Frames to fields

Theory behind the Frames to Fields and Fields to Frames plugins

Historically, CRT-type TVs used interlaced signals to save bandwidth. An interlaced video consists of two fields that are read and drawn on the screen one after the other. Each field must be played at a framerate double that of the resulting video. In two steps the complete frame will be reconstructed.

Frame 1 $\implies$ F1-field1 (Top or Odd), F1-field2 (Bottom or Even)

Frame 2 $\implies$ F2-field1, F2-field2

Interlaced video reading: F1 - f1 then F1 - f2 then F2 - f1 then F2 - f2 ... There may be visual problems if the Top type interlacing is read according to a Bottom scheme. So it's important to know if a video is Top or Bottom. Generally an HD video is Top; a DV video (both PAL and NTSC) is Bottom; SD (PAL) is Top; SD (NTSC) is Bottom (but not always). Instead, high-definition videos need to be more compressed and this contrasts with the interlacing that is little and badly compressible, so modern videos are mostly progressive.

In CINELERRA-GG

1. upload an interlaced video to the Timeline and Resources and play it for viewing.
2. The video presents visual artifacts because PC monitors are progressive.
3. In the Resources window, open the media Info with the right mouse button. Below you can see that the asset's interlacing is active. It has four options for settings interlacing type: Unknown, Top Fields first, Bottom Fields first, and Not interlaced. If the file is (H)DV type, recognition and configuration is done automatically. All other media types will be set unknown. We need to set the type of interlacing, so we have to manually set the interlacing.
4. Now we can to use the Frames to Fields plugin, but we have to configure it and act manually.

Now for the practical use of this plugin which applies the operation reverse to the Fields to Frames plugin. It extracts the two interlaced fields stored in alternating lines of interlaced source footage and outputs them as separate full frames. The alternating lines missing on each output frame are interpolated.

This plugin is only useful if its output is pulled with doubled framerate with respect to the source footage. One typical usage scenario is to do masking, scaling and translating on interlaced footage without the need to destroy the additional temporal information contained in such source material. This is helpful if your intended target format is interlaced. If on the other hand, you just want to target a progressive display (for example, you create video for display on a computer monitor solely) then it is much more convenient to de-interlace the source material prior to any further processing.

Processing interlaced footage without deinterlacing

1. Create a new project with doubled frame rate. That is, make it 50 fps if your source footage is 25i. In Resources → Media → Info set the interlaced type (or unknown)
2. Insert your source footage onto a video track in the timeline. Now, CINELERRA-GG will playback each frame of your footage twice. There will be visual artifacts because the video is interlaced and the monitor is progressive.
3. Apply the Frames to Fields effect. Be sure to choose the correct field order. If we know or believe that the original video is Top First let's try it first, but it doesn't have to be the right solution. The only way is to playback and look for visual artifacts.
4. Then apply any further effects afterwards, including translations, scaling, slow motion, precise frame-wise masking or use of the motion tracker plugin.
5. Render your project to an intermediate clip. Be sure to choose a rather lossless video codec, for example Motion-JPEG-A or even uncompressed YUV if you have plenty of storage.
6. Insert the intermediate clip into your original project. Make sure the doubled framerate has been detected correctly by CINELERRA-GG (by looking in the clip's media info in the media resources folder).
7. Apply the Fields to frames effect to the intermediate clip. This will combine two adjacent fields into one interlaced field with the original frame rate.
8. Do the final render on your original project. Now there will be no visual artifacts on the monitor.