Diierential Coding of Alpha Planes with Adaptive Quantization

Alpha planes are used to compose objects in order to build synthetic images. They contain the shape and the transparency of the diierent objects. The scene is composed by superimposing objects in a given order. The alpha planes specify the transparencies of the objects. In order to independently compress these objects, the alpha planes must also be coded. In this paper, we present a way of coding alpha planes using 2D diierential coding. This technique quantizes alpha planes without introducing too many artifacts in the composed image. The quantization must be done carefully. The proposed method has been compared to the scheme used in the MPEG-4 Ver-iication Model (VM) for several sequences using diierent types of alpha planes. Depending on the sequence, the PSNR can improve up to 3 dB compared to the VM. The scheme is also much simpler than the VM.