Automatic (unwanted) deinterlacing?

Help/How To
1

Hi all,

I have problems getting proper interlaced output with Shotcut 20.07.11.

I am working on a few old, but personally important SD videos (Super-8, VHS - i.e. all interlaced) using some advanced Avisynth scripts for cleaning them up. Furthermore, I am always using FFV1 as intermediate codec for all steps until the final x264 encoding.

I worked with FFV1 files before in shotcut and set every clip to “Interlace” and “Top/Bottom Field” manually after having loaded them into the playlist. On export, I chose FFV1, again “Interlace” and “Top/Bottom Field” enabled, and it works when I just put the clips next to each other in the timeline and add simple cross-dissolve transitions and fade-in/fade-out. Project settings are of course also set to PAL DV (i.e. 720x576, interlace enabled, 25fps etc.)

Now, I wanted to “repair” an stabilization error in one of my videos using the scale and rotate filters and keyframe editing. However, whatever export option I chose, I now always get an progressive output file with 25p instead of 50i.
I know that scaling filters need deinterlacing before being applied to interlace video material, but when I select interlace output, I assume that the affected parts of the video would be re-interlaced after internal composition, while the scenes, where the scaling filters are not applied, are output “1:1”.

Any hints what could have went wrong in my setup?
If this is a limitation of Shotcut (found a hint saying “use filters in shortcut for automatic deinterlacing”), I would regard this being a bug because I am not used to get progressive output (with half frame rate) when I select “interlaced” in the output settings or doubled frames when selecting progressive output with the frame rate set to 50p.

Thanks in advance for any answer!

2

This is by design because these filters are not field-aware. Basically, anything that affects the vertical resolution causes a deinterlace. This is not a bug; it is a limitation with no plans to change it as interlace output is not important to the current developers. Anyone interested in this must work on it theirself.

3

It´s confusing to not be informed about forced deinterlacing and having the export settings ignored. Setting the export to interlace and getting a non-interlaced result is a bug in my opinion. Or not having the option to output it progressive with double frame rate.
I call this a bug because the described behaviour (having material deinterlaced for effects and the reinterlaced on output) has been a standard for >25 years, and interlace material is available for ages.

But if there is no interest for the developers for now, I will have to deal with it.

4

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