// Physically based lightness calculation
// for a rod light perpendicular to a
// matte (Lambertian) ground plane, expanding up and down.
// details: https://hyperknot.com/blog/area_lights_in_shaders/

float rod_light_antideriv(vec2 uv, float i, float h) {
  return i * uv.x / (dot(uv,uv) + h*h);
}

float rod_light(vec2 uv, float i, float h_top, float h_bottom) {
  // h_top and h_bottom - the light's top and bottom above the ground
  // i - light's intensity
  return rod_light_antideriv(uv, i, h_top) - rod_light_antideriv(uv, i, h_bottom);
}


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 + 0.002;
    float v = rod_light(uv, 0.1, h, 0.001);
    v = lin_to_srgb(v);

    fragColor=vec4(v);
}