Operation Manual
74 Working with Graphics, Animation, and Multimedia
If you’re being considerate of users with dial-up modem connections
you can work out your own rule of thumb. Assuming these users are
willing to wait up to 20 seconds (because they’re probably accustomed
to waiting) you can multiply that by 4K bytes per second, the
throughput of a 1995-vintage modem. By that logic, if your pages
exceed about 80KB in size you will start to alienate some fraction of
your intended visitors. For comparison, a high-speed connection is
about ten times as fast.
You can determine the actual size of your files by publishing one
page at a time to a local folder (using File/Publish Site/Publish to
Disk Folder...), then using Windows Explorer to examine the
contents of the folder. View the files by date and Shift-select the
most recent batch, then right-click and choose Properties to see
the aggregate byte count. Obviously, the lower the better.
Is there anything you can do to reduce the total size of your graphics,
aside from using fewer graphics? An obvious suggestion is to make
them no larger than they need to be to get your point across. Since file
size increases as the square of each dimension, shrinking both height
and width by 50% reduces the file size by 75%. Try to use the correct
dpi setting for images before importing, rather than scaling large
images down on the page. You can use the Size and Resolution dialog
(accessible from the Picture toolbar) after importing to scale pictures
and/or resample them so excessive dpi doesn’t waste bytes. Avoid large
regions of transparency if possible.
If you save graphics as GIFs (see above), you can take advantage of the
fact that GIFs, unlike most other 256-color (8-bits-per-pixel “bit
depth”) formats, don’t insist on using 8-bit pixels. If the number of
colors in the image is 128, GIFs will encode using 7 bits; with 64
colors, 6 bits, and so on. As an experiment, we tried starting with a
small image (some anti-aliased text) that only used 14 colors. With the
paint program set for 256 colors, we saved as a .GIF, yielding a file size
of 1204 bytes. After reducing the number of colors displayed in the
paint program to 16—still sufficient to display all those in the image—
we saved again, and the file size went down to 420. If you’ve got a
dozen or more small GIFs per page, those little savings can really add
up!
Paint programs handle color reduction in various ways. Some let you
set the image to either 16 or 256 colors, but not to 64 or 128; that’s still
a help if your images require 16 or fewer. Ideally, you can save with an
arbitrary number of colors, and the program will attempt to optimize
the image using that value. So you can pick any intermediate bit depth,
and find the one that works best for the specific image. To sum up: