define function using new_x,new_y which is an expr of x,y

[HuaMuLanChina]

x, y = hl.Var("x"), hl.Var("y")

new_x = expr_of_xy_0...
new_y = expr_of_xy_1...

dst_img = hl.Func("dst_img")
dst_img[x,y] = hl.select(cond,expr_of_new_x_and_new_y,bg_color)

-------------------is this possible?--------------------------------------------------------_
bool_mapping_flag= hl.Func("bool_mapping_flag")
bool_mapping_flag[newx,newy] = hl.select(cond,True,False)

expr_of_xy0,expr_of_xy1 are some of kind of transformation(rotation or scale) on xy grid.
x,y and new_x,new_y is images with same width height.
use newx,newy for bool_mapping_flag got error like:
Error: Argument 1 in initial definition of "bool_mapping_flag" is not a Var.