Abstract: In order to evaluate image scrambling quality good or bad actually，new image scrambling quality evaluation method based on DCT transform is proposed.Original and scrambled images are firstly divided into a number of blocks with same size，and their blocks with equal size are transformed into coefficients in frequency field by means of Fourier transform.Then difference matrixes are constructed by means of the frequency field coefficients of original and scrambled images and their discrete probability models are defined.In the end，the image scrambling quality evaluation function is constructed by information energy of the discrete probability distribution for difference coefficient matrixes.The evaluation method proposed in this paper is very well capable of measuring scrambling degree of scrambled image，and effective to describe the relation between the scrambling quality and the number of iterations in the scrambling techniques，which is largely consistent with human vision.For different images，when some scrambling transformation is used，this evaluation method can reflect the scrambling quality in each scrambling stage to some extent.

Key words: image scrambling, scrambling degree, DCT transform, information energy

摘要： 提出了基于DCT变换的图像置乱效果评价新方法，以便真实地评价图像置乱效果的好坏。首先将置乱前后两图像分成大小相同的子块并对其进行DCT变换，其次利用置乱前后相同位置子块图像所对应频域系数来构造偏差矩阵并定义其离散概率分布，最后通过计算离散概率分布的信息能量作为图像置乱程度的评价。提出的评价方法能够很好地度量图像的置乱程度，反映了加密次数与置乱程度之间的关系，与人的视觉基本相符。而且对于不同的图像，该评价方法能在一定程度上反映所用的置乱变换在各置乱阶段的效果。

关键词: 图像置乱, 置乱度, DCT变换, 信息能量