Two sets of images were used in this study. In the first set, 15
eyes were selected from a set of over 150 subjects photographed in a
major screening of aborigines in the northwest of Western Australia.
To test the limits of image compression, eyes with subtle abnormalities
were selected; in most cases these abnormalities were of low clinical
significance, requiring only to be noted for future observation. The
abnormalities included small nerve fiber layer and macular hemorrhages,
macular and peripheral drusen, and cotton wool spots. Four of the 15
eyes were normal controls, taken from the same set of eyes. Images with
extreme abnormalities were not used because some preliminary
investigations indicated that these images could be compressed to well
over 1:300 using JPEG, and abnormalities in the retina could still be
detected.
The 35-mm slides were digitized with a Polaroid SprintScan35
(Cambridge, MA) at a resolution of 675 dots/inch with 24-bit color,
resulting in a file size of 1.5 MB (752 × 680 pixels). This
produced a high-quality digital image, without making the image size
too large to be unmanageable. The images were stored as TIFF (Tagged
Interchange File Format) files. The images were all compressed to five
different levels using JPEG and Wavelet algorithms. The goal was to
produce some compressed images in which the image quality was too low
to be useful for assessment.
JPEG compression was performed by an algorithm developed from standards
set by the Joint Photographic Experts Group (http://www.jpeg.org/). The
JPEG algorithm is also available in many imaging software programs,
although there appears to be a variation among some of these in the
definition of the compression level. The compression level is
determined by selecting a quality (Q) value, and the exact compressed
file size cannot be predicted. Q values of 20, 40, 60, 80, and 100 were
used, which resulted in images that were approximately 350 KB, 49 KB,
29 KB, 21 KB, and 14 KB, respectively, in size.
The Wavelet compression was applied by using a baseline wavelet
transform coder with an Antonini filter
15 in custom
written software. The resultant file size after Wavelet compression
could be predicted; compression ratios were selected to produce files
equivalent to the JPEG compressed images, but still using a “round”
compression ratio. The compression ratios were 1:5, 1:30, 1:50, 1:70,
and 1:100, resulting in image sizes of approximately 300 KB, 52 KB, 31
KB, 22 KB, and 15 KB, respectively.
After compression, each image was decompressed and saved as a 752 by
680 pixel TIFF image to ensure that each image was loaded for viewing
at the same rate.
Four methods were used to assess the quality of the 165 images: