import megamu.shapetween.*;

BlendShaper blend;
Shaper lens;

void setup(){
  lens = new CircularShaper(Shaper.SEAT);
  blend = new BlendShaper(lens);
}

void draw(){
  background(255);
  
  beginShape();
  for( float i=0; i<=1; i+= 0.05 )
    vertex( i*width, blend.shape( i )*height );
  endShape();
}

void mouseMoved(){
  float transition = norm( mouseX, 0, width );
  float mix = norm( mouseY, height, 0 );
  lens.setTransitionPoint( transition );
  blend.setBlend( mix );
}

proportion

The amount to mix the second shaper with the first shaper