Detect A4 sheet of paper for example at runtime on Android.

Hello everyone! I am trying to automatically launch the android camera when I recognize a sheet of paper. I have tried some approaches such as color, templete, and geometry (rectangle), but none have been successful. My sheet will contain some features that may help, such as: Size: A4 Anchors: as in the image below.  I thought about using CascadeClassifier :: detectMultiScale but I have no idea where to start or if this is a good approach. Thanks for any help!