Preserving Video Metadata When Watermarking

[tim-janik]

Hi.
When using videowmark to watermark an MP4 file, it successfully reads the MP4 and adds the watermark, but seems to loose some of the metadata, according to mediainfo.

$ videowmark add BigBuckBunny_320x180.mp4 BigBuckBunny_WM.mp4 0000111100002222
$ mediainfo BigBuckBunny_320x180.mp4 >x1
$ mediainfo BigBuckBunny_WM.mp4 >x2
$ diff -u x1 x2

The differences are as follows:

--- x1	2024-04-19 12:05:44.013193748 +0200
+++ x2	2024-04-19 12:05:49.757202964 +0200
@@ -1,19 +1,16 @@
 General
-Complete name                            : BigBuckBunny_320x180.mp4
+Complete name                            : BigBuckBunny_WM.mp4
 Format                                   : MPEG-4
 Format profile                           : Base Media
-Codec ID                                 : isom (mp41)
+Codec ID                                 : isom (isom/iso2/avc1/mp41)
 File size                                : 61.7 MiB
 Duration                                 : 9 min 56 s
-Overall bit rate mode                    : Variable
 Overall bit rate                         : 867 kb/s
 Movie name                               : Big Buck Bunny
 Performer                                : Blender Foundation
 Composer                                 : Blender Foundation
 Recorded date                            : 2008
-Encoded date                             : UTC 1970-01-01 00:00:00
-Tagged date                              : UTC 1970-01-01 00:00:00
-Writing application                      : Lavf52.14.0
+Writing application                      : Lavf58.76.100
 
 Video
 ID                                       : 1
@@ -39,8 +36,6 @@
 Scan type                                : Progressive
 Bits/(Pixel*Frame)                       : 0.508
 Stream size                              : 50.0 MiB (81%)
-Encoded date                             : UTC 1970-01-01 00:00:00
-Tagged date                              : UTC 1970-01-01 00:00:00
 Codec configuration box                  : avcC
 
 Audio
@@ -49,15 +44,18 @@
 Format/Info                              : Advanced Audio Codec Low Complexity
 Codec ID                                 : mp4a-40-2
 Duration                                 : 9 min 56 s
-Bit rate mode                            : Variable
-Bit rate                                 : 160 kb/s
+Source duration                          : 9 min 56 s
+Bit rate mode                            : Constant
+Bit rate                                 : 161 kb/s
 Channel(s)                               : 2 channels
 Channel layout                           : L R
 Sampling rate                            : 48.0 kHz
 Frame rate                               : 46.875 FPS (1024 SPF)
 Compression mode                         : Lossy
-Stream size                              : 11.4 MiB (18%)
-Encoded date                             : UTC 1970-01-01 00:00:00
-Tagged date                              : UTC 1970-01-01 00:00:00
+Stream size                              : 11.4 MiB (19%)
+Source stream size                       : 11.4 MiB (19%)
+Default                                  : Yes
+Alternate group                          : 1
+mdhd_Duration                            : 596224

I.e. "Encoded date" and "Tagged date" are lost for both streams, Video and Audio.
Also, is it expected that the audio stream turns from "Bit rate mode: Variable" before watermarking into "Bit rate mode: Constant" after watermarking?

For the record, I've tried to adjust videowmark to preserve the metadata by adding -movflags +faststart+use_metadata_tags -map_metadata 0 to the audio+video merging step. But that tends to also rename matadata tags (e.g. "Movie name"->"title" or "Performer"->"artist) without preserving any dates, so it is not really an improvement.

As for the above metadata loss, is that purely due to the different codecs used during re-encoding, or can metadata loss go beyond that?