Identify texture slice within pixel shader

hi all,

I had to split my output image (because very large on X). Now I have a texture spread, containing the slices of my final output.

the pixel shader does the final fx pass, and now I have to recalculate back to the original uvs. (to show images that span across my texture slices).

how can I identify on which texture of my spread the PS is executed? so that I can offset and scale my uvs correctly?

maybe one of those annotations?