Hardware-accelerated image composition for sort-last parallel rendering has received increasing attention as an effective solution to increased performance demands brought about by the recent advances in commodity graphics accelerators. So far, several different hardware solutions for alpha and depth compositing have been proposed and a few of them have become commercially available. They share impressive compositing speed and high scalability. However, the cost makes it prohibitively expensive to build a large visualization system. In this paper, we used a hardware image compositor marketed by Mitsubishi Precision Co., Ltd. (MPC) which is now available as an independent device enabling the building of our own visualization cluster. This device is based on binary compositing tree architecture, and the scalable cascade interconnection makes it possible to build a large visualization system. However, we focused on a minimal configuration PC Cluster using only one compositing device while taking cost into consideration. In order to emulate this cascade interconnection of MPC compositors, we propose and evaluate the hybrid hardware-assisted image composition method which uses the OpenGL alpha blending capability of the graphics boards for assisting the hardware image composition process. Preliminary experiments show that the use of graphics boards diminished the performance degradation when using an emulation based on image feedback through available interconnection network. We found that this proposed method becomes an important alternative for providing high performance image composition at a reasonable cost.