Computer Vision - Algorithm for reconstructing tilted image Recently I've seen this very good apps

https://play.google.com/store/apps/details?id=com.kingjim.shotnote&amp;hl=en
https://itunes.apple.com/JP/app/id411332997?mt=8

this apps reconstruct the image from the camera based on the marker (this is what I think :) ) it also have a special paper that have marker. What I've still haven't got is what is the algorithm to do this? what computer vision theory used in this one? I understand artificial intellegence and basic computer vision, is there anyone have the ide how can this be done? at first I thought this based on stereo image but based on what I've studied before stereo vision need two camera. I hope my question is clear enough but if is there any information needed please ask in the comment :)

Thank you!
It detects 4 markers lying on the corners of the note. One of the markers is unique to able to determine the orientation. Because the actual size and layout of the form is known, a perspective warp can be computed, bringing the scanned image to a canonical (fronto-parallel) form. Then, OCR is carried out in the region where date and time is expected. The rest of the content are taken as image and saved on the smartphone.

For OCR, character segmentation and classification might be your key words.
For Marker Detection, generally black-white polygonal regions are detected and expected to be quad-formed. The rest of the blobs are filtered out and you end up with the markers.