Contrast Enhancement of an Image by DWT-SVD and DCT-SVD

In this paper a novel contrast stretching technique is proposed that is based on two methods: (a) Discrete Wavelet Transform (DWT) followed by SVD and (b) Discrete Cosine Transform (DCT) followed by SVD where SVD refers to Singular Value Decomposition. In DWT-SVD technique, DWT is applied on an image resulting in the conversion of that entire image into four distinct frequency subbands (i.e. LL, LH, HL and HH subbands) followed by the application of SVD on LL sub-band of DWT processed image (because LL subband contains illumination coefficients). In this way, the values of illumination coefficients are normalized and LL subband is reformed using updated coefficients. Afterwards, image is reconstructed by using inverse DWT. In 2nd method, image is processed with DCT followed by SVD and after the reconstruction of the image in frequency domain, finally image is reconstructed by taking inverse DCT. This paper provides modified technique of DWT-SVD technique as DWT-SVD technique alone cannot produce appreciable results for some images having low contrast. Based upon the quality of contrast within an image, DWT-SVD or DCT-SVD can be used.