Feature request: Support for `cjxl` to handle text chunks in PNGs

[eddiezato]

Currently cjxl can't handle text chunks (tEXt/iTXt/zTXt) in PNG files and transfer metadata to JXL. It would be useful to have support of this.

[kmilos]

To be more specific, this is about recognizing Exif/XMP/ICC metadata being stored in text chunks (as raw payload under Raw profile type ... keyword) since early PNG spec didn't have dedicated metadata chunks like eXIf.

https://exiftool.org/TagNames/PNG.html#TextualData

This is still the case for XMP using the XML:com.adobe.xmp keyword.

[lectrode]

Is this why cjxl "0911152001.png" -d 0 -e 9 "0911152001.png.d0e9.jxl" doesn't preserve the metadata from this image? or is that a separate issue?

convert 0911152001.png -quality 100 -define jxl:effort=9 0911152001.png.q100e9.jxl from imagemagick, on the other hand, does preserve the metadata when converting to jxl

(original image was jpg, which was converted to png, provided below)

[kmilos]

Is this why cjxl "0911152001.png" -d 0 -e 9 "0911152001.png.d0e9.jxl" doesn't preserve the metadata from this image?

Yes.

$ exiv2 -pS ../210919341-5df28b79-3339-48da-a5b8-a6f19733d96b.png
STRUCTURE OF PNG FILE: ../210919341-5df28b79-3339-48da-a5b8-a6f19733d96b.png
 address | chunk |  length | data                           | checksum
       8 | IHDR  |      13 | ...@...$....                   | 0x8d002add
      33 | zTXt  |    5426 | Raw profile type exif..x...Wr# | 0x2075c0fd
    5471 | iCCP  |     387 | ICC profile..x.}.=H.@.._S."..; | 0x4db1a3cb
    5870 | iTXt  |    3448 | XML:com.adobe.xmp.....<?xpacke | 0x048f4a5c
    9330 | pHYs  |       9 | .........                      | 0x009a9c18
    9351 | tIME  |       7 | .......                        | 0x712bdafa
    9370 | tEXt  |      81 | Comment.charset=InvalidCharset | 0x66195c5f
    9463 | IDAT  |    8192 | x.T...%[email protected]..... | 0xe64a5dbc

If you convert the jpg to png using (a relatively recent version of) ImageMagick as well you will see the Exif is no longer stored in a "Raw profile type exif" zTXt chunk but a proper eXIf chunk instead, and cjxl would detect the metadata then.

[alistair7]

This is working since 0d89f89, but it hasn't made it into a release yet. Prior to that change, cjxl(_ng) didn't add metadata to the JXL output.

I believe the library has supported reading "Raw profile type" chunks for both Exif and XMP for quite a while - it's just that release 0.7.0 doesn't do anything with them after reading them.

Here's a new-ish version of cjxl converting that file:

$ cjxl 210919341-5df28b79-3339-48da-a5b8-a6f19733d96b.png 210919341-5df28b79-3339-48da-a5b8-a6f19733d96b.jxl -e 1
JPEG XL encoder v0.8.0 0a6bafee [AVX2,SSE4,SSSE3,Unknown]
Read 4160x2340 image, 8684443 bytes, 196.0 MP/s
Encoding [Container | VarDCT, d1.000, effort: 1 | 5174-byte Exif | 3426-byte XMP], 
Compressed to 844598 bytes including container (0.694 bpp).
4160 x 2340, 9.33 MP/s [9.33, 9.33], 1 reps, 8 threads.

$ jxlinfo 210919341-5df28b79-3339-48da-a5b8-a6f19733d96b.jxl 
JPEG XL file format container (ISO/IEC 18181-2)
JPEG XL image, 4160x2340, lossy, 8-bit RGB
Brotli-compressed Exif metadata: 4637 compressed bytes
Brotli-compressed xml  metadata: 696 compressed bytes
Color space: RGB, D65, sRGB primaries, sRGB transfer function, rendering intent: Perceptual

(Not all decoders understand Brotli-compressed boxes, so you might want to use --compress_boxes=0)

[lectrode]

Thank you - very informative.

I can confirm that converting with the latest cjxl git does indeed retain the metadata as expected.

The main concern is making sure the metadata exists in the new/replacement image. If I have to compile a newer metadata viewer for now to view that info, that's not a blocker. Hopefully that won't be required for too much longer.

[kmilos]

FYI, IM 7.1.0-58 should have much improved Exif/XMP support for JPEG XL files.