(2) Viewers, Browsers, and WWW Performance
by Huntley, Bittorf, and Taragin
Dermatology Online Journal, July 1995
Volume 1, Number 1
COMMENT
What determines the image format being used on WWW? The standard browser, NCSA
Mosaic only supports XBM, Compuserve-GIF, and JPEG formats. XBM is for two
color icon files, so not under consideration here. Compuserve-GIF generally
opens much faster than JPEG images, and this is the only format for inline
images supported by earlier versions of NCSA Mosaic. However, GIF image files are
much larger than JPEG compressions of the same images, and JPEG images may be
better for medical photography.
Fig. 1a: JPEG/JFIF versus GIF format for image size and opening times.
Shown for illustration for a single image is the difference in file size and
opening time according to the file format. The image was extracted from Kodak
CD-Rom archive into a raw PPM 768*512*24 file which was then converted to LZW/GIF
and JPEG/JFIF compressed files. Opening times were generated by the Unix time
application measuring time to image display with the xv application on a Hewlett
Packard 50 MHz workstation. Each data point represents the average of 3 or more
trials. The standard error bars were too small to be shown on the graphThe raw
image file was 1.18 megabytes, whereas the GIF format only used 250 kilobytes and
the JPEG only 43 kilobytes. Comparing processor time to open the file, the GIF
file took 1.2 seconds and the JPEG file 2.9 seconds.
Display times are generally faster for images saved in Compuserve GIF format
than for the same image saved in JPEG format, even though the corresponding
image files in JPEG are much smaller (about one fifth the size) of their GIF
counterpart. This was significant especially for the less powerful processors.
GIF seems to be a better format for images with few colors or for reproducing
sharp edges, as represented by the graphs in this report. JPEG format appears
to be better for a gradation of colors.
You may be able to see some differences in the formats by looking at enlarged
images.
Here are two GIF images and one JPEG/JFIF image which is linked:
BASE IMAGE (AS A GIF FILE)
This image is of herpetic vessicles on the palm. It is cropped to only show the
central part of the image. Note the top row of vessicles in this image. The
middle one is shown enlarged 32 times in the second image. The same vessicle is
shown in JPEG/JFIF format in image #3. Image 2 and 3 are the same
magnification, although they vary in in the size of cropping.
GIF ENLARGEMENT OF #1
JPEG/JFIF ENLARGEMENT
The morphology of the lesions cannot be discerned at this magnification. What
this magnification does demonstrate is the gradation in colors in the JPEG/JFIF
image as compared to the GIF image.
All contents copyright (C), 1995.
Dermatology Online Journal
University of California Davis