Fix #3400: Smarter thumbnail generation for videos

app/logical/ffmpeg.rb

@@ -0,0 +1,25 @@
+class FFmpeg
+  attr_reader :file
+
+  # Operate on a file with FFmpeg.
+  # @param file [File, String] a webm, mp4, gif, or apng file
+  def initialize(file)
+    @file = file.is_a?(String) ? File.open(file) : file
+  end
+
+  # Generate a .jpg preview image for a video or animation. Generates
+  # thumbnails intelligently by avoiding blank frames.
+  #
+  # @return [MediaFile] the preview image
+  def smart_video_preview
+    vp = Tempfile.new(["video-preview", ".jpg"], binmode: true)
+
+    # https://ffmpeg.org/ffmpeg.html#Main-options
+    # https://ffmpeg.org/ffmpeg-filters.html#thumbnail
+    ffmpeg_out, status = Open3.capture2e("ffmpeg -i #{file.path} -vf thumbnail=300 -frames:v 1 -y #{vp.path}")
+    raise "ffmpeg failed: #{ffmpeg_out}" if !status.success?
+    Rails.logger.debug(ffmpeg_out)
+
+    MediaFile.open(vp)
+  end
+end