The math is escaping me at the moment but to map a sprite to this would require a perspective transform. Google has a lot on the subject, but it may take some tinkering to get the transform to match.
You could modify the fx so that you get the image to repeat in only a line by putting // in front of one of these lines:
[code]xx=clamp(xx, 0.0, 1.0/scale_x);
yy=clamp(yy, 0.0, 1.0/scale_y);[/code]