[jdiwnab]

The ceiling isn't necessarly perpendicular to the robot. You're driving down a hallway. Theres a ramp. Robot goes up ramp, but becuase the camera is now pointed a little back, it goofs. And ceilings arn't all the same. There are many places with the rectangles for the ceiling as you say, but what if you want to go to Lowes with the bot. Their ceiling is much higher and is more werehouse style. Now you take your robot home with you ("He followed me home. Can I keep him mommy?" lol) and you have lower ceilings and no regular squares. Again, you have an attachment that moves around above your bot. It swings past the camera and confuses the bot. This could also be a freshmen waving his hands over the bot, or going under something.

While I think it is worth the effort to try it, I don't think it would be very practical. I think that if you are just tracking where you are, then a sensor on the floor, like optical mice, would work better. There are fewer variations that would mess it up. They are all right near the sensor (if you make it that way) and with a set distance, that size of objects can be determined and you can still calculate distances. If you have it close to the ground, you would need to either flip the bot or give it one of the few surfaces that you can't track. It still uses the same basic techniques, but would be more reliable. And might be more usable in an event if you so choose.