BiRefNet matte

Give it a video and a span [start, end) in seconds; get back that span's foreground as a VP9 WebM with alpha (yuva420p), short-GOP so the compositor seeks it fast. Frame k of the cutout is frame start × fps + k of the source, so laid over the source in a compose spec it covers the type behind the people exactly (SSIM 0.98 against the source on a real job, no ghost edges). Three steps: cut the segment, matte it on fal, merge the mask in as alpha. Use the cutout from hn4a-compositor-in-scene-titles (type behind people) or hn4a-compositor-motion-graphics (a presenter cut out over a slide).

Inputs

The video (the finished cut, or the presenter footage) and its frame rate (ffprobe-inspect-media).
The span, snapped to the shot's cut times (ffmpeg-scene-cuts, with the low threshold that finds dissolves), so the cutout starts and ends with the shot.

Run it

1. The segment, frame-exact at the source's own rate, near-lossless, no audio. Seek before the input; -r is the source's rate. For 0 to 9.0833 s of a 24 fps cut:

exec ffmpeg -v error -y -ss 0 -i <job>/cut.mp4 -t 9.0833 -an
  -c:v libx264 -crf 12 -pix_fmt yuv420p -r 24
  <job>/mattes/A_cutout_src.mp4

2. The mask. Call describe("fal/fal-ai/birefnet/v2/video") before the first run in a job and check these fields against it; they are the ones proven on real jobs.

run(
  model="fal/fal-ai/birefnet/v2/video",
  input={
    "video_url": "<job>/mattes/A_cutout_src.mp4",
    "model": "General Use (Heavy)",
    "operating_resolution": "2048x2048",
    "output_mask": true,
    "refine_foreground": true,
    "video_quality": "maximum",
    "video_output_type": "X264 (.mp4)"
  },
  into="<job>/mattes",
  unit="A_cutout_mask"
)

The output's mask_video is the mask (white = foreground). Models on offer: General Use (Heavy), General Use (Light), General Use (Light 2K), General Use (Dynamic), Matting, Portrait; resolutions 1024x1024, 2048x2048, 2304x2304. Heavy at 2048 is the proven setting: it keeps furniture in front of the back wall (a podium, a chair), which is right when the type sits on that wall.

3. The alpha merge: the segment plus the grey mask, a slight feather on the edge (Gaussian sigma 0.8; 0 for none), VP9 with a keyframe every 12 frames:

exec ffmpeg -v error -y
  -i <job>/mattes/A_cutout_src.mp4 -i <job>/mattes/A_cutout_mask.mp4
  -filter_complex "[1:v]format=gray,gblur=sigma=0.8[m];[0:v][m]alphamerge,format=yuva420p"
  -c:v libvpx-vp9 -b:v 0 -crf 18 -g 12 -auto-alt-ref 0 -row-mt 1
  <job>/mattes/A_cutout.webm

Keep -auto-alt-ref 0 as written (the proven VP9 alpha encode); -g 12 keeps seeks short.

What you get

A_cutout.webm, plus the segment (A_cutout_src.mp4) and the mask (A_cutout_mask.mp4) beside it. Keep all three: re-merging with another feather is a local ffmpeg run, while a new mask is another paid run.

Tips and traps

Never colour-key the source for a cutout: it eats skin and hair edges and leaves a visible fringe. Matte it, or keep the footage in a window where its own background shows.
ffmpeg's native VP9 decoder drops alpha. Put -c:v libvpx-vp9 before -i in any command that reads the WebM. ffprobe reports such a file as yuv420p with the tag alpha_mode=1; that is a cutout with alpha, not a broken one.
One cutout per shot that needs one, cut at that shot's boundaries and placed in the comp at the same start.

Check it

Frame count of the WebM equals the segment's (count framemd5 lines, decoding with libvpx): exec ffmpeg -v error -c:v libvpx-vp9 -i <cutout.webm> -map 0:v -f framemd5 -
Edges: one frame over a flat colour, looked at for fringes and missing hair:

exec ffmpeg -v error -y -c:v libvpx-vp9 -i <job>/mattes/A_cutout.webm
  -filter_complex "color=c=0xB8E62E:s=1920x1080:r=24[bg];[bg][0:v]overlay=shortest=1,select=eq(n\,48)"
  -frames:v 1 -update 1 <job>/mattes/A_cutout_check.png

Alignment: the cutout over its own segment should be near identical to the segment (SSIM about 0.98 or better, read from the log): exec ffmpeg -i <segment> -c:v libvpx-vp9 -i <cutout.webm> -lavfi "[0:v][1:v]overlay=shortest=1[o];[o][0:v]ssim" -f null -

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