Abstract
Most modern color digital cameras are equipped with a single image sensor with a color filter array (CFA). One of the most important stages of preprocessing is noise reduction. Most research related to this topic ignores the problem associated with the actual color image acquisition process and assumes that we are processing the image in the sRGB space. In the presented paper, the real process of developing raw images obtained from the CFA sensor was analyzed. As part of the work, a diverse database of test images in the form of a digital negative and its reference version was prepared. The main problem posed in the work was the location of the denoising and demosaicing algorithms in the entire raw image processing pipeline. For this purpose, all stages of processing the digital negative are reproduced. The process of noise generation in the image sensors was also simulated, parameterizing it with ISO sensitivity for a specific CMOS sensor. In this work, we tested commonly used algorithms based on the idea of non-local means, such as NLM or BM3D, in combination with various techniques of interpolation of CFA sensor data. Our experiments have shown that the use of noise reduction methods directly on the raw sensor data, improves the final result only in the case of highly disturbed images, which corresponds to the process of image acquisition in difficult lighting conditions.
Highlights
The vast majority of today’s digital cameras use a single sensor with a color filter array (CFA).It is necessary to interpolate the missing pixels using so-called demosaicing algorithms to obtain a complete RGB image
There are many modifications to these algorithms that significantly improve their speed. Both non-local means (NLM) and BM3D have been adapted to work with CFA data, we decided to test the solution presented in the work [22]—pseudo four-channel filtering technique (P4Ch)
The presented paper analyzes the influence of location of filtering algorithms in the whole pipeline digital negative development on the final image quality
Summary
The vast majority of today’s digital cameras use a single sensor with a color filter array (CFA). The standard filtering quality assessment procedure assumes the use of a set of sRGB test images with 8-bit color depth for each channel These images are distorted by simulated noise, the distribution of which usually does not fully correspond to the processes occurring in the real imaging sensors. Many commonly used test images have been obtained using CFA sensors and may contain interpolation artifacts, and often contain significant noise These images usually have a limited bit depth compared to real raw sensor data. It seems, that such data should not be used either to test the entire process of developing digital negatives, or to test its individual stages.
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