My confusion regarding rectifying Map Frames

I was wondering how to relate frames captured from Tablets and Video cameras for rectification, Michael said rectification can be done linearly with the finger tips' coordinates. But actually we wont relate or compare frames from Tablets with those of Video cameras rather we would rectify those separately. It my misinterpretation from the pipeline-sketch. Now I got it !!! We are gonna do the capturing separately. The rectification now makes sense.