Abstract

Blockwise transform image enhancement techniques are discussed. Previously, transform image enhancement has usually been based on the discrete Fourier transform (DFT) applied to the whole image. Two major drawbacks with the DFT are high complexity of implementation involving complex multiplications and additions, with intermediate results being complex numbers, and the creation of severe block effects if image enhancement is done blockwise. In addition, the quality of enhancement is not very satisfactory. In this paper, it is shown that the best transforms for transform image coding, namely, the scrambled real discrete Fourier transform, the discrete cosine transform, and the discrete cosine-III transform, are also the best for image enhancement. Three enhancement techniques discussed in detail are alpha-rooting, modified unsharp masking, and filtering based on the human visual system response (HVS). With proper modifications, it is observed that unsharp making and HVS filtering are basically equivalent. Block effects are completely removed by using an overlap-save technique in addition to the best transform. In conclusion, transform image enhancement yields highly satisfactory performance, it is biologically sound, provides parallel models for implementation, and can be performed simultaneously with transform image coding.

© 1991 Optical Society of America