Is it a waste to learn OpenGL? My view on Direct X is your view on OpenGL. I can't see why Direct X would be the future in any way. OpenGL ES/WebGL is the future!
AR app: Mapping 3D object to 2D plane? Won't the vectors LL,UL and LL,LR need to be perpendicular to each other( 90 degree angle ) in order for the cross product to give the normal to the plane? In 3D space they are, but in 2D space they are not. I could be wrong though.