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Decimate nth frame every nth frame

https://forum.blackmagicdesign.com/viewtopic.php?f=22&t=151839

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Decimate nth frame every nth frame

PostPosted: Fri Dec 03, 2021 11:27 am
by Interlacedgeek
Hi!

I'd like to exclude/decimate n frames every nth frame.
In fact every 6th frames from an interlaced NTSC footage to obtain/recover an interlaced PAL.

I tried the good old:

footage -> NTSC to PAL ... but the result is not precise enough

I guessed that the best way would be this:
footage -> fields (deinterlace) -> "tool or expression to decimate" -> fields (interlace)

Who could help me with this?

Thanks, Takk, Merci, Dank u!

Re: Decimate nth frame every nth frame

PostPosted: Fri Dec 03, 2021 2:56 pm
by Sander de Regt
Are you in Fusion or Fusion in Resolve?

Because - if I recall correctly - what your describing is a pulldown sequence. There is an option in the loader to deal with this automatically.

Just go into the Loader's import settings and adjust it to your needs.
This is a quote from the manual. It sounds a lot like what you're after.

Import Mode
This menu provides options for removing pull-up from an image sequence. Pull-up is a reversible
method of combining frames used to convert 24 fps footage into 30 fps. It is commonly used to
broadcast NTSC versions of films.
– Normal: This passes the image without applying pull-up or pull-down 2:3.
– Pull Up: This removes existing 3:2 pull-down applied to the image sequence, converting from 30
fps to 24 fps 2:3.
– Pull Down: The footage has pull-down applied, converting 24 fps footage to 30 fps by creating
five frames out of every four. The process mode of a Loader set to Pull Down should always be
Full Frames.
First Frame
This menu appears when the Import Mode is set to either Pull Up or Pull Down. It is used to determine
which frame of the 3:2 sequence is used as the first frame of the loaded clip.
Detect Pull-Down Sequence
This button is used to detect and set the pull-up sequence of the footage automatically. It only works if
Pull-Up or Pull-Down is first selected from the Import Mode menu. If it succeeds in detecting the order,
the First Frame control automatically sets to the correct value.


Hope it helps

Re: Decimate nth frame every nth frame

PostPosted: Sun Dec 05, 2021 10:13 am
by Interlacedgeek
Hi!
Thanks for your reply.
I'm actually looking for 29,97 to 25 ... not 24.

Re: Decimate nth frame every nth frame

PostPosted: Mon Dec 06, 2021 4:10 am
by birdseye
In that case you can use the Loader setting 2:3 Pull up to remove the Telecine, as Sander stated, check that the telecine has been removed correctly and set the saver to 25 fps.

Actually, on checking I see there is a 'Detect Pulldown Sequence' which should help line up the frames. I can't understand why Americans didn't get rid of that idiotic nonesense when digital video was introduced. It's only purpose appears to be to cause confusion.

Re: Decimate nth frame every nth frame

PostPosted: Tue Dec 07, 2021 9:35 am
by Interlacedgeek
birdseye wrote:In that case you can use the Loader setting 2:3 Pull up to remove the Telecine, as Sander stated, check that the telecine has been removed correctly and set the saver to 25 fps.

Actually, on checking I see there is a 'Detect Pulldown Sequence' which should help line up the frames. I can't understand why Americans didn't get rid of that idiotic nonesense when digital video was introduced. It's only purpose appears to be to cause confusion.


Not working.
This is a video footage that was converted from 25 to 29,97 ... and I'm trying to revert the process.

In After Effects, I would use the script:

frame=Math.round(time/thisComp.frameDuration)
group=Math.floor(frame/5)
offset=frame%5
choose=group*6+offset
choose*thisComp.frameDuration

With success.

I s there a script to do that in Fusion?

Re: Decimate nth frame every nth frame

PostPosted: Thu Dec 09, 2021 9:34 am
by birdseye
It depends on how it was converted, so it wasn't slowed down to 24 or 23.976 and Telecined. Is every 5th frame repeated or some other method. It should not be difficult if you know exactly what method has been used and what the field order is etc but you would probably be better leaving it as it is. Yet another conversion will likely make it look a lot worst.