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image/png: DecodeConfig gives incorrect result for paletted images with transparency #54325

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depp opened this issue Aug 7, 2022 · 2 comments
Closed

image/png: DecodeConfig gives incorrect result for paletted images with transparency #54325

depp opened this issue Aug 7, 2022 · 2 comments
Labels
FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Go1.20

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@depp
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depp commented Aug 7, 2022

The color model returned from png.DecodeConfig is different from the one returned from png.Decode, because it incorrectly omits the alpha channel data.

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

$ go version
go version go1.19 darwin/amd64

Does this issue reproduce with the latest release?

Yes.

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

go env Output
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/depp/Library/Caches/go-build"
GOENV="/Users/depp/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/depp/Projects/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/depp/Projects/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.19/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.19/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.19"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/1l/g8_2q7j95rndy_wpjnd5b1h00000gn/T/go-build1338501950=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

See go.dev/play snippet

When run, prints

Color 0 (png.DecodeConfig): {0 0 0 255}
Color 0 (png.Decode): {0 0 0 0}

Program exited.

What did you expect to see?

Same color model, no matter whether I used DecodeConfig or Decode

What did you see instead?

Palette entry 0 has alpha = 255 (incorrect) if returned by DecodeConfig, alpha = 0 (correct) if returned by Decode.
@ianlancetaylor ianlancetaylor changed the title image/png: png.DecodeConfig gives incorrect result for paletted images with transparency image/png: DecodeConfig gives incorrect result for paletted images with transparency Aug 7, 2022
@ianlancetaylor
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CC @nigeltao

@thanm thanm added this to the Backlog milestone Aug 9, 2022
@thanm thanm added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Aug 9, 2022
@gopherbot
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Change https://go.dev/cl/424917 mentions this issue: image/png: have DecodeConfig read tRNS chunks

@dmitshur dmitshur modified the milestones: Backlog, Go1.20 Aug 19, 2022
@dmitshur dmitshur added NeedsFix The path to resolution is known, but the work has not been done. and removed NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. labels Aug 19, 2022
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
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Milestone
Go1.20
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@depp @dmitshur @ianlancetaylor @gopherbot @thanm