Abstract: The concept of Alpha channel is summarized in detail. Formulas for multilayer overlay composition of video objects are deduced and verified with Adobe After Effects software, the meaning of premultiplied color composition is presented. By proving that the filtering operation of video object in single layer is carried out on premultiplied color, its rotation and zooming operations should be carried out on premultiplied color too. It comes out, not the non-premultiplied color, but the premultiplied color should be stored in the frame buffer.