Timebase independent video GIF previewer

lib/philomena_media/gif_preview.ex

@@ -0,0 +1,117 @@
+defmodule PhilomenaMedia.GifPreview do
+  @moduledoc """
+  GIF preview generation for video files.
+  """
+
+  @type duration :: float()
+  @type dimensions :: {pos_integer(), pos_integer()}
+
+  @type num_images :: integer()
+  @type target_framerate :: 1..50
+  @type opts :: [
+          num_images: num_images(),
+          target_framerate: target_framerate()
+        ]
+
+  @doc """
+  Generate a GIF preview of the given video input with evenly-spaced sample points.
+
+  The input should have pre-computed duration `duration`. The `dimensions`
+  are a `{target_width, target_height}` tuple of the largest dimensions desired,
+  and the image will be resized to fit inside the box of those dimensions,
+  preserving aspect ratio.
+
+  Depending on the input file, this may take a long time to process.
+
+  Options:
+  - `:target_framerate` - framerate of the output GIF, must be between 1 and 50.
+    Default 2.
+  - `:num_images` - number of images to sample from the video.
+    Default is determined by the duration:
+    * 90 or above: 20 images
+    * 30 or above: 10 images
+    * 1 or above: 5 images
+    * otherwise: 2 images
+  """
+  @spec preview(Path.t(), Path.t(), duration(), dimensions(), opts()) :: :ok
+  def preview(video, gif, duration, dimensions, opts \\ []) do
+    target_framerate = Keyword.get(opts, :target_framerate, 2)
+
+    num_images =
+      Keyword.get_lazy(opts, :num_images, fn ->
+        cond do
+          duration >= 90 -> 20
+          duration >= 30 -> 10
+          duration >= 1 -> 5
+          true -> 2
+        end
+      end)
+
+    {_output, 0} =
+      System.cmd(
+        "ffmpeg",
+        commands(video, gif, clamp(duration), dimensions, num_images, target_framerate)
+      )
+
+    :ok
+  end
+
+  @spec commands(Path.t(), Path.t(), duration(), dimensions(), num_images(), target_framerate()) ::
+          [String.t()]
+  defp commands(video, gif, duration, {target_width, target_height}, num_images, target_framerate) do
+    # Compute range [0, num_images)
+    image_range = 0..(num_images - 1)
+
+    # Generate input list in the following form:
+    #   -ss 0.0 -i input.webm
+    input_arguments =
+      Enum.flat_map(image_range, &["-ss", "#{&1 * duration / num_images}", "-i", video])
+
+    # Generate graph in the following form:
+    #   [0:v] trim=end_frame=1 [t0]; [1:v] trim=end_frame=1 [t1] ...
+    trim_filters =
+      Enum.map_join(image_range, ";", &"[#{&1}:v] trim=end_frame=1 [t#{&1}]")
+
+    # Generate graph in the following form:
+    #   [t0][t1]... concat=n=10 [concat]
+    concat_input_pads =
+      Enum.map_join(image_range, "", &"[t#{&1}]")
+
+    concat_filter =
+      "#{concat_input_pads} concat=n=#{num_images}, settb=1/#{target_framerate}, setpts=N [concat]"
+
+    scale_filter =
+      "[concat] scale=width=#{target_width}:height=#{target_height}:" <>
+        "force_original_aspect_ratio=decrease [scale]"
+
+    split_filter = "[scale] split [s0][s1]"
+
+    palettegen_filter =
+      "[s0] palettegen=stats_mode=single:max_colors=255:reserve_transparent=1 [palettegen]"
+
+    paletteuse_filter =
+      "[s1][palettegen] paletteuse=dither=bayer:bayer_scale=5:new=1:alpha_threshold=255"
+
+    filter_graph =
+      [
+        trim_filters,
+        concat_filter,
+        scale_filter,
+        split_filter,
+        palettegen_filter,
+        paletteuse_filter
+      ]
+      |> Enum.join(";")
+
+    # Delay in centiseconds - otherwise it will be computed incorrectly
+    final_delay = 100.0 / target_framerate
+
+    ["-loglevel", "0", "-y"]
+    |> Kernel.++(input_arguments)
+    |> Kernel.++(["-lavfi", filter_graph])
+    |> Kernel.++(["-f", "gif", "-final_delay", "#{final_delay}", gif])
+  end
+
+  defp clamp(duration) when duration <= 0, do: 1.0
+  defp clamp(duration), do: duration
+end

