This paper proposes a new method for estimating quantization steps (QSs) from an image that has been previously JPEG-compressed and stored in a lossless format. In this method, DCT coefficients of each frequency band of JPEG-compressed image are aggregated in the QS and its multiples. The entire estimation process can be grouped into two categories: alternating and direct current bands. Considering that DCT coefficients under different QSs show different periodicity, QS estimation for each band is then further divided into three steps, which involve identifying whether the QS is one, two, or another value. For each step, the periodicity of DCT coefficients can be well exploited with the analyses of the DCT-coefficient histogram and its corresponding frequency magnitude spectrum. Experimental results demonstrate the efficacy of the proposed method and the superiority in QS estimation for previously JPEG-compressed images, especially in the case that the actual QSs are higher than two.

本文提出了一种新方法，用于从先前经过JPEG压缩并以无损格式存储的图像中估计量化步长（QS）。在这种方法中，将JPEG压缩图像的每个频带的DCT系数汇总到QS及其倍数中。整个估算过程可以分为两类：交流频带和直流频带。考虑到不同QS下的DCT系数显示出不同的周期性，然后将每个频段的QS估计进一步分为三个步骤，这涉及确定QS是一个，两个还是另一个值。对于每个步骤，通过对DCT系数直方图及其对应的频率幅值频谱进行分析，可以很好地利用DCT系数的周期性。