  In this work, we theoretically assume that a compact object (CO) can have a
dark surface so that the CO is simplified to have no emissions and reflections.
Considering that the radius of the surface can be located inside or outside the
photon region, which is closely related to the shadow curve, we investigate if
a CO without an event horizon could produce shadow structures similar to black
holes and figure out how different of shadows of COs with and without horizons.
In particular, by introducing the (possible) observational photon region, we
analytically construct an exact correspondence between the shadow curves with
the impact parameters of photons and find that there are indeed several
differences for shadows of COs without horizons and black holes. More
precisely, We found the shadow curve is still determined by the photon region
when the radius of the surface is small enough to retain a whole photon region
outside the shell. When only part of the photon region remains, the shadow
curve is partially determined by the photon region, and the remaining portion
of the shadow curve is partly controlled by the impact parameters of photons
which has a turning point on the surface. When there's no photon region outside
the surface, the shadow curve is totally controlled by the impact parameters of
photons which has a turning point on the surface.
