[LoRd_MuldeR]

Whenever you apply filters that operate on progressive frames, you have to deinterlace first (if input is interlaced). And of course it's a good idea to do that with a "high quality" deinterlacer, like QTGMC.

Applying a filter, which assumes progressive frames, on interlaced video will most likely destroy the content! So you have to deinterlace first, no matter what. You could re-interlace afterwards, if really needed.

Only time to can apply filters directly on interlaced video is when (a) the filter explicitly was written to operate on interlaced material or (b) the filter has a special "interlaced" mode you can use.

If, in case (b), the filter simply does a Deinterlace+Filter+Weave, then you are most likely better off by applying QTGMC first and then calling the filter in "progressive" mode. Although that probably is slower.

(BTW: Depending on what filter you are applying, it may also be reasonable to use SeparateFields() + YourProgressiveFilterHere() + Weave() instead of deinterlacing+re-interlacing)