Quality Control for the BPG Lossy Compression of Three-Channel Remote Sensing Images

Abstract

This paper deals with providing the desired quality in the Better Portable Graphics (BPG)-based lossy compression of color and three-channel remote sensing (RS) images. Quality is described by the Mean Deviation Similarity Index (MDSI), which is proven to be one of the best metrics for characterizing compressed image quality due to its high conventional and rank-order correlation with the Mean Opinion Score (MOS) values. The MDSI properties are studied and three main areas of interest are determined. It is shown that quite different quality and compression ratios (CR) can be observed for the same values of the quality parameter Q that controls compression, depending on the compressed image complexity. To provide the desired quality, a modified two-step procedure is proposed and tested. It has a preliminary stage carried out offline (in advance). At this stage, an average rate-distortion curve (MDSI on Q) is obtained and it is available until the moment when a given image has to be compressed. Then, in the first step, an image is compressed using the starting Q determined from the average rate-distortion curve for the desired MDSI. After this, the image is decompressed and the produced MDSI is calculated. In the second step, if necessary, the parameter Q is corrected using the average rate-distortion curve, and the image is compressed with the corrected Q. Such a procedure allows a decrease in the MDSI variance by around one order after two steps compared to variance after the first step. This is important for the MDSI of approximately 0.2–0.25 corresponding to the distortion invisibility threshold. The BPG performance comparison to some other coders is performed and examples of its application to real-life RS images are presented.

Providing a Desired Compression Ratio for Better Portable Graphics Encoder of Color Images: Design and Analysis

“…Since then, many other, more efficient lossy compression techniques have been designed [10][11][12]. In particular, better portable graphics (BPG) encoder has been designed and tested [3,[12][13][14]. It has been shown as a possible alternative to JPEG due to many useful features, such as considerably better rate-distortion characteristics [13,14].…”

“…In particular, better portable graphics (BPG) encoder has been designed and tested [3,[12][13][14]. It has been shown as a possible alternative to JPEG due to many useful features, such as considerably better rate-distortion characteristics [13,14]. In addition, BPG supports most Web browsers, different chroma formats, and image data from 8 to 14 bits per channel [12].…”

“…Such a property of Q leads to the fact that peak signal-to-noise ratio (PSNR) can be provided with a limited accuracy characterized by error from -0.5 dB to 0.5 dB. Similarly, it is even potentially impossible to provide desired values of visual quality metrics without errors [14]. It is also impossible to provide a desired CR.…”

“…Note that the problem of reaching a desired CR is typical for many coders based on discrete cosine transform [6,8,14]. It stems from the fact that rate-distortion curves (e.g., dependences of PSNR on CR in general or on Q for the BPG encoder) are very individual and depend on image complexity [15].…”

“…Fortunately, simple and fast two-stage procedures have been proposed recently [6,14]. They have been designed to provide a desired visual quality of compressed images according to a chosen metric (e.g., MDSI [17] in [14]). Meanwhile, we propose to use the same approach to reach a desired CR.…”