Option to keep specific metadata?

[c6y]

I'd like to keep keywords and some IPTC and EXIF metadata. Is this possible?

[lvandeve]

You can keep metadata with the --keepchunks=<chunknames...> flag. For this you need to know the names of the PNG chunks the metadata you wish to keep is stored in in your file (see http://dev.exiv2.org/projects/exiv2/wiki/The_Metadata_in_PNG_files)

For example if it's in iTXt, tEXt and/or zTXt chunks, you can try adding the flag
--keepchunks=iTXt, tEXt,zTXt

Does this work?

[c6y]

Thanks Lode! Keeping the uTXt chunk works:

tmpfile=$(mktemp)
zopflipng -m -y --keepchunks=iTXt $1 $tmpfile
mv $tmpfile $1

In this case I just need to keep the Create Date metadata. The rest of the metadata could be removed. Is this possible?

Preserved metadata after compression — output via exiftool:

======== ./myfile.png
ExifTool Version Number         : 10.47
File Name                       : myfile.png
Directory                       : .
File Size                       : 2.5 kB
File Modification Date/Time     : 2017:03:22 15:22:59-07:00
File Access Date/Time           : 2017:03:22 15:23:27-07:00
File Inode Change Date/Time     : 2017:03:22 15:22:59-07:00
File Permissions                : rw-------
File Type                       : PNG
File Type Extension             : png
MIME Type                       : image/png
Image Width                     : 54
Image Height                    : 43
Bit Depth                       : 4
Color Type                      : Palette
Compression                     : Deflate/Inflate
Filter                          : Adaptive
Interlace                       : Noninterlaced
XMP Toolkit                     : Adobe XMP Core 5.6-c014 79.156797, 2014/08/20-09:53:02
Creator Tool                    : Adobe Photoshop CC 2014 (Macintosh)
Create Date                     : 2014:10:28 23:35:57-07:00
Metadata Date                   : 2015:05:15 20:22:12-07:00
Modify Date                     : 2015:05:15 20:22:12-07:00
Format                          : image/png
Instance ID                     : xmp.iid:28698FBAF36411E4B1AFCA9E1202FD7D
Document ID                     : xmp.did:28698FBBF36411E4B1AFCA9E1202FD7D
Original Document ID            : xmp.did:80d5d65d-f02a-404c-9e88-f04fc2f1f2f6
History Action                  : created, saved, saved
History Instance ID             : xmp.iid:80d5d65d-f02a-404c-9e88-f04fc2f1f2f6, xmp.iid:f7eb5048-aee4-428c-b37b-e2290465a66f, xmp.iid:fbc28a4e-df67-4994-9e9b-55a1e8f1d31b
History When                    : 2014:10:28 23:35:57-07:00, 2014:10:28 23:36:46-07:00, 2015:05:15 13:13:01-07:00
History Software Agent          : Adobe Photoshop CC 2014 (Macintosh), Adobe Photoshop CC 2014 (Macintosh), Adobe Photoshop CC 2014 (Macintosh)
History Changed                 : /, /
Derived From Instance ID        : xmp.iid:fbc28a4e-df67-4994-9e9b-55a1e8f1d31b
Derived From Document ID        : adobe:docid:photoshop:2e0c3f68-3bcd-1178-8e18-e53a97ba8791
Document Ancestors              : xmp.did:089A9EDF0920681183BA8760C1477831, xmp.did:80d5d65d-f02a-404c-9e88-f04fc2f1f2f6
Palette                         : (Binary data 45 bytes, use -b option to extract)
Transparency                    : (Binary data 15 bytes, use -b option to extract)
Image Size                      : 54x43
Megapixels                      : 0.002