Question about filters and codecs

[DeezNutz]

Can someone explain to me what exactly a filter is? I hear a lot about different directshow filters, etc., but I never figured out what they are.

Also, what is the difference between a directshow decoding filter like ffdshow and the DivX decoding codec? How come ffdshow, as a filter, can do the same thing that the codec can?

Finally, the ffdshow filter says it can use xvid.dll instead of libavcodec to decode DivX content if it is availible. Is it a good idea to use xvid.dll or libavcodec?

Thanks in advance.

[Blueseb]

Can someone explain to me what exactly a filter is?
Avi and ogm files are streams, that means different information (video, audio, subtitles, etc.) are 'merged' together in a way they can 'flow' smooth during playback. So you need something to split these information, and treat each of them rightway: here comes the filters. They are distinct piece of software each doing a specific work.
You may need to do many handlings on each information, before the informations comes to the hardware, so the output of a filter is used as input in another filter. To look at this kind of chain, you can open an avi file into graphedit, it would be very interesting for you (i suppose ;-)

How come ffdshow, as a filter, can do the same thing that the codec can?
'cause, in the end, they are the same thing (I'm talking about the decoder portion of the codec, obviously): both get a single coded video stream and both give to the video render the reconstructed data to be displayed. What is different is the how the reconstruction is implemented (and who wrote the filter!:D).

Is it a good idea to use xvid.dll or libavcodec?
Have a look at this (http://forum.doom9.org/showthread.php?s=&postid=147716#post147716)

[DeezNutz]

Thanks Blueseb.

About using xvid.dll for decoding... the author of ffdshow told me in an email that he actually found that libavcodec seems a bit more efficient in general; he said to use xvid.dll if I have problems playing certain xvid content.

By the way, would ffdshow work in Windows NT 4.0 SP6? NT4 SP6 supposedly has most of the functionality of DirectX 3.0a.