K(p)=det(S(p))
S(p)=-DpN
N=rsxrt/|rsxrt|=rsxrt/d
rs=(txs,txs,0)
rt=(x,x,1)
N=rsxrt/d=(txs,-txs,0)/txs=(1,-1,0)
-DN=-NsSxdx-NsSydy-Ntdt=0
det(0,0,0)=0=K