How to Use FFmpeg Weave Filter to Merge Fields

This article provides a straightforward guide on how to use the weave video filter in FFmpeg to merge individual interlaced fields into full video frames. You will learn the correct command-line syntax, understand how the field order parameters work, and see practical examples of how to reconstruct interlaced video streams from split-field sources.

The weave filter in FFmpeg takes a sequence of fields (where each input frame represents a single field, containing only the odd or even lines of a video) and interleaves them to reconstruct full-height interlaced frames. This process doubles the vertical resolution of the video while halving the frame rate.

Basic Syntax and Parameters

The basic syntax for applying the weave filter is as follows:

ffmpeg -i input.mp4 -vf "weave=first_field=top" output.mp4

The filter accepts one primary parameter:

• first_field (or f): Specifies which field is treated as the first line of the output frame.
  • top (or 0): The first input frame is treated as the top field (even-numbered lines, starting at line 0). This is the default setting.
  • bottom (or 1): The first input frame is treated as the bottom field (odd-numbered lines, starting at line 1).

Practical Command Examples

If you have a video that has been previously split into separate fields (for example, by using the separatefields filter) and you want to merge them back into a single interlaced video, you can chain the filters or apply weave directly:

ffmpeg -i split_fields.mp4 -vf "weave=first_field=bottom" output_interlaced.mp4

In scenarios where you are processing raw interlaced video streams and need to ensure the correct field dominance is maintained during the weave process, you can explicitly define the option like this:

ffmpeg -i input_fields.yuv -vf "weave=f=top" output_interlaced.mp4

By default, if you do not specify the first_field parameter, FFmpeg will assume top field dominance. Use bottom if your source material was captured with bottom-field-first (BFF) dominance to avoid rendering artifacts like judder or comb-like ghosting on moving objects.