Class: ActiveStorage::HotCell::Server::Previewers::Video::Ffmpeg

Inherits:
ToolOperation show all
Defined in:
lib/active_storage/hot_cell/server/previewers/video/ffmpeg.rb

Overview

What ActiveStorage::Previewer::VideoPreviewer does, moved out of the application.

Rails runs ffmpeg <input args> -i <file> <output args> - and lets an application set both argument lists as shell strings it splits with Shellwords. This honours the input half: video_preview_input_arguments arrives split, and it goes before -i exactly as it does in Rails, because that is where an input option has to be. The output half is fixed here — see RELEVANT_FRAME — and the only other thing a caller says is how many seconds in to seek.

Constant Summary collapse

MAX_SEEK = 
86_400
RELEVANT_FRAME =

Rails' own frame selection, verbatim from the 7.0 defaults: frame 0, keyframes, and scene changes over 0.015 are selected, and the loop/trim pair yields the second of those with the first as the fallback — so a video that opens on black previews as its first scene rather than as the black. This is the output half of video_preview_arguments, and it stays fixed: the input half is what an application needs to shape, and that travels separately as input_arguments.

'select=eq(n\,0)+eq(key\,1)+gt(scene\,0.015),loop=loop=-1:size=2,trim=start_frame=1'

Constants inherited from Operation

Operation::CONTENT_TYPES

Instance Method Summary collapse

Instance Method Details

#perform(inputs, outputs, seek: 0, input_arguments: []) ⇒ Object 



34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
 
# File 'lib/active_storage/hot_cell/server/previewers/video/ffmpeg.rb', line 34

def perform(inputs, outputs, seek: 0, input_arguments: [])
  source, = inputs
  destination, = outputs

  seek = seek!(seek)
  input_arguments = arguments!(:input_arguments, input_arguments)

  # JPEG rather than PNG, because Rails' own video previewer yields image/jpeg and this is meant to
  # drop into its place. A preview that changed the attached blob's content type would not be a
  # replacement.
  #
  # -ss before -i seeks by keyframe, which is orders of magnitude cheaper on a long file than
  # decoding up to the point. -nostdin because ffmpeg reads the terminal otherwise, and a worker has
  # no terminal. The application's input arguments go directly before -i, so they cannot drift
  # past it as the fixed flags around them change.
  #
  # Both descriptors go to ffmpeg: it reads /dev/fd for the source and writes /dev/fd for the frame,
  # so neither the input nor the output touches scratch. Writing the descriptor directly leaves
  # partial bytes in the caller's file on a mid-write failure where a staged output would have left
  # it empty — harmless here, because run! turns a non-zero exit into a refusal and the previewer
  # discards its output on any failure, but it is a real property of the direct write.
  run! "ffmpeg", "-nostdin", "-loglevel", "error", "-ss", seek.to_s,
       *input_arguments, "-i", source.fd_path,
       "-vf", RELEVANT_FRAME,
       "-frames:v", "1", "-f", "image2", "-c:v", "mjpeg", "-y", destination.fd_path,
       pass: [ source.to_io, destination.to_io ]

  { format: "jpg", content_type: "image/jpeg", seek: seek,
    bytes: produced!(destination.to_io.stat.size, "ffmpeg") }
end