A compact representation of relightable images for the web

Relightable images have demonstrated to be a valuable tool for the study and the analysis of coins, bas-relief, paintings, and epigraphy in the Cultural Heritage (CH) field. Reflection Transformation Imaging (RTI) are the most diffuse type of relightable images. An RTI image consists in a per-pixel function which encodes the reflection behavior, estimated from a set of digital photographs acquired from a fixed view. Even if web visualization tools for RTI images are available, high fidelity of the relighted images still requires a high amount of data to be transmitted. To overcome this limit, we propose a web-friendly compact representation for RTI images which allows very high quality of the rendered images with a relatively small amount of data required (in the order of 6-9 standard JPEG color images). The proposed approach is based on a joint interpolation-compression scheme that combines a PCA-based data reduction with a Gaussian Radial Basis Function (RBF) interpolation. We will see that the proposed approach can be adapted also to other data interpolation schemes, and it is not limited to Gaussian RBF. The proposed approach has been compared with several techniques, demonstrating its superior performance in terms of quality/size ratio. Additionally, the rendering part is simple to implement and very efficient in terms of computational cost. This allows real-time rendering also on low-end devices.