Graphics File Formats for Your Web Page
By Stephen Bucaro
Displaying graphics on your web page can make it more interesting. The images that
you put on your web page must be transferred from a Web server, across the Internet,
to your Web site visitors computer. To keep the transfer delays short, you must keep
the image files small. The Graphics Interchange Format (GIF) developed by CompuServe
uses a compression technique to reduce the size of graphics files.
Image in GIF format
A GIF image can contain up to 256 different colors. The GIF image format works well
because most simple Web graphics contain less than 256 different colors. When you
convert an image to the GIF format, if the image contains more than 256 different colors,
some color information will be lost.
Complex images such as photographs can contain millions of different colors. Converting
a photograph to GIF format may degrade the image's appearance. The Joint Photographic
Experts Group (JPEG) format uses a compression method that is more suitable for photographic images.
When you convert an image to the JPEG format, you must select a compression level,
sometimes referred to as the quality. A lower compression level results in a higher
quality image, but the file size is not reduced as much. A high compression level
results in a smaller file size, but the image quality is poorer.
The JPEG format uses a lossy compression technique. This means that the image
file size is reduced by throwing away some of the details in the image. As you
convert your image with higher compression levels, more image details are lost,
resulting in a blurry image.
Compession Level: 30
File Size: 13KB
Compession Level: 80
File Size: 4B
The secret to converting an image to the JPEG format is to set the compression level
to the highest setting where the loss of detail in the image is not noticeable, or at
least is acceptable for your purpose.
Which image format to use, GIF or JPEG, depends upon the character of the image you
want to convert. If the image contains blocks of solid color, use the GIF format. Using
the complex JPEG compression method on an image that contains large blocks of solid
color would actually result in a larger file than would the GIF format. If the image
contains lots of detail, like a photograph, use the JPEG format.
In 1995 Unisys surprised Compuserve and the Internet community by announcing that
they held a patent on the Lempel-Ziv-Welch (LZW) compression technique used in the GIF
file format. Anyone wishing to use the LZW technique would have to pay Unisys a $0.20
per unit royalty.
In response to this, the Portable Network Graphics (PNG) format was developed. The
PNG format was placed in the public domain so that no-one is required to pay royalties
to use it. Unfortunately the designers of the PNG format did not create a simple
replacement for the GIF format. Instead they threw in everything but the kitchen sink.
For this reason the PNG format has not received wide acceptance.
Note: As a Web graphics user or designer you do not have to pay royalties to Unisys
in order to use the GIF format. The royalties apply mainly to software developers who
use the LZW compression algorithm in the code of their graphics applications.