lib/philomena_media/processors/webm.ex

@@ -3,6 +3,7 @@ defmodule PhilomenaMedia.Processors.Webm do
 
   alias PhilomenaMedia.Intensities
   alias PhilomenaMedia.Analyzers.Result
+  alias PhilomenaMedia.GifPreview
   alias PhilomenaMedia.Processors.Processor
   alias PhilomenaMedia.Processors
   import Bitwise
@@ -28,12 +29,11 @@ defmodule PhilomenaMedia.Processors.Webm do
     duration = analysis.duration
     stripped = strip(file)
     preview = preview(duration, stripped)
-    palette = gif_palette(stripped, duration)
     mp4 = scale_mp4_only(stripped, dimensions, dimensions)
 
     {:ok, intensities} = Intensities.file(preview)
 
-    scaled = Enum.flat_map(versions, &scale(stripped, palette, duration, dimensions, &1))
+    scaled = Enum.flat_map(versions, &scale(stripped, duration, dimensions, &1))
     mp4 = [{:copy, mp4, "full.mp4"}]
 
     [
@@ -82,12 +82,12 @@ defmodule PhilomenaMedia.Processors.Webm do
     stripped
   end
 
-  defp scale(file, palette, duration, dimensions, {thumb_name, target_dimensions}) do
+  defp scale(file, duration, dimensions, {thumb_name, target_dimensions}) do
     {webm, mp4} = scale_videos(file, dimensions, target_dimensions)
 
     cond do
       thumb_name in [:thumb, :thumb_small, :thumb_tiny] ->
-        gif = scale_gif(file, palette, duration, target_dimensions)
+        gif = scale_gif(file, duration, target_dimensions)
 
         [
           {:copy, webm, "#{thumb_name}.webm"},
@@ -199,53 +199,14 @@ defmodule PhilomenaMedia.Processors.Webm do
     mp4
   end
 
-  defp scale_gif(file, palette, duration, {width, height}) do
+  defp scale_gif(file, duration, dimensions) do
     gif = Briefly.create!(extname: ".gif")
-    scale_filter = "scale=w=#{width}:h=#{height}:force_original_aspect_ratio=decrease"
-    palette_filter = "paletteuse=dither=bayer:bayer_scale=5:diff_mode=rectangle"
-    rate_filter = rate_filter(duration)
-    filter_graph = "[0:v]#{scale_filter},#{rate_filter}[x];[x][1:v]#{palette_filter}"
 
-    {_output, 0} =
-      System.cmd("ffmpeg", [
-        "-loglevel",
-        "0",
-        "-y",
-        "-i",
-        file,
-        "-i",
-        palette,
-        "-lavfi",
-        filter_graph,
-        "-r",
-        "2",
-        gif
-      ])
+    GifPreview.preview(file, gif, duration, dimensions)
 
     gif
   end
 
-  defp gif_palette(file, duration) do
-    palette = Briefly.create!(extname: ".png")
-    palette_filter = "palettegen=stats_mode=diff"
-    rate_filter = rate_filter(duration)
-    filter_graph = "#{rate_filter},#{palette_filter}"
-
-    {_output, 0} =
-      System.cmd("ffmpeg", [
-        "-loglevel",
-        "0",
-        "-y",
-        "-i",
-        file,
-        "-vf",
-        filter_graph,
-        palette
-      ])
-
-    palette
-  end
-
   # x264 requires image dimensions to be a multiple of 2
   # -2 = ~1
   def box_dimensions({width, height}, {target_width, target_height}) do
@@ -255,8 +216,4 @@ defmodule PhilomenaMedia.Processors.Webm do
 
     {new_width, new_height}
   end
-
-  # Avoid division by zero
-  def rate_filter(duration) when duration > 0.5, do: "fps=1/#{duration / 10},settb=1/2,setpts=N"
-  def rate_filter(_duration), do: "setpts=N/TB/2"
 end