How can I modify the bulge effect to only be a half circle?

Hello,

I would like to use the bulge effect but only half of it. For example I would like to bulge just the right side of the area getting the bulge effect but leave the left side normal without the bulge (as if a line was drawn down the center of the effect and the effect was only applied to one side). How could I modify the code to do this?

Thanks!