Quadratic filters for image contrast enhancement

The classical unsharp masking method of image contrast enhancement is based on the addition of an amplitude scaled linear highpass filtered version of the image to itself. However, the highpass filter also enhances noise components present in the image. Some improvement in the quality of the enhancement has been obtained using a fairly simple quadratic filter in place of the linear highpass filter. The authors first explain why the linear unsharp masking performs well for contrast enhancement of noise free images. A further modification to make the overall approach considerably less sensitive to noise is then proposed. A general class of quadratic filters utilizing Weber´s law is also introduced and their important properties are discussed. The modified unsharp masking scheme together with the proposed class of quadratic filters is shown to provide significant improvement in the quality of enhanced images compared to that obtained using similar approaches