A survey on lossy compression of DSC raw data

Abstract

The study investigates the lossy compression of DSC raw data based upon the 12 bit baseline JPEG compression. Computational simulations disclose that JPEG artefacts originate from the quantization of the DCT coefficients. Input noise is shown to serve as an appropriate means to avoid these artefacts. Stimulated by such a noise, the JPEG encoder simply acts as an high frequency noise generator. The processing structure of a general compression model is introduced. The four color planes of an image sensor are separately compressed by a 12 bit baseline JPEG encoder. One-dimensional look-up-tables allow for an optimized adaptation of the JPEG encoder to the noise characteristics of the input signals. An idealized camera model is presumed to be dominated by photon noise. Its noise characteristics can optimally be matched to the JPEG encoder by a common gamma function. The gamma adapted compression model is applied to an exemplary set of six raw images. Its performance concerning the compression ratio and compression noise is examined. Optimally adjusted to the input noise, the compression procedure offers excellent image quality without any perceived loss referring to sharpness or noise. The results show that this method is capable to achieve compression ratios of about factor 4 in practice. The PSNR reaches about 60 dB over the complete signal range.