Progressive content access to databases of JPEG-compressed images

Progressive content access provides a mode that allows a coarse version of an image being viewed at a lower computing cost and then gradually refined by subsequent resolution enhancement if required. This proves extremely useful when millions of compressed images and video sequences need to be browsed manually or processed in pixel domain, saving the cost and removing the necessity of full decompression. Such a progressive content access algorithm suitable for all discrete-cosine-transform-based (DCT-based) JPEG and MPEG-compressed files is proposed here. First a theoretical model in approximation of cosine function used in inverse discrete cosine transform (IDCT) with various orders is developed. Following that a progressive content access algorithm, which comprehends both the successive approximation and the spectral selection, is proposed. Further analysis and experiments are reported to show that our proposed algorithm saves computational cost in comparison with JPEG full decompression. Extensive experiments also support that the proposed algorithm achieves encouraging peak signal-to-noise ratio (PSNR) values for reconstructed images even with lower order approximation.