some problem about APN

[deepblue0822]

I have some question about the APNcrop , tx,ty is the Center point of APN，and tl is half of the APN
,and tl is the Upper left of the picture,br is lower right of the picture,The X axis of the upper left corner and the lower right corner of the region obtained by crop is （tx(tl)、tx(br)） y axis （ty(tl)、ty(br)）:
tx(tl) =tx - tl ty(tl) = ty -tl
tx(br) =tx + tl ty(br) = ty +tl
but the real should be:
tx(tl) =tx - tl ty(tl) = ty +tl
tx(br) =tx + tl ty(br) = ty -tl
I don't know where is wrong,could you tell me where my thought is wrong?

[deepblue0822]

I know it.What I thought is wrong.