A method and system for large image matting, where alpha mask αR for a large image is built with the help of alpha mask αR found for the reduced image. In the current context, a large image is an image for which data structures required by the closed form method do not fit available memory. To overcome memory limitation the large image is divided into a plurality of smaller size sub-images. To find an alpha mask for every sub-image a sparse linear system is solved where the pixel values sampled from reduced image alpha mask αR are used as constraints on a sparse linear system.