Writing is not supported, and never will.

I welcome PRs with fixes, but please raise an issue first if you want to add new features.

JPEG XL files often store EXIF and XMP Brotli compressed (e.g. cjxl does this by default). We didn't want to force a Brotli decoder into everyone's build, so you need to provide one via the DecompressBrotli option to read these, e.g. using andybalholm/brotli:

imagemeta.Decode(imagemeta.Options{
	// ...
	DecompressBrotli: func(r io.Reader) io.Reader { return brotli.NewReader(r) },
})

If not set, compressed metadata is skipped with a warning.

Note that this library has no dependencies outside of the standard library and golang.org/x/text. The Brotli decoder is only imported by our tests, so it will not be compiled into your binary, but as with our other test dependencies, it will show up in e.g. go mod graph, go list -m all and your go.sum.

Extracting EXIF performs well, ref. the benhcmark below. Note that you can get a significant boost if you only need a subset of the fields (e.g. only the Orientation). The last line is with the library that Hugo used before it was replaced with this.

BenchmarkDecodeCompareWithGoexif/bep/imagemeta/exif/jpeg/alltags-10                68658             15825 ns/op            3034 B/op        128 allocs/op
BenchmarkDecodeCompareWithGoexif/bep/imagemeta/exif/jpeg/orientation-10           444249              2567 ns/op             501 B/op         11 allocs/op
BenchmarkDecodeCompareWithGoexif/rwcarlsen/goexif/exif/jpg/alltags-10              27206             44110 ns/op          141977 B/op        816 allocs/op

The output of this library is tested against exiftool -n -json. This means, for example, that:

• We use f-numbers and not APEX for aperture values.
• We use seconds and not APEX for shutter speed values.
• EXIF field definitions are fetched from this table: https://exiftool.org/TagNames/EXIF.html
• IPTC field definitions are fetched from this table: https://exiftool.org/TagNames/IPTC.html
• The XMP handling is currently very simple, you can supply your own XMP handler (see the HandleXMP option) if you need more.

There are some subtle differences in output:

• Exiftool prints rationale number arrays as space formatted strings with a format/precision that seems unnecessary hard to replicate, so we use strconv.FormatFloat(f, 'f', -1, 64) for these.

Many of the tests depends on generated golden files. To update these, run:

 go generate ./gen

Note that you need a working exiftool and identify(ImageMagick) in your PATH for this to work. identify will fail for the truncated RAW fixtures unless ImageMagick is built with libraw (Homebrew's is not); the generator then keeps the committed .config.json for those. This was tested OK with:

exiftool -ver
12.76

Debugging tips:

 exiftool testdata/goexif_samples/has-lens-info.jpg -htmldump > dump.html