Fix the bitrate scale factor for h264-to-hevc transcoding (#9485)

MediaBrowser.Controller/MediaEncoding/EncodingHelper.cs

@@ -2021,14 +2021,20 @@ namespace MediaBrowser.Controller.MediaEncoding
 
         private static double GetVideoBitrateScaleFactor(string codec)
         {
+            // hevc & vp9 - 40% more efficient than h.264
             if (string.Equals(codec, "h265", StringComparison.OrdinalIgnoreCase)
                 || string.Equals(codec, "hevc", StringComparison.OrdinalIgnoreCase)
-                || string.Equals(codec, "vp9", StringComparison.OrdinalIgnoreCase)
-                || string.Equals(codec, "av1", StringComparison.OrdinalIgnoreCase))
+                || string.Equals(codec, "vp9", StringComparison.OrdinalIgnoreCase))
             {
                 return .6;
             }
 
+            // av1 - 50% more efficient than h.264
+            if (string.Equals(codec, "av1", StringComparison.OrdinalIgnoreCase))
+            {
+                return .5;
+            }
+
             return 1;
         }
 
@@ -2036,7 +2042,9 @@ namespace MediaBrowser.Controller.MediaEncoding
         {
             var inputScaleFactor = GetVideoBitrateScaleFactor(inputVideoCodec);
             var outputScaleFactor = GetVideoBitrateScaleFactor(outputVideoCodec);
-            var scaleFactor = outputScaleFactor / inputScaleFactor;
+
+            // Don't scale the real bitrate lower than the requested bitrate
+            var scaleFactor = Math.Min(outputScaleFactor / inputScaleFactor, 1);
 
             if (bitrate <= 500000)
             {