From CrossWire Bible Society
The anatomy of an SWFilter
At the most basic level, a SWORD filter performs some distinct and concise transformation to the data stream.
The actual signature of a filter is very minimal, and looking at the SWFilter interface itself... http://crosswire.org/svn/sword/trunk/include/swfilter.h
... there is simply 1 method you need to implemented to create a filter:
char SWFilter::processText( SWBuf &text, const SWKey *key = 0, const SWModule *module = 0) = 0;
Basically, you're sent a buffer of text, possibly along with some context in the form of an SWKey and SWModule-- in case you care, and then you make some small change to the buffer.
Easy, right? Well, sortof.
Anyone can write a filter. The great thing about filters is that they partition up a very complicated task: processing a complex document, into tiny manageable chunks that anyone can tackle.
"Handle <title> tags in a buffer? Sure, I can do that! That's easy.... Wait.... What should I _DO_ with a <title> tag?"
That's when the real world hits and things get more complex. A SWORD filter is an easy to grasp, easy to code against, abstract concept.
How filters are orchestrated together inside the SWORD engine-- what distinct roles they are expected to fill, when, and in what order-- is hidden knowledge that for years only few have been privy to yield...
Stay tuned for "The Red Pill" coming soon to a listserv near you.