gistfile1.c

//_pos is in pixel coordinates in a quad
float2 offsetFunc(float2 _pos)
{
  //distance from the center (the quad is (radius + width) * 2 pixels wide)
  _pos -= radius + width; 
	_pos /= scale;

	float x = length(_pos);
  
  //function, made in GeoGebra: http://imgur.com/zMbqAcs
	float val = cos((x - currentRadius)/(width/PI)) * strength/2 + strength/2;
	if(abs(x - currentRadius) > width)
		val = 0;
	
	return normalize(_pos) * val;
}