Metadata: rudimentary support for XMP metadataa in MP4-based videos

XMP is a media-metadata standard based on XML which may be used
across a variety of media formats. Some video-processing software
writes XMP data without updating the native metadata fields.
Therefore, we should aim at reading XMP metadata and give priority
of XMP data over native fields.

Pros:
	- Support for *all* common media formats.
Cons:
	- XML (complex, verbose, chaotic).
	- Does not even come close to fulfilling its promise of being
	  well defined (see below).

Implement a simple XMP-parser using libxml2. Connect the XMP-parser to
the existing Quicktime/MP4 parser.

First problem encountered: According to the spec, XMP data supposed
to be put in the 'XMP_' atom. But for example exiftools instead
writes an 'uuid' atom with a special 16-byte uid. Implement both,
more options will probably follow.

Second problem: two versions of recording the creation date were found
  1) The content of a <exif:DateTimeOriginal> tag.
  2) The xmp::CreateDate attribute of a <rdf:Description> tag.

Here too, more versions are expected to surface and will have
to be supported in due course (with an obvious priority problem).

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

1 files changed, 35 insertions, 4 deletions

diff --git a/core/metadata.cpp b/core/metadata.cpp
index 2457c69c0..b549a5b3b 100644
--- a/core/metadata.cpp
+++ b/core/metadata.cpp
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include "metadata.h"
+#include "xmp_parser.h"
 #include "exif.h"
 #include "qthelper.h"
 #include <QString>
@@ -111,6 +112,15 @@ static bool parseExif(QFile &f, struct metadata *metadata)
 	}
 }
 
+// Parse an embedded XMP block. Note that this is likely generated by
+// external tools and therefore we give priority of XMP data over
+// native metadata.
+static void parseXMP(const char *data, size_t size, metadata *metadata)
+{
+	if (timestamp_t timestamp = parse_xmp(data, size))
+		metadata->timestamp = timestamp;
+}
+
 static bool parseMP4(QFile &f, metadata *metadata)
 {
 	f.seek(0);
@@ -170,8 +180,9 @@ static bool parseMP4(QFile &f, metadata *metadata)
 
 		if (!memcmp(type, "moov", 4) ||
 		    !memcmp(type, "trak", 4) ||
-		    !memcmp(type, "mdia", 4)) {
-			// Recurse into "moov", "trak" and "mdia" atoms
+		    !memcmp(type, "mdia", 4) ||
+		    !memcmp(type, "udta", 4)) {
+			// Recurse into "moov", "trak", "mdia" and "udta" atoms
 			atom_stack.push_back(atom_size);
 			continue;
 		} else if (!memcmp(type, "mdhd", 4) && atom_size >= 24 && atom_size < 4096) {
@@ -203,10 +214,30 @@ static bool parseMP4(QFile &f, metadata *metadata)
 				metadata->duration.seconds = lrint((double)duration / timescale);
 			// Timestamp is given as seconds since midnight 1904/1/1. To be convertible to the UNIX epoch
 			// it must be larger than 2082844800.
-			if (timestamp >= 2082844800) {
+			// Note that we only set timestamp if not already set, because we give priority to XMP data.
+			if (!metadata->timestamp && timestamp >= 2082844800) {
 				metadata->timestamp = timestamp - 2082844800;
-				// Currently, we only know how to extract timestamps, so we might just quit parsing here.
+				// We got our timestamp and duration. Nevertheless, we continue
+				// parsing, as there might still be an XMP atom.
+			}
+		} else if (!memcmp(type, "XMP_", 4) && atom_size > 32 && atom_size < 100000) {
+			// Parse embedded XMP data.
+			std::vector<char> d(atom_size);
+			if (f.read(&d[0], atom_size) != static_cast<int>(atom_size))
 				break;
+
+			parseXMP(&d[0], atom_size, metadata);
+		} else if (!memcmp(type, "uuid", 4) && atom_size > 32 && atom_size < 100000) {
+			// UUID atoms with uid "BE7ACFCB97A942E89C71999491E3AFAC" contain XMP blocks
+			// according the JPEG 2000 standard. exiftools produces mp4-style videos with such
+			// an UUID atom.
+			std::vector<char> d(atom_size);
+			if (f.read(&d[0], atom_size) != static_cast<int>(atom_size))
+				break;
+
+			static const char xmp_uid[17] = "\xBE\x7A\xCF\xCB\x97\xA9\x42\xE8\x9C\x71\x99\x94\x91\xE3\xAF\xAC";
+			if (!memcmp(&d[0], xmp_uid, 16)) {
+				parseXMP(&d[16], atom_size - 16, metadata);
 			}
 		} else {
 			// Jump over unknown atom