x/image/bmp: support to decode 8-bit format with up to 256 color palette

[entooone]

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

$ go version
go version go1.20.5 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//.cache/go-build"
GOENV="/home//.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home//go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home//go"
GOPRIVATE=""
GOPROXY="direct"
GOROOT="/usr/lib/golang"
GOSUMDB="off"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/golang/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20.5"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build1874621086=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I got bmp: unsupported BMP image when I tried to Decode the following image.

• colormap-251.bmp
  • colormap-251.bmp.zip
  • 8-bit format, 251 color palette
  • I made it by editing testdata/colormap.bmp with GIMP.

Sample code:

package main

import (
	"os"

	"golang.org/x/image/bmp"
)

func main() {
	f, err := os.Open("colormap-251.bmp")
	if err != nil {
		panic(err)
	}
	defer f.Close()

	_, err = bmp.Decode(f)
	if err != nil {
		panic(err)
	}
}

What did you expect to see?

Decode success

What did you see instead?

$ go run .
panic: bmp: unsupported BMP image
...

Additional information

Current implementation of reader.go assumes that the number of palette is 256. Therefore, bmp: unsupported BMP image occurs when the number of palette is not 256.

I think that the number of palette should depend on biClrUsed of BITMAPINFOHEADER instead of fixing it to 256. Note that if biClrUsed is 0, the number of palette is 2 to the power of bit per pixel (e.g. 256 colors for 8-bit format, 16 colors for 4-bit format).

I will modify reader.go to support 8-bit format with up to 256 color palette.

References:

• https://learn.microsoft.com/en-us/windows/win32/api/wingdi/ns-wingdi-bitmapinfoheader
• https://en.wikipedia.org/wiki/BMP_file_format#DIB_header_(bitmap_information_header)

[gopherbot]

Change https://go.dev/cl/508575 mentions this issue: bmp: support to decode 8-bit format with up to 256 color palette