The unsharp masking filter (UMF) has been widely used in image processing front ends for contrast enhancement. The filter, being easy to implement, is based on the concept of augmenting a scaled and high-passed version of the image to itself. The UMF performance is critically dependent on the generation of the high-passed signal to be added as well as its associated scale factor. However, the optimal choice of filter parameters still remains a challenging task due to possible intensity clipping problems where the filtered pixel magnitude is vulnerable to be out of the permitted display ranges. In this research, an adaptive scheme is formulated such that the scale is derived from the pixel intensity of the input image. Specifically, pixels in the mid-range intensity will be assigned a larger scaling factor according to a Gaussian-like profile. In addition, the optimal profile coefficients and the width of the high-pass generator window are determined by adopting the particle swarm optimization algorithm. Satisfactory simulation results obtained from a collection of a large set of images have shown the effectiveness of the proposed image contrast enhancement approach.