Abstract: We present an image-space acceleration technique that allows real-time direct volume rendering of large unstructured volumes. Our algorithm operates as a simple post-process and can be used to improve the performance of any existing volume renderer that is sensitive to image size. Using a joint bilateral upsampling filter, images can be rendered efficiently at a fraction of their original size, then upsampled at a high quality using properties that can be quickly computed from the volume. We show how our acceleration technique can be efficiently implemented with current GPUs and used as a post-process for a wide range of volume rendering algorithms and volumetric datasets.