The World Wide Web is incorporating recent advances in Internet technology and 3-D graphics, promoting the spread of visualization over networks. To increase the efficiency of web-based 3-D graphics systems including scientific visualizations, we present a method that utilizes previously rendered and transmitted images instead of transmitting 3-D models to the client. Our image-based rendering and compression (IBRAC) method exploits spatial and temporal coherence between new and previously rendered synthetic images. Reprojecting color and visibility data accelerates the computation of new images, while reprojecting the surface orientation eliminates inconsistencies in shading and lighting changes that plague other image-based rendering methods. We also present a new image data structure called an isomap, containing classification-interval data that enables a remote user to interactively classify volume data without loading the 3-D dataset. Our approach produces rendering speedups of seven and more without visible degradation, and compression ratios are a factor of 2 to 10 times better than MPEG2 in our test cases. The approach is best suited for remote rendering applications, where the client has limited rendering resources and network bandwidth, including wireless or palm computing applications.