Scientific visualization evolved up to the stage where collaborative work in virtual coexistence of various experts becomes unavoidable to identify and solve specific visualization tasks. These problems encouraged us to research and define key aspects of visualization service which will enable collaborative work over the Internet. In this paper we define architecture of the whole system and unified interface for all requirements in such environment. Unified requests are sent to visualization processing unit to execute required visualization routine. Collaborative work on the volumetric medical data is especially demanding task, so we present development of VMI (Visualization of Medical data over Internet) system, where segmentation for visualization is based on joint histogram.