Projector camera systems are basically systems that can serve as improvement to projector based applications by addition of visual feedback to the projector. However, unless the projector is precisely aligned to the presentation screen, the resulting image suffers from perspective (keystone) distortions requiring manual optical or digital correction. This process is manually hard during continuous movement of projector and screen of projection. Misalignment of projector and projected surface cause projective transform which change the shape of image. This paper propose a method to find largest area rectangle inside any quadrilateral. An uncalibrated, low-resolution digital camera is used to infer the projector-screen geometry and to automatically determine the parameters.