#include <flutter/runtime_effect.glsl>

// The engine supplies the backing texture and its physical size.
uniform vec2 u_texture_size;
uniform vec2 u_origin;
uniform vec2 u_extent;
uniform float u_strength;
uniform sampler2D u_background;
out vec4 frag_color;

vec2 sampleUV(vec2 point) {
  vec2 uv = point / u_texture_size;
  #ifdef IMPELLER_TARGET_OPENGLES
    uv.y = 1.0 - uv.y;
  #endif
  return clamp(uv, vec2(0.001), vec2(0.999));
}

void main() {
  vec2 point = FlutterFragCoord().xy;
  vec2 centre = u_origin + u_extent * 0.5;
  vec2 local = point - centre;
  float radius = max(u_extent.y * 0.5, 1.0);
  float spine = max(u_extent.x * 0.5 - radius, 0.0);
  vec2 outward = local - vec2(clamp(local.x, -spine, spine), 0.0);
  float distance = length(outward);
  vec2 normal = outward / max(distance, 0.001);
  float depth = radius - distance;
  float rim = 1.0 - smoothstep(0.0, radius * 0.46, depth);
  // Curved glass bends rays most at its edge, with subtle centre magnification.
  vec2 ray = centre + local / (1.0 + u_strength * 0.075)
      - normal * rim * rim * radius * u_strength * 0.23;
  vec4 colour = texture(u_background, sampleUV(ray));
  float dispersion = u_strength * rim * 0.65;
  colour.r = texture(u_background, sampleUV(ray + normal * dispersion)).r;
  colour.b = texture(u_background, sampleUV(ray - normal * dispersion)).b;
  // The curved rim catches nearby background light as well as transmitting it.
  // Reflection follows live pixels instead of being a painted white outline.
  float facing = max(0.0, dot(normal, normalize(vec2(-0.5, -0.8))));
  float edge_light = pow(rim, 4.0) * u_strength * facing;
  vec4 reflected = texture(u_background,
      sampleUV(point + normal * radius * 0.40));
  colour = mix(colour, reflected, min(edge_light * 0.18, 0.22));
  // A grazing-angle white reflection gives the curved rim visible thickness;
  // the broad centre continues to transmit the live, lightly softened scene.
  colour.rgb = mix(colour.rgb, vec3(colour.a), min(edge_light * 0.12, 0.16));
  float reflection = edge_light * 0.07;
  colour.rgb = min(colour.rgb + reflection * colour.a, vec3(colour.a));
  frag_color = colour;
}