Lossless Compression Efficiency of JPEG-LS, PNG, QOI and JPEG2000: A Comparative Study

By Dimitrios Bozikas, CAST

Abstract

Determining the best fitting lossless image format for a specific application is a process involving the examination of multiple variables in order to make an informed decision. From the compression ratio a codec is able to achieve for a specific image type, to algorithmic complexity, speed or memory requirements for a software application, to size and power requirements for a hardware implementation, all constitute deciding factors that will shape the form of the final product. In this paper we attempt to compare the achievable compression ratio of four lossless image formats across a data set of 2814 images exhibiting high variance in their key characteristics. The results are then categorized based on the image type to further illustrate the potential effectiveness of each image format for specific use cases.

Introduction

The continuous trend of increasing the image resolution in the majority of commercial and professional applications necessitates an ever-evolving set of solutions that facilitate the efficient encoding of images, reducing their size without compromising on quality. From state-of-the-art 4K and 8K video, to ultra-low-power applications, the ratio between the raw image data size and the encoded image size is considered one of the primary metrics to determine the effectiveness of each format, as it is the deciding factor for the storage, and bandwidth in the case of real-time processing, requirements of the application.

Compression ratio (CR) may vary between image formats, as well as the type of image data. Selecting the appropriate format for the target data type might become crucial to the performance of the entire application. With the introduction of novel image formats, such as QOI, the Quite OK Image format [1], along with the continuous improvement of existing formats and implementations, measuring the performance of each format for data sets displaying a significant variance in image types is a useful guide to determine the correct codec to pair with each application.

Methodology

The methodology used to assess the CR of each format aimed at establishing a point of reference common to all codecs, regardless of the initial encoding of the chosen image set. As such, all images were encoded to raw RGB format, which was subsequently used as the point of reference for all compression measurements. Since JPEG-LS and JPEG2000 do not support an alpha channel, a filter mapping the alpha channel to a white background was applied during the RGB encoding process.

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