Slit camera weirdness

I was just daydreaming about slit cameras, thinking about their application to 'photo-finish' judging in horse races. The basic idea is that all the horses run past the slit, and the image you get makes it clear which one was first. You get a picture that, rather than having x,y axes has y,t axes.

So far so good.

But there is a weird thing about slit cameras -- because they 'lose' the x axis, they are effectively symmetric. If an object moves left to right or right to left, it shouldn't really matter (modulo obvious corner cases).

So what would a photo finish look like if one horse and jockey was running in the opposite direction to the rest of the horses?

(Easy enough to figure out, but a bit brainbending)