image/gif: transparency lost when encoding a wrapped *image.Paletted

[splace]

go 1.12.1 linux/amd64

What did you do?

package main

import "image"
import "image/png"
import "image/gif"
import "os"
import "encoding/base64"
import "strings"


type offsetImage struct{
	image.Image
	Rect image.Rectangle	
}

func (i offsetImage) Bounds() image.Rectangle{
	return i.Rect
}

func main() {
	tgif:=`R0lGODlhAAGAAPcAAAAAAP///1UATQBFAE4AVABTAFwAUABJAFIAQQBUAEUAIABGAEwAQQBHAC4A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=`

	i,_,err:=image.Decode(base64.NewDecoder(base64.StdEncoding, strings.NewReader(tgif)))

	if err!=nil{panic(err)}

	w,_:= os.Create("1.png")
	png.Encode(w,i)
	w,_= os.Create("1.gif")
	gif.Encode(w,i,nil)

	i2:=offsetImage{i,i.Bounds()}   // no offset, same image

	w,_= os.Create("2.png")
	png.Encode(w,i2)
	w,_= os.Create("2.gif")
	gif.Encode(w,i2,nil)
	
}

What did you expect to see?

images the same

What did you see instead?

gif has transparency converted to black, png retains origin transparency.

output:

• 
• 
• 
• 

[dmitshur]

@splace Thanks for the issue report.

When you say "images the same" under "what did you expect", can you clarify? There are 4 images, which ones did you expect to be the same?

[splace]

the images are in the order the code produced them, they should all be the same since the image is not being modified at any point.

details....
the image (made from inline gif in base64) has transparency, when this is saved as a png or a gif the transparency is there in both files, but when the image is embedded in a new type, offsetImage, (still the same Image to the interface) then saving as png is fine, but saved as gif causes the transparency to be replaced by black. so it seems the gif encoder is in some way 'bypassing' the interface, being aware and varying what it does depending on the specific type.

[dmitshur]

The concrete type returned by image.Decode in your snippet is *image.Paletted.

I suspect this issue is happening because wrapping the *image.Paletted in a concrete type such as offsetImage makes the type assertion in gif.Encode no longer report positively:

go/src/image/gif/writer.go

Line 436 in c8aaec2

pm, ok := m.(*image.Paletted)

/cc @robpike @nigeltao Do you think this is working as intended, or is this a bug in encoding/gif? Should it handle situations where the concrete type of the passed image.Image is not precisely *image.Paletted the same as if it were?

[splace]

FYI this was encountered when i put together a simple image geometry transforming utility. (any supported format input, any output.) with the very simple technique of wrapping the Image and modifying the x and y of the method At(x,y) then calling the embedded Image. works as expected, except for this.

[nigeltao]

Yeah, it's a bug in encoding/gif, although not one that's trivial to fix. The

pm, ok := m.(*image.Paletted)
if !ok || len(pm.Palette) > opts.NumColors {
  etc
}

check should be something like

pm, _ := m.(*image.Paletted)
if pm == nil {
  // Either one or both of
  if pm2, ok := m.(image.PalettedImage) {
    // Create a new pm, based on pm2.
    // Note that image.PalettedImage is an interface; image.Paletted is a concrete type.
    // Yeah, the names are confusing. In hindsight, I'd do it differently.
  }
  // or
  if _, ok := m.ColorModel().(color.Palette); ok {
    // Create a new pm, based on m.
  }
}
if (pm == nil) || (len(pm.Palette) > opts.NumColors) {
  etc
}

But that's not great. The first approach might not be feasible if it requires your wrapper type to also or conditionally implement the image.PalettedImage interface.

The second approach would probably work. It won't be super-efficient to, for each pixel, call At and look up the color.Palette to turn it back into an index. But given the Go 1.x image interfaces that we have, it could be feasible.

[rsc]

@nigeltao, what do you suggest we do?