H00=-0.8769524;
H01=-1.3081547;
H11=-0.5959477;
H10=-0.616868258;

H00=-1.3081547;
H01=-0.19655551;
H11=0.342594951;
H10=-0.5959477;

pos={ 497.326416,156.01,-1.10000038,12.7279205 };
pos=pos-Floor[pos]
dir={1.0,0.0,-1.0,1.41421354};

pos+dir

a=H00;
b=H01-H00;
c=H10-H00;
d=H11+H00-H01-H10;
Plot3D[a+b*x+c*y+d*x*y,{x,0,1},{y,0,1},PlotRange->{-1.4,0.3}]

Px[t_]:=pos[[1]]+dir[[1]]*t;
Py[t_]:=pos[[2]]+dir[[2]]*t;
Pz[t_]:=pos[[3]]+dir[[3]]*t;
H[x_,y_]:=a+b*x+c*y+d*x*y;
H2[t_]:=H[Px[t],Py[t]]-Pz[t];
Plot[H2[t],{t,0,1}]