[Playerstage-users] newbie: walls of 'cave.pnm' not appearing in blobfinder?

[Carlos V. <ca...@us...>]

As a new user to stage/player, I've been familiarizing myself with this
awesome simulator.

I added a camera

 ptz
 (
   pose [0.09 0.00 1]
   vision ( )
 )
                                                                      
to the simple.world file (just like the one in the table.world).
I ran playerv and subscribed to the blobfinder:0(vision) device. I was
expecting to see the walls of the cave.pnm but didn't. I thought
something was wrong, so then I defined a ball an placed it in the cave:

   define ball puck( size [0.15 0.15] )
   define blue_ball ball( color "blue" )
   blue_ball( pose [3.00 3.00 0] )

***AND*** to my pleasant surprise I saw the ball in the playerv. This is
cool. So What I want to do is to see walls (or something similar) in the
playerv. 

What am I doing wrong? 
Thanks,
Carlos