Chapter 12. IMAGE FUNCTIONS
Analyzing Images.............................................................................................
Creating JPEG, PNG, and WBMP Images................................................
The majority of the image functions draw their functionality from the GD library, free
software for manipulating images. These require the GD extension to be loaded, via
php3.ini or the dl function. The four functions in the first section of this chapter are not
part of the GD extension and should always be available to you.
The GD library was created at Boutell.com, a company that has contributed several Open
Source tools to the Web community. The library historically supported GIF image
creation, but in 1999 this functionality was pulled in favor of PNG format files. The
compression algorithm used in GIF creation is patented, which means permission must be
granted to software authors who use it. PNG, on the other hand, is an open specification.
It also happens to be technically superior to GIF. Support for PNG was added to the
fourth generations of the two most popular browsers, Netscape Navigator and Microsoft
Internet Explorer, so using PNG is feasible. In early 2000, support for JPEG and WBMP
images was added to GD.
The GD library's home on the Web is http://www.boutell.com/gd/. The URL for
PNG's home page is http://www.cdrom.com/pub/png/.
Two of the functions in this section require a special library to deal with TrueType fonts:
imagettfbbox and imagettftext. Likewise, the functions that work with PostScript
fonts require their own library. Consequently, these functions may not be available to
you, depending on how PHP was compiled.
Chapter 19, "Generating Graphics," makes use of the functions in this chapter to explore
some practical uses.
Analyzing Images
These functions are part of PHP's core and do not require loading an extension.
string gamma_correct_tag(string color, double original, double
new)
The gamma_correct_tag function adjusts an HTML color from one gamma to another.
Video display hardware is given a gamma rating that describes relatively how bright
images appear. Identical images appear lighter on Macintosh hardware than on the typical
Wintel clone. The W3C has a nice discussion about color spaces that includes
information about gamma values: http://www.w3.org/Graphics/Color/sRGB.html.