ellipse_with_feather.glsl.frag

// simple glsl ellipse function based on circle form BookOfShaders (https://thebookofshaders.com/07/)

float ellipse(in vec2 st, in vec2 position, in float radius, in float feather, in vec2 aspectratio){
 
  // receive bigger of both numbers from aspect ratio
  float max_val = max(aspectratio.x, aspectratio.y);

  // clamping the aspect ratio ensures that the ellipse will deform but won't scale
  // scaling should only result via changing the radius
  vec2 clamped_ar = aspectratio / max_val;
  
  // invert to ensure that higher numbers result in more stretching
  vec2 ar = 1.0 / clamped_ar; 

  // calculate distance from center point, like in circle function
  vec2 dist = (st * ar) - (position * ar); 
  
  // create a smooth feather with total size of 'feather'
  float ellipse =  1.0 - smoothstep(radius - feather * 0.5, 
                                    radius + feather * 0.5,
                                    dot(dist,dist) * 4.0);
  return ellipse;
}