You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering 
Chapter Title:  Future Challenges 
1. Abstract of Cited Paper (BIB001):  We present a direct interblock interpolation technique that enables direct volume rendering of blocked, multiresolution volumes. The proposed method smoothly interpolates between blocks of arbitrary block-wise level-of-detail (LOD) without sample replication or padding. This permits extreme changes in resolution across block boundaries and removes the interblock dependency for the LOD creation process. In addition the full data reduction from the LOD selection can be maintained throughout the rendering pipeline. Our rendering pipeline employs a flat block subdivision followed by a transfer function based adaptive LOD scheme. We demonstrate the effectiveness of our method by rendering volumes of the order of gigabytes using consumer graphics cards on desktop PC systems.

Literature Review:
Though the techniques presented in this article allow realistic volume rendered images to be generated at interactive frame rates, still several challenges exist to be addressed in future work. When reviewing existing techniques it can be noticed that, although the underlying optical models already contain simplifications, there is sometimes a gap between the model and the actual implementation. In the future it is necessary to narrow this gap by providing physically more accurate illumination models. Apart from that, large data sets still pose a driving challenge. Here, the illumination computation time increases and it is not possible to store large intermediate data sets due to memory limitations. Although memory capacity on GPUs has increased during the last couple of years, the speed at which data can be transferred for calculations has not increased at the same rate. Multiresolution data structures BIB001 BIB002 and explicit caches [JGY * 12] could alleviate these problems but more work in relation to volumetric illumination is needed.