  We deal with a set of autonomous robots moving on an infinite grid. Those
robots are opaque, have limited visibility capabilities, and run using
synchronous Look-Compute-Move cycles. They all agree on a common chirality, but
have no global compass. Finally, they may use lights of different colors, but
except from that, robots have neither persistent memories, nor communication
mean. We consider the infinite grid exploration (IGE) problem. For this problem
we give two impossibility results and three algorithms, including one which is
optimal in terms of number of robots. In more detail, we first show that two
robots are not sufficient in our settings to solve the problem, even when
robots have a common coordinate system. We then show that if the robots'
coordinate systems are not self-consistent, three or four robots are not
sufficient to solve the problem. Finally, we present three algorithms that
solve the IGE problem in various settings. The first algorithm uses six robots
with constant colors and a visibility range of one. The second one uses the
minimum number of robots, i.e., five, as well as five modifiable colors, still
under visibility one. The last algorithm requires seven oblivious anonymous
robots, yet assuming visibility two. Notice that the two last algorithms also
satisfy achieve exclusiveness.
