All intermediate frames must be in RGB32.
This means that filters which work in alien formats, such as YUV, must contain
private code for conversion to and from RGB.
Future frames cannot be accessed.
Because VirtualDub requires one frame out per each frame input, filters which
would require frames ahead of the one currently being processed, such as those
with bidirectional prediction, cannot properly run. Delay tricks will work,
but will skew the video stream from the audio.
Frame count cannot change.
Again, because of the one-out-per-one-in limitation, it is also not currently
possible to create filters that alter the frame count by adding or dropping
frames. This precludes making interpolation filters, such as slow-motion,
and decimation filters, like telecine removers. Such filters require source-code
modification to VirtualDub itself.