// fadegrayscale — generated from https://github.com/gl-transitions/gl-transitions/blob/master/transitions/fadegrayscale.glsl
// Author: gre
// License: MIT
// Description: A crossfade through grayscale

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 intensity; 
 
vec3 grayscale (vec3 color) {
  return vec3(0.2126*color.r + 0.7152*color.g + 0.0722*color.b);
}
 
vec4 transition (vec2 uv) {
  vec4 fc = getFromColor(uv);
  vec4 tc = getToColor(uv);
  return mix(
    mix(vec4(grayscale(fc.rgb), 1.0), fc, smoothstep(1.0-intensity, 0.0, progress)),
    mix(vec4(grayscale(tc.rgb), 1.0), tc, smoothstep(    intensity, 1.0, progress)),
    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);
}