Specifications

Image Formats

The GD library supports JPEG, PNG, and WBMP formats. It no longer supports the GIF for-

mat. Let’s briefly look at each of these formats.

JPEG

JPEG (pronounced “jay-peg”) actually stands for Joint Photographic Experts Group and is the

name of a standards body. The file format we mean when we refer to JPEGs is actually called

JFIF, which corresponds to one of the standards issued by JPEG.

In case you are not familiar with them, JPEGs are usually used to store photographic or other

images with many colors or gradations of color. This format uses lossy compression, that is, in

order to squeeze a photograph into a smaller file, some image quality is lost. Because JPEGs

should contain what are essentially analog images, with gradations of color, the human eye can

tolerate some loss of quality. This format is not suitable for line drawings, text, or solid blocks

of color.

You can read more about JPEG/JFIF at the official JPEG site:

http://www.jpeg.org/public/jpeghomepage.htm

PNG

PNG (pronounced “ping”) stands for Portable Network Graphics. This file format is seen as

being the replacement for GIF (Graphics Interchange Format) for reasons we’ll discuss in a

minute. The PNG Web site describes it as “a turbo-studly image format with lossless compres-

sion”. Because it is lossless, this image format is suitable for images that contain text, straight

lines, and simple blocks of color such as headings and Web site buttons—all the same pur-

poses for which you previously might have used GIFs.

It offers better compression than GIF as well as variable transparency, gamma correction, and

two-dimensional interlacing. It does not, however, support animations—for this you must use

the extension format MNG, which is still in development.

You can read more about PNG at the official PNG site:

http://www.freesoftware.com/pub/png/

WBMP

WBMP stands for Wireless Bitmap. It is a file format designed specifically for wireless

devices. Although gd supports this format, there are no PHP functions at present that take

advantage of this functionality.

Generating Images

HAPTER 19

ENERATING

IMAGES

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