import megamu.shapetween.*;

BlendShaper blend;
Shaper lens;
Shaper lens2;

void setup(){
  lens = new CircularShaper(Shaper.SEAT);
  lens2 = new QuadraticShaper(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 );
}

void mousePressed(){
  if( blend.getShaperTwo() == lens2 )
    blend.setShaperTwo( lens2 );
  else
    blend.setShaperTwo( new LinearShaper() );
}

shaper

The shaper to use as the second shaper in the blender