code: protected float getYSource(int y, float destTop, float srcTop,
  			float heightRatio) {
  
  		float dest = getYEncodedLocation(y, encoding);
  		float source = getSource(dest, destTop, srcTop, heightRatio);
  
  		return source;
  	}
docs: Determine the y source pixel location
  
  @param y
  y pixel
  @param destTop
  destination top most pixel
  @param srcTop
  source top most pixel
  @param heightRatio
  source over destination height ratio
  @return y source pixel