I've got a question for the 3d guns. Im making a ramp in a mode7 3d space... Its using a distortImage routine to draw the image. Im looking for a way to determine from the view whether to use the top image or the bottom image.

Best shown with a couple of pictures. The first shows the view that has the top view, the second picture though I want to show a different image. I need a formula based on the bike position and the coordinates of the ramp (all listed on the images) that I can use to determine whether its a top of bottom view. I had a look at some back face culling but my attempts at implementing were.... how does one say... fruitless ..... Just wondering if there is some wicked cool routine that I don't know about yet ...