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114 lines
3.4 KiB
Elixir
114 lines
3.4 KiB
Elixir
defmodule PhilomenaMedia.GifPreview do
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@moduledoc """
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GIF preview generation for video files.
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"""
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@type duration :: float()
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@type dimensions :: {pos_integer(), pos_integer()}
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@type num_images :: integer()
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@type target_framerate :: 1..50
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@type opts :: [
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num_images: num_images(),
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target_framerate: target_framerate()
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]
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@doc """
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Generate a GIF preview of the given video input with evenly-spaced sample points.
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The input should have pre-computed duration `duration`. The `dimensions`
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are a `{target_width, target_height}` tuple.
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Depending on the input file, this may take a long time to process.
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Options:
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- `:target_framerate` - framerate of the output GIF, must be between 1 and 50.
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Default 2.
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- `:num_images` - number of images to sample from the video.
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Default is determined by the duration:
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* 90 or above: 20 images
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* 30 or above: 10 images
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* 1 or above: 5 images
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* otherwise: 2 images
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"""
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@spec preview(Path.t(), Path.t(), duration(), dimensions(), opts()) :: :ok
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def preview(video, gif, duration, dimensions, opts \\ []) do
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target_framerate = Keyword.get(opts, :target_framerate, 2)
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num_images =
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Keyword.get_lazy(opts, :num_images, fn ->
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cond do
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duration >= 90 -> 20
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duration >= 30 -> 10
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duration >= 1 -> 5
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true -> 2
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end
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end)
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{_output, 0} =
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System.cmd(
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"ffmpeg",
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commands(video, gif, clamp(duration), dimensions, num_images, target_framerate)
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)
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:ok
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end
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@spec commands(Path.t(), Path.t(), duration(), dimensions(), num_images(), target_framerate()) ::
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[String.t()]
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defp commands(video, gif, duration, {target_width, target_height}, num_images, target_framerate) do
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# Compute range [0, num_images)
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image_range = 0..(num_images - 1)
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# Generate input list in the following form:
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# -ss 0.0 -i input.webm
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input_arguments =
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Enum.flat_map(image_range, &["-ss", "#{&1 * duration / num_images}", "-i", video])
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# Generate graph in the following form:
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# [0:v] trim=end_frame=1 [t0]; [1:v] trim=end_frame=1 [t1] ...
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trim_filters =
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Enum.map_join(image_range, ";", &"[#{&1}:v] trim=end_frame=1 [t#{&1}]")
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# Generate graph in the following form:
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# [t0][t1]... concat=n=10 [concat]
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concat_input_pads =
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Enum.map_join(image_range, "", &"[t#{&1}]")
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concat_filter =
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"#{concat_input_pads} concat=n=#{num_images}, settb=1/#{target_framerate}, setpts=N [concat]"
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scale_filter =
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"[concat] scale=width=#{target_width}:height=#{target_height},setsar=1 [scale]"
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split_filter = "[scale] split [s0][s1]"
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palettegen_filter =
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"[s0] palettegen=stats_mode=single:max_colors=255:reserve_transparent=1 [palettegen]"
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paletteuse_filter =
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"[s1][palettegen] paletteuse=dither=bayer:bayer_scale=5:new=1:alpha_threshold=255"
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filter_graph =
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[
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trim_filters,
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concat_filter,
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scale_filter,
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split_filter,
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palettegen_filter,
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paletteuse_filter
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]
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|> Enum.join(";")
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# Delay in centiseconds - otherwise it will be computed incorrectly
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final_delay = 100.0 / target_framerate
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["-loglevel", "0", "-y"]
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|> Kernel.++(input_arguments)
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|> Kernel.++(["-lavfi", filter_graph])
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|> Kernel.++(["-f", "gif", "-final_delay", "#{final_delay}", gif])
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end
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defp clamp(duration) when duration <= 0, do: 1.0
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defp clamp(duration), do: duration
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end
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