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

app/logical/media_file/image.rb

@@ -38,11 +38,15 @@ class MediaFile::Image < MediaFile
   # @see https://github.com/jcupitt/libvips/wiki/HOWTO----Image-shrinking
   # @see http://jcupitt.github.io/libvips/API/current/Using-vipsthumbnail.md.html
   def preview(width, height)
-    output_file = Tempfile.new(["image-preview", ".jpg"])
-    resized_image = image.thumbnail_image(width, height: height, **THUMBNAIL_OPTIONS)
-    resized_image.jpegsave(output_file.path, **JPEG_OPTIONS)
+    if is_animated?
+      FFmpeg.new(file).smart_video_preview
+    else
+      output_file = Tempfile.new(["image-preview", ".jpg"])
+      resized_image = image.thumbnail_image(width, height: height, **THUMBNAIL_OPTIONS)
+      resized_image.jpegsave(output_file.path, **JPEG_OPTIONS)
 
-    MediaFile::Image.new(output_file)
+      MediaFile::Image.new(output_file)
+    end
   end
 
   def crop(width, height)

app/logical/media_file/ugoira.rb

@@ -36,6 +36,7 @@ class MediaFile::Ugoira < MediaFile
   # XXX should take width and height and resize image
   def convert
     raise NotImplementedError, "can't convert ugoira to webm: ffmpeg or mkvmerge not installed" unless self.class.videos_enabled?
+    raise RuntimeError, "can't convert ugoira to webm: no ugoira frame data was provided" unless frame_data.present?
 
     Dir.mktmpdir("ugoira-#{md5}") do |tmpdir|
       output_file = Tempfile.new(["ugoira-conversion", ".webm"], binmode: true)
@@ -87,10 +88,8 @@ class MediaFile::Ugoira < MediaFile
   end
 
   def preview_frame
-    tempfile = Tempfile.new("ugoira-preview", binmode: true)
-    zipfile.entries.first.extract(tempfile.path) { true } #  'true' means overwrite the existing tempfile.
-    MediaFile.open(tempfile)
+    FFmpeg.new(convert).smart_video_preview
   end
 
-  memoize :zipfile, :preview_frame, :dimensions
+  memoize :zipfile, :preview_frame, :dimensions, :convert
 end

app/logical/media_file/video.rb

@@ -32,9 +32,7 @@ class MediaFile::Video < MediaFile
   end
 
   def preview_frame
-    vp = Tempfile.new(["video-preview", ".jpg"], binmode: true)
-    video.screenshot(vp.path, seek_time: 0)
-    MediaFile.open(vp.path)
+    FFmpeg.new(file).smart_video_preview
   end
 
   memoize :video, :preview_frame, :dimensions, :duration, :has_audio?