Use FFmpeg scene detection for video previews

Generating a video preview by capturing only the first frame of a video
is problematic for videos that begin with a fade in from black.  By
using keyframe and scene detection that is built in to FFmpeg, we can
generate a more representative preview.

activestorage/lib/active_storage/previewer/video_previewer.rb

@@ -28,7 +28,21 @@ module ActiveStorage
 
     private
       def draw_relevant_frame_from(file, &block)
-        draw self.class.ffmpeg_path, "-i", file.path, "-y", "-vframes", "1", "-f", "image2", "-", &block
+        ffmpeg_args = [
+          "-i", file.path,
+          "-vf",
+            # Select the first video frame, plus keyframes and frames
+            # that meet the scene change threshold.
+            'select=eq(n\,0)+eq(key\,1)+gt(scene\,0.015),' +
+            # Loop the first 1-2 selected frames in case we were only
+            # able to select 1 frame, then drop the first looped frame.
+            # This lets us use the first video frame as a fallback.
+            "loop=loop=-1:size=2,trim=start_frame=1",
+          "-frames:v", "1",
+          "-f", "image2", "-",
+        ]
+
+        draw self.class.ffmpeg_path, *ffmpeg_args, &block
       end
   end
 end

guides/source/active_storage_overview.md

@@ -559,7 +559,7 @@ the box, Active Storage supports previewing videos and PDF documents.
 </ul>
 ```
 
-WARNING: Extracting previews requires third-party applications, FFmpeg for
+WARNING: Extracting previews requires third-party applications, FFmpeg v3.4+ for
 video and muPDF for PDFs, and on macOS also XQuartz and Poppler.
 These libraries are not provided by Rails. You must install them yourself to
 use the built-in previewers. Before you install and use third-party software,