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x/image: bmp decode of attached image will result in all alpha values to be 0 #54368

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maaft opened this issue Aug 10, 2022 · 7 comments
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x/image: bmp decode of attached image will result in all alpha values to be 0 #54368

maaft opened this issue Aug 10, 2022 · 7 comments
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FrozenDueToAge WaitingForInfo Issue is not actionable because of missing required information, which needs to be provided.
Milestone
Unreleased

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@maaft
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maaft commented Aug 10, 2022 •
edited

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

$ go version

go version go1.19 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="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"
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-build115352423=/tmp/go-build -gno-record-gcc-switches"

What did you do?

download this BMP image

execute this bmp-to-png conversion "script":

package main

import (
	"bytes"
	"image/png"
	"os"

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

func must(err error) {
	if err != nil {
		panic(err)
	}
}

func main() {

	f, err := os.Open("<path to bmp file>")

	must(err)

	bmp_decode, err := bmp.Decode(f)

	must(err)

	buffer := &bytes.Buffer{}
	err = png.Encode(buffer, bmp_decode)

	must(err)

	err = os.WriteFile("test.png", buffer.Bytes(), 0644)

	must(err)

}

What did you expect to see?

I expect to see the original image in png format or an error while opening the bmp file if its corrupt.

What did you see instead?

I saw no errors and the resulting png image is completely transparent. All alpha values of bmp_decode are 0.
@gopherbot gopherbot added this to the Unreleased milestone Aug 10, 2022
@thanm thanm added the WaitingForInfo Issue is not actionable because of missing required information, which needs to be provided. label Aug 10, 2022
@thanm
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thanm commented Aug 10, 2022

I am getting a 404 on the link cited (https://file.io/3QG9p08hAz71)

@maaft
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maaft commented Aug 16, 2022

@thanm Oh, sorry about that. That provider seems to delete the file after first download. Since I can't upload BMP files here directly, any idea what to use for a hosting service?

@seankhliao
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zip the file and upload here

@maaft
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maaft commented Aug 18, 2022

lol, yes. how could I not think about zipping it -.- thanks
30Sep23-84.BMP.zip

@ZekeLu
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ZekeLu commented Aug 25, 2022

Here is the information of the image:

Offset Size Hex value Value Description
BMP Header
00 2 42 4D "BM" ID field
02 4 96 38 01 00 80022 bytes (0x013896) Size of the BMP file (54 bytes header + 79968 bytes data)
06 2 00 00 Unused Application specific
08 2 00 00 Unused Application specific
0A 4 36 00 00 00 54 bytes (14+40) Offset where the pixel array (bitmap data) can be found
DIB Header (BITMAPINFOHEADER)
0E 4 28 00 00 00 🔴 40 bytes Number of bytes in the DIB header (from this point)
12 4 88 00 00 00 136 pixels Width of the bitmap in pixels
16 4 93 00 00 00 147 pixels Height of the bitmap in pixels
1A 2 01 00 🔴 1 plane Number of color planes being used
1C 2 20 00 🔴 32 bits Number of bits per pixel
1E 4 00 00 00 00 🔴 0 BI_RGB, no pixel array compression used
22 4 60 38 01 00 79968 bytes (0x013860) Size of the raw bitmap data
26 4 00 00 00 00 horizontal resolution Not provided
2A 4 00 00 00 00 vertical resolution
2E 4 00 00 00 00 0 colors Number of colors in the palette
32 4 00 00 00 00 0 important colors 0 means all colors are important
Start of pixel array (bitmap data)
36 4 7A 7A 7A 00 🔴 The last byte (00) in the pixel
is incorrectly treated as the alpha value
3A 4 7A 7A 7A 00
3E 4 78 78 78 00
pixel array truncated...

We are interested in these attributes:

  • The size of the DIB header is 40, which means it's a BITMAPINFOHEADER
  • The bits per pixel (bpp) is 32
  • The type of compression is BI_RGB (no pixel array compression used)

For BITMAPINFOHEADER with compression set to BI_RGB, the alpha channel is not supported (see the references in the end of this comment). So the alpha component in the decoded image data should be 0xFF.

I think this is a bug in the x/image/bmp package. I will send a CL to fix it.

It's interesting that the x/image repository already contains two test images which looks different but decoded to the same image data (covered by a test):

  • testdata/yellow_rose-small.bmp:

  • testdata/yellow_rose-small.png:

My fix will make this test fail. Obviously, the two images look different and in my opinion, the failure is expected. I will modify testdata/yellow_rose-small.png to make it pass again.

BTW, GraphicsMagick 1.3.38 has the same issue. Running the following command will generate a transparent image:

gm convert 30Sep23-84.BMP 30Sep23-84.png

References:

@ZekeLu ZekeLu mentioned this issue Aug 25, 2022
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@gopherbot
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Change https://go.dev/cl/425418 mentions this issue: bmp: ignore alpha in Decode when the alpha channel is not supported

@thanm
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thanm commented Aug 25, 2022

@ZekeLu thanks for sending that CL, looks good to me (although I have very little familiarity with image/bmp).

@golang golang locked and limited conversation to collaborators Sep 2, 2023
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bmp: ignore alpha in Decode when the alpha channel is not supported ZekeLu/image
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@ZekeLu @maaft @gopherbot @seankhliao @thanm