x/image/tiff: Unable to decode DNG files with external package

[mholt]

What version of Go are you using (go version)?

$ go version
go version go1.19.1 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/matt/.cache/go-build"
GOENV="/home/matt/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/matt/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/matt/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.19.1"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build1397311274=/tmp/go-build -gno-record-gcc-switches"

What did you do?

The x/image/tiff package is in conflict with DNG decoder packages because it registers its format on init(), even though TIFF files and DNG files look similar.

I suppose one of these solutions must work:

• x/image/tiff should support decoding DNG files
• or it should use a more specific "magic" header if possible
• or it should not register its format on init().

What did you expect to see?

I expect that I should be able to call image.Decode() on a DNG file and have it successfully work using a registered DNG decoder.

The problem is that DNG and TIFF files look similar at the beginning, so registering both the TIFF and DNG formats will cause the TIFF to be used instead of DNG.

I'm not actually decoding TIFF files, but I'm using TIFF APIs because DNG shares some in common.

What did you see instead?

"tiff: unsupported feature: color model"

Currently it doesn't seem possible to use image.Decode() for DNG files.

[seankhliao]

Given how the DNG package you linked has the author saying

// DNG is a bastardized TIFF file.
// This package is a stripped back version of golang.org/x/image/tiff.

perhaps it should be on the DNG package to also support tiffs, or choose a more specific header?

[seankhliao]

no longer registering by default would be a breaking change.

cc @nigeltao

[mholt]

If DNG is "basically" a TIFF file, I think my favorite solution would be for x/image/tiff to support DNG 😃