Membrane FFmpeg SWScale plugin View Source API Docs CircleCI

Plugin providing an element scaling raw video frames, using SWScale module of FFmpeg library.

It is a part of Membrane Multimedia Framework.


Add the following line to your deps in mix.exs. Run mix deps.get.

{:membrane_ffmpeg_swscale_plugin, "~> 0.5.0"}

You also need to have FFmpeg library with development headers installed on your system.


Scaler needs input in the YUV420p format, processes one frame at a time and requires getting caps with input video width and height. To meet all requirements either Membrane.Element.RawVideo.Parser or some decoder (e.g. Membrane.H264.FFmpeg.Decoder) have to precede Scaler in the pipeline.

There are two options that have to be specified when creating the element:

  • output_width - desired scaled video width.
  • output_height - desired scaled video height.

Both need to be even numbers.

Scaling consists of two operations:

  • scaling itself - resizing video frame with keeping original ratio. After that operation at least one of the dimensions of the input frame match the respective dimension of the desired output size. The second one (if does not match) is smaller than its respective dimension.
  • adding paddings - if one dimension does not match after scaling, paddings have to be added. They are put on both sides of the scaled frame equally. They are either above and below the frame or on the left and right sides of it. It depends on the dimension that did not match after scaling.

The output of the element is also in the YUV420p format. It has the size as specified in the options. All caps except for width and height are passed unchanged to the next element in the pipeline.

Sample usage

Scaling encoded (using H.264 standard) video requires parser and decoder because Scaler scales only raw video.

defmodule Scaling.Pipeline do
  use Membrane.Pipeline

  @impl true
  def handle_init(_) do
    children = [
      file_src: %Membrane.File.Source{location: "/tmp/input.h264"},
      parser: Membrane.H264.FFmpeg.Parser,
      decoder: Membrane.H264.FFmpeg.Decoder,
      scaler: %Membrane.FFmpeg.SWScale.Scaler{output_width: 640, output_height: 640},
      encoder: Membrane.H264.FFmpeg.Encoder,
      file_sink: %Membrane.File.Sink{location: "/tmp/output.h264"}

    links = [
      |> to(:parser)
      |> to(:decoder)
      |> to(:scaler)
      |> to(:encoder)
      |> to(:file_sink)

    {{:ok, spec: %ParentSpec{children: children, links: links}}, %{}}

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Software Mansion

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