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Blob Grabbing
We are now able to somewhat reliably identify a blue ball. We can steer towards the blue
ball by centering on its Center of Gravity and move in such a way to ensure that the
COG of the ball stays in the center of the screen. We already have a tutorial about
tracking the center of gravity at Color Object Tracking so
we will not get into detail about that here.
We do, however, have a new problem that once we get really close to the ball we need to know
when to grab it. The issue we have is that when we are ready
to grab the ball the ball is now out of camera view!
Knowing when to grab the ball requires that the robot 'know' without seeing the ball. We
accomplish this by keeping track of the COG of the ball. When the ball becomes quite large
within the image we know we are getting closer and begin a different kind of tracking. This
tracking will notify us when the ball's COG suddenly disappears out of sight. At this point (
assuming the ball was not suddenly picked up by someone) the ball should have just slipped by
under the
camera view and is now just in range of the grippers. Based on this assumption we can now close
the grippers.
Below you can see this from the Robots perspective. The first image is when the ball is outside of the
robots grippers. You can see the gripper ends on the left and right side of the image. The second
image shows when the ball is within the grippers and almost out of view.
 
Interestingly enough, when the grippers close, the ball pops back into the camera view!
While not done in this tutorial this is a nice effect that can be used to ensure that
we actually grabbed the ball, i.e. if the grippers close and no COG of blue is found then
we probably missed the ball. Best to backup and try again.
Below we can see what the image looks like when it had grabbed the ball. Notice that the top of the
ball is once again in view.
Once we have grabbed the ball we need to find our destination cone ...
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