In this paper we propose a new image randomness measure using Shannon entropy over local image blocks. The proposed local Shannon entropy measure overcomes several weaknesses
of the conventional global Shannon entropy measure, including unfair randomness
comparisons between images of different sizes, failure to discern image randomness before
and after image shuffling, and possible inaccurate scores for synthesized images. Statistical
tests pertinent to this new measure are also derived. This new measure is therefore both
quantitative and qualitative. The parameters in the local Shannon entropy measure are further
optimized for a better capture of local image randomness. The estimated statistics and
observed distribution from 50,000 experiments match the theoretical ones. Finally, two
examples are given, applying the proposed measure to image randomness among shuffled
images and encrypted images. Both examples show that the proposed method is more
effective and more accurate than the global Shannon entropy measure
