A survey on lossy compression of DSC raw data

The study investigates the lossy compression of DSC raw data based upon the 12 bit baseline JPEG compression. The processing structure of a general compression model is introduced. Two configurations of model parameters are applied to an exemplary high-resolution raw image and compared concerning their performance to compression rate and compression noise. The results show that this method is capable to achieve compression rates of about 1:5 in practice. The PSNR reaches nearly 70 dB on average of the complete image area and has a similar magnitude as the dynamic range of the used image sensor. Moreover, the sensor noise is being reduced by the JPEG compression procedure without any visual losses of sharpness or details. Introduction In digital photography, raw data capturing and processing more and more becomes a common practice. Modern raw converters offer strong tools to optimize image quality after image exposure. A problem of handling and workflow using raw data is the amount of image data especially for large sensor sizes resulting in long transmission times between internal RAM and external storage media. This applies to the side of the camera during exposure as well as at the side of the PC for post-processing. Additionally, the memory requirements of raw data considerably reduce the maximum number of images that can be stored on memory cards compared to the common JPEG file format. Up to now, camera manufacturers use lossless or quasilossless compression methods by entropy coding and/or nonlinear quantization to reduce the file sizes of raw data. By using these techniques, only low compression rates of about 1:2 are being achieved. Higher compression rates are assumed to be attainable only by lossy compression procedures. This study investigates the potential and general characteristics of the lossy compression of raw data using the baseline JPEG algorithm. In still picture technology, the JPEG and JPEG2000 compression techniques have been widely established. The characteristics of both methods have been extensively researched and compared to each other. The results [1-3] show that JPEG has advantages in perceptual image quality for low compression rates of up to about 1:20. JPEG2000 gains at increasing compression rates. As low compression rates are of particular interest to raw data compression, our approach focuses on lossy compression using the JPEG method. Compression Model We propose the following structure of a compression model to be applied to DSC raw data: CFA Sensor Data Color Separation 1D Compression LUT