

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 shader displacementMap;
vec4 _texture_displacementMap(vec2 uv) { return vec4(unpremul(displacementMap.eval(vec2(uv.x, 1.0 - uv.y) * _resolution))); }

uniform float strength; 

vec4 transition (vec2 uv) {
  float displacement = _texture_displacementMap( uv).r * strength;

  vec2 uvFrom = vec2(uv.x + progress * displacement, uv.y);
  vec2 uvTo = vec2(uv.x - (1.0 - progress) * displacement, uv.y);

  return mix(
    getFromColor(uvFrom),
    getToColor(uvTo),
    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);
}