Norm here is some information for GIF, JPEG, and PNG the primary Web image
formats.
For archive versions of images, PNG may be the best format for Web use. Some
software with early support for PNG file creation such as Photoshop 5.0, had
certain problems with creating correctly encoded images.

---Terence de Giere
Human Factors International

GIF IMAGES
Best for diagrams and line art.
Supports a maximum of 256 colors. Not recommended for full color images and
photographs.

Web-safe colors and dither
If you need a color in a GIF image that is not one of the browser-safe
colors, you can create your own dither pattern using only web-safe colors.
Such pre-dithered patterns will reproduce predictably, since each pixel is
still a browser-safe color. If you do not use a Web-safe color, it may
dither unexpectedly.

Size
You can reduce the size of GIF image files by reducing the number of colors
in the palette, by reducing the resolution of the image, or by reducing the
size of the image. Experiment to determine the best effect for the smallest
file size. Size operations may be performed easily in Adobe Photoshop 3.0 or
higher, and similar image-processing programs.

JPEG IMAGES
Best for photographs and other full color images
Supports full color photographic images.

Web-safe colors and dither
On machines with 256 or fewer colors, browsers generally do a good job of
simulating colors in JPEG files by dithering using the available colors. You
don't have to worry much about browser-safe colors. Photographs and other
true-color images should normally be compressed with JPEG.

Size
Do not reduce the amount of color in an image before compressing it with
JPEG. Doing so will usually increase file size, while worsening the image
quality. There is a trade-off between compression and image quality which
varies with the content of the image. For Internet use, try to get the
smallest image with acceptable quality for fastest download times. The JPEG
standard specifies how an image is to be decompressed-how it is compressed
is not specifically defined. Some utilities make use of this principle to
allow manual compression of some areas of an image (such as an unimportant
background) with a different compression scheme than another part of the
image, such as a person's face. This allows even greater compression of the
full file than could otherwise be achieved.
Progressive JPEG provides slightly better compression than standard JPEG,
but tends to have a rather unpleasant appearance when displaying the
progressive buildup during download. Also note that the JPEG compression for
color images should be the last processing step, since it results in a loss
of quality and information. Any preprocessing of the image should be done
with a lossless file representation, such as TIFF or the Adobe PhotoShop
native file format.

PNG IMAGES
Best for high quality full-color images.

Avoid PNG on the Internet as a primary image format until support improves.
Use GIF or JPEG first. Even though PNG is an excellent replacement for GIF
it has more limited browser support.

Designed to replace non-animated 256 color GIF files, PNG also can encode
photographs in high-quality true color, with gamma (contrast) correction for
correct display on different platforms. PNG includes an alpha channel for
feathered transparency effects.
If an important image-for example, a detailed color photograph of a surgical
procedure-must be viewed with maximum fidelity in true color, use PNG but
display the photograph on the main page using JPEG, and provide a link to a
separate page with the higher quality PNG version.

Web-safe colors and dither
If you need a color in a 256-color PNG image that is not one of the
browser-safe colors, you can create your own dither pattern using only
Web-safe colors. Such pre-dithered patterns will reproduce predictably,
since each pixel is still a browser-safe color. If you do not use a Web-safe
color, it may dither unexpectedly.
For true-color PNG images, the browser will usually produce an acceptable
dither on machines that do not support true color.

Size
256-color PNG files will normally be smaller than a corresponding GIF
file.While 256 and fewer-color PNG files may be smaller than GIF files, true
color PNG files may be quite large. PNG uses a lossless compression
algorithm. For links with large sized images, provide an alternate JPEG
version as well as the PNG version and warn the user of the file size by
placing the file size next to the link.

Example:
Surgical procedure Full-size JPEG image (75kB)
Surgical procedure Full-size, true-color PNG image (1.4MB)
In this example, the true-color PNG file takes 19 times longer to download
than a highly compressed JPEG file, but the PNG image will not contain any
compression artifacts to distort shapes or color.

----- Original Message -----
From: Norm Wolcott <nwolcott(at)kreative.net>
To: Gutenberg list <hwg-gutenberg(at)hwg.org>
Sent: 2000/March/16 / 22:40
Subject: png?


Can anyone describe a .png file, portable network graphics, what is it and
where are the specs and how do you make one and why.

Internet Mail:  nwolcott(at)post.harvard.edu -- this is a mail forwarding
address for messages lessthan 2 MB. For longer messages or attachments use
nwolcott(at)kreative.net -- the current ISP.

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