Using DirectShow filters outside DirectShow?

Question

I'm currently dealing with Windows Media Foundation. However, due to some problems with the Microsoft H.264 decoder and some missing decoders for custom format, I'd like to know if it would be possible to instantiate a DirectShow Decoder directly using CLSID, and build a proxy around it that exposes IMFTransform to get a decoder for Media Foundation. So here is my question:

Can i instantiate a Directshow filter (preferrably decoders) directly and use them for decoding (i.e. put some compressed frames and get uncompressed ones) to create a MFT?

I know how to instantiate the filter itself using its CLSID. However, I have no clue how to use the actual decoding functionality.

Any ideas, hints, links whatever will be appreciated. Thanks,

J.

Answer 1

(disclaimer: I have never actually done this, but I see no technical reason it cannot be done. So YMMV)

If the decoder is a DMO filter, then it'll be a lot easier--you can talk to it through IMediaObject. This isn't really much different from how DirectShow uses DMOs; it simply wraps the DMO with another transform filter that handles the media type negotiation and sample passing, but there's nothing really stopping you from doing this in your own application.

There's one catch: for IMediaObject::ProcessInput and IMediaObject::ProcessOutput, you'll need your own buffer class that implements IMediaBuffer. But it's a pretty basic interface, so I don't think you'll have too much trouble implementing it. Here's a basic implementation.

For regular directshow filters, it's actually going to be a lot more difficult, because most DirectShow filters really depend on there being an external graph available (case in point: all of the directshow eventing sort of presumes the existence of this graph). If you really want to use a single DShow filter standalone, you'd probably have to wrap the entire filter graph, and then have a custom source filter to feed samples in. You could use the sample grabber (or a custom render filter) to yank samples out of the graph and expose to the rest of the application. (one sort of crazy idea would even be to wrap this graph in a DMO filter implementation, and then use IMediaObject to talk to it--this might be tricky, however)

Luckily most decoders tend to be implemented as DMO filters, so I think there's a strong likelihood that you can just use IMediaObject.

Answer 2

I'm unsure as to why you would want to do this. You don't really want a filter living outside of a graph.

If you don't want to use the traditional file / network source filters, or the traditional renderers, you can write buffer renderers, and buffer source filters, that you pass pointers to, and get pointers from. Then you can drop the whole mess into a graph and run it, and get the use of the decoder pretty much directly without anything else. This wouldn't be difficult to do. The decoder is probably expecting a PES packet stream though.