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Writing a transform filter can be broken into the following discrete steps.
The fewer copies in the media stream, the better. However, some filters require a copy operation; this influences the choice of base classes.
In this step, create the header or headers for your filter. In many cases, you can use the transform base classes, derive your class from the correct transform filter class, and override a few member functions. In other cases, you can use the more generic base classes. These classes implement most of the connection and negotiation mechanism; but these classes also allow more flexibility at the cost of overriding more member functions.
This step requires adding a static CreateInstancemember function to your derived class and also a global array that contains the name of the filter, a CLSID, and a pointer to that member function.
This step addresses the Component Object Model (COM) aspects of implementing interfaces, other than those in the base classes.
This includes writing the transform function that is unique to
your filter and overriding a few member functions that are
necessary for the connection process, such as setting the allocator
size or providing media types.
Last updated on Tuesday, May 18, 2004