Determine Which Base Classes to Use

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Before choosing a base class for your transform filter, you must first decide whether your filter needs more than one input and output pin. If it does, you should derive your filter class from CBaseFilter.

If your filter needs to perform a video transform, you should derive your filter class from CVideoTransformFilter.

Once you determine whether the transform filter will copy media samples or transform them in place, you must decide which base class or classes to use and which member functions you must override and implement. You can then define your derived classes.

Some member functions in the base classes must be overridden in your derived class because they are either declared as pure virtual in the base classes (they have no implementation), or have default implementations that do nothing but return an error value.

You derive your filter class from the transform base classes CTransformFilter, CTransInPlaceFilter, or CVideoTransformFilter, or from the more generic CBaseFilter filter class. Most of the connection, media type, and allocator negotiation code is handled in the base classes and inherited by the transform classes. The transform classes make it possible to create a filter by deriving just one filter class (no pin classes). The transform classes make assumptions about the workings of transform filters that make the process of creating a transform filter easier.

To learn more about CTransformFilter and CTransInPlaceFilter and which of their member functions are typically overridden by the derived class, see Using the CTransformFilter and CTransInPlaceFilter Transform Base Classes.

See Also

Write a Transform Filter

Last updated on Wednesday, April 13, 2005

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