Apache: The Definitive Guide, 3rd Edition

Image Negotiation

Image negotiation is a special corner of general content negotiation because the Web has a variety of image files with different levels of support: for instance, some browsers can cope with PNG files and some can’t, and the latter have to be sent the simpler, more old-fashioned, and bulkier GIF files. The client’s browser sends a message to the server telling it which image files it accepts:

HTTP_ACCEPT=image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, */*

Browsers almost always lie about the content types they accept or prefer, so this may not be all that reliable. In theory, however, the server uses this information to guide its search for an appropriate file, and then it returns it. We can demonstrate the effect by editing our ... /htdocs/catalog_summer.html file to remove the .jpg extensions on the image files. The appropriate lines now look like this:

...
<img src="bench" alt="Picture of a Bench">
...
<img src="hen" alt="Picture of a hencoop like a pagoda">
...

When Apache has the Multiviews option turned on and is asked for an image called bench, it looks for the smaller of bench.jpg and bench.gif — assuming the client’s browser accepts both — and returns it.

Apache v2 introduces a new directive, which is related to the Filter mechanism (see later in this chapter, Section 6.6).