Lake Verona -- via Luminance Masks P1210805

Hi Everybody,
This is the lake in Verona Park.
I confess that it didn't really look this. The sky is imported since
my camera burns out skies.
Now I am playing with Luminance masks. You can get similar effects
with "Blend If" or whatever its real name is. (Double click on an object
and there it is.) The advantage of a Luminance mask is that you can use
all the tools on it -- especially contrast controls to determine where the
top object and bottom object show up.
I just can't understand how to get a luminance mask of just an
object -- get a full screen preview -- and then position it as if it were an
object.
Phil