Modified Laplacian enhancement of low-resolution digital images

This paper describes improved methods for enhancing low resolution images with the aim of extracting the desired image from a noisy background, while preserving its surface features. A novel modified Laplacian filter (Laplace-8) was used. This was based on the classical Laplacian filter (Laplace-4) and operates in a similar fashion by using the Laplace coefficient to determine the level of enhancement. Laplace-4 has a major shortfall in that it only enhances gradients, and so the gray levels of a smooth image surface are left unchanged. This results in severe image surface distortion. The improved filter (Laplace-8), aims to alleviate the distortion by enhancing the whole surface of the image as well as producing good contour enhancement. This is possible even if the image surface is totally smooth. Two controlling techniques were used to compare the two filters, namely, limited gray level (limited between 0 -255) and unlimited gray level. Results were based on the correlations of original and Laplace filtered images, and the statistics of their contours and surfaces. The images produced by Laplace-8 filtering were shown to be superior to that of Laplace-4, showing little image distortion in the case of unlimited gray level enhancement. Laplace-8 is also an effective contour extractor, producing higher contour gray levels for a given enhancement (Laplace) coefficient. The paper describes in detail the performance of the Laplace-8 method with aid of examples.