Distance and position detection seems odd

[martialgallorini]

In UI, detected positions seems erratic :

• when object approaches right border of ROI, distance rises up !
• nothing detected otherwise

Notes :

• maybe a centroid position error due to a scale problem.
• when ROI = size of kinect image there are no problem. Once ROI is resized, problem appears.

[martialgallorini]

blobs are shifted by the ROI offset, so if the ROI starts at x = 100, then a blob that normally is at position x = 150 is now at position x = 50, so we need to add roi.x to blob x position to have it in the right place again.