Add URL references for IPTC information

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/doc/stdmetadata.rst

@@ -1063,6 +1063,16 @@ avoid conflicts with other plugins or standards.
     The history of the image or document.
 
 
+References for more information on IPTC metadata:
+
+* https://www.iptc.org/std/photometadata/specification/IPTC-PhotoMetadata
+* https://www.iptc.org/std/photometadata/specification/IPTC-PhotoMetadata#iptc-core-schema-1-5-specifications
+  This is the one where you can find the length limits
+* ExifTool's documentation about IPTC tags (caveat: not a definitive
+  reference, could be outdated or incorrect):
+  https://exiftool.org/TagNames/IPTC.html
+
+
 SMPTE metadata
 ==============
 

src/libOpenImageIO/iptc.cpp

@@ -84,6 +84,15 @@ static IIMtag iimtag[] = {
 // N.B. All "Date" fields are 8 digit strings: CCYYMMDD
 // All "Time" fields are 11 digit strings (what format?)
 
+// IPTC references:
+//
+// * https://www.iptc.org/std/photometadata/specification/IPTC-PhotoMetadata
+// * https://www.iptc.org/std/photometadata/specification/IPTC-PhotoMetadata#iptc-core-schema-1-5-specifications
+//   This is the one where you can find the length limits
+// * ExifTool's documentation about IPTC tags (caveat: not a definitive
+//   reference, could be outdated or incorrect):
+//   https://exiftool.org/TagNames/IPTC.html
+
 }  // anonymous namespace