This paper presents a new method for unsharp masking for contrast enhancement of images. The approach employs an adaptive filter that controls the contribution of the sharpening path in such a way that contrast enhancement occurs in high detail areas and little or no image sharpening occurs in smooth areas.

https://www.eecis.udel.edu/~barner/courses/eleg675/papers/Enhancement%20in%20the%20Spatial%20Domain/Image%20Enhancement%20via%20Adaptive%20Unsharp%20Masking.pdf

Image enhancement via adaptive unsharp masking

Assumptions about image continuity lead to oversmoothed edges in common image interpolation algorithms. A wavelet-based interpolation method that imposes no continuity constraints is introduced. The algorithm estimates the regularity of edges by measuring the decay of wavelet transform coefficients across scales and preserves the underlying regularity by extrapolating a new subband to be used in image resynthesis. The algorithm produces visibly sharper edges than traditional techniques and exhibits an average peak signal-to-noise ratio (PSNR) improvement of 2.5 dB over bilinear and bicubic techniques.

Regularity-preserving image interpolation

Teager's energy operators are defined in both the continuous and discrete domains and are very useful tools for analyzing single component signals from an energy point of view. A number of important properties of these operators are shown that make it possible to determine the energy functions of quite complicated functions, provided these functions can be expressed as products of simpler functions, this operation of function multiplication being typical of a modulation process. Some of the eigenfunction properties of the energy operator that illustrate the special role of the trigonometric, Gaussian, and single soliton functions are also given. >

Some useful properties of Teager's energy operators

A technique for enhancing the perceptual sharpness of an image is described. The enhancement algorithm augments the frequency content of the image using shape-invariant properties of edges across scale by using a nonlinearity that generates phase coherent higher harmonics. The procedure utilizes the Laplacian transform and the Laplacian pyramid image representation. Results are presented depicting the power-spectra augmentation and the visual enhancement of several images. Simplicity of computations and ease of implementation allow for real-time applications such as high-definition television (HDTV).

/pdf/image-enhancement-by-nonlinear-extrapolation-in-frequency-4cek4btt5l.pdf

Image enhancement by nonlinear extrapolation in frequency space

Some useful properties of Teager's energy operator

Two types of very simple two-dimensional nonlinear filters are introduced and applied to image contrast enhancement. The first type is based on a generalization of the Teager's algorithm. A theoretical analysis has shown that this type of nonlinear filter works like a local-mean-weighted highpass filter. Based on this analysis, a second type of nonlinear filter has been developed which works like local-mean-weighted bandpass filter. The proposed image contrast enhancement technique is based on combining the original image with its filtered version obtained using one of the two nonlinear filters. Very high quality enhancement has been achieved for natural images. >

I.-S. Lin

Papers

A new class of nonlinear filters for image enhancement