Talking about keypad matrix, with the 8051 port pins I can also identify
two or more keys pressed at the same time without diodes. Rows and
Columns are tied to the 8051 port pins in such way that they could be
used as inputs or outputs on fly, it means that driving a single column
line and reading the other column lines I could identify shorts,
multiple keypress, etc. The same for rows.

Just don't forget that the original posting was not "key contact" but
"obscuring a light beam", but I don't understand why you said that "with
a laser system you can't select a row at a time".  With a laser, I just
need two stepper motors (or galvos) with mirrors and reflective plates
installed at the each object cell position at the floor (or at the
board).  I can read all objects sites and report which object is missing
or existent. The laser would be installed anywhere in the ceiling or a
high wall.

Last month I saw at the Brazilian boot at the International Center at
Epcot (Disney), a pretty system that use lasers (infra-red or UV,
because you can't see it) to identify a reflective color tag each person
is holding in a room grid.  More than 50 persons should stand each one
right over a spot on the floor, holding this hand tag. The tag has two
reflective sides, red and green.  Holding the tag towards the big
projection screen (the sensors are located above the screen), it senses
not only where there is a person, but also which tag side (color) each
person selected. Using this technique, they play several group computer
games, where people use the tags to control the game, move characters,
navigate a boat, etc.  Each game use the "majority" of green or red to
move things on the screen. At the soccer game, the left side of the room
controls players of the left side of the field and so on. It is not only
funny, it is hilarious because half of the persons standing in the room
don't have the minimum idea why they are turning the tags red to green
and so on, they can't understand why and how just turning a piece of
plastic in their hand can control a computer game... it is easy to
recognize the "duh!" expression in people's face. :)

>
[snip]
> > With a laser system, though, the problem is that you can't select a row at a
> > time.