JPEG and wavelet compression of ophthalmic images

Abstract

This study was designed to determine the degree and methods of digital image compression to produce ophthalmic images of sufficient quality for transmission and diagnosis. The photographs of 15 subjects, which included eyes with normal, subtle and distinct pathologies, were digitised to produce 1.54MB images and compressed to five different levels using JPEG and wavelet image compression. The images were analysed by three different methods: (i) objectively by calculating the RMS error between the uncompressed and compressed image, (ii) semi-subjectively by assessing the visibility of blood vessels, and (iii) subjectively by asking a number of experienced observers to assess the images for quality and clinical interpretation. Results showed that as a function of compressed image size, wavelet compressed images produced less RMS error than JPEG compressed images. Blood vessel branching could be observed to a greater extent after Wavelet compression compared to JPEG compression to the same image size. Assessment of the compressed images by experienced observers showed that Wavelet compression produced better images than JPEG compression for a given image size. Overall, it was shown that images had to be compressed to below 2.5% for JPEG and 1.7% for Wavelet compression (image size about 37 kbytes and 25kbytes respectively) before fine detail was lost, or when image quality was too poor to make a reliable diagnosis.