// Physically based lightness calculation
// for a point light above a 100% matte (Lambertian) ground plane,
// moving up and down.
// details: https://hyperknot.com/blog/area_lights_in_shaders/

float point_light(vec2 uv, float h, float i) {
  // h - light's height over the ground
  // i - light's intensity
  return i * h * pow(dot(uv,uv) + h*h, -1.5);
}

float lin_to_srgb ( float val ) {
  if( val < 0.0031308 )
    return val * 12.92;
  else
    return 1.055 * pow(val,1.0/2.4) - 0.055;
}

void mainImage( out vec4 fragColor, in vec2 fragCoord ){
    vec2 uv = ( fragCoord - .5*iResolution.xy ) / iResolution.y;

    float h = pow(sin(iTime/3.), 2.) * 0.1 + 0.001;
    float v = point_light(uv, h, 0.005);
    v = lin_to_srgb(v);

    fragColor=vec4(v);
}