ISP2HRNet: Learning to Reconstruct High Resolution Image from Irregularly Sampled Pixels via Hierarchical Gradient Learning

While image signals are typically defined on a regular 2D grid, there are scenarios where they are only available at irregular positions. In such cases, reconstructing a complete image on regular grid is essential. This paper introduces ISP2HRNet, an end-to-end network designed to reconstruct high resolution image from irregularly sampled pixels that do not fall on a regular grid. To handle the challenges brought by irregular sampling, we propose an architecture to extract gradient structure hierarchically and learn continuous image representation. Specifically, we derive image gradient for each irregularly sampled pixel and further learn higher order gradient structural features according to the geometric and photometric information at the vertices of neighboring triangles. To convert the features from irregular pixels to regular grid, we propose a dual branch content-dependent weight generator to adaptively fuse the information from neighboring irregular pixels. Subsequently, an encoder captures deep structural details on regular grid and forms latent codes. Implicit neural representation parameterized by multi-layer perceptron decodes the latent codes and coordinates to pixel values for generating high resolution image. Experimental results demonstrate that the proposed network can effectively solve the problem of high resolution image reconstruction from irregularly sampled pixels and achieve promising results. The code will be made publicly available.