image/gif: last color in per-image Palette is different from global Palette

[pierrre]

Go version: 1.5.2 & 1.6beta1

I tried to decode an animated GIF image. (https://raw.githubusercontent.com/pierrre/imageserver/master/testdata/animated.gif)
I get a GIF with a Palette ColorModel, and a slice of Paletted images.
If I compare the global palette, with the palette of each frame, the last color is different.

I wrote a small program:

package main

import (
    "bytes"
    "fmt"
    "image/color"
    "image/gif"
    "io/ioutil"
)

func main() {
    b, err := ioutil.ReadFile("/home/pierre/Go/src/github.com/pierrre/imageserver/testdata/animated.gif")
    if err != nil {
        panic(err)
    }
    g, err := gif.DecodeAll(bytes.NewReader(b))
    if err != nil {
        panic(err)
    }
    if g.Config.ColorModel == nil {
        fmt.Println("no global color model")
        return
    }
    pa, ok := g.Config.ColorModel.(color.Palette)
    if !ok {
        fmt.Println("global color model is not a Palette")
        return
    }
    if len(pa) == 0 {
        fmt.Println("global palette is empty")
        return
    }
    for i, p := range g.Image {
        if len(p.Palette) != len(pa) {
            fmt.Printf("image %d: palette length is not equal: current=%d, global=%d\n", i, len(p.Palette), len(pa))
            continue
        }
        for j := range p.Palette {
            if p.Palette[j] != pa[j] {
                fmt.Printf("image %d: palette index %d: color is not equal: current=%#v, global=%#v\n", i, j, p.Palette[j], pa[j])
            }
        }
    }
    fmt.Printf("background index: %d\n", g.BackgroundIndex)
}

I get:

image 0: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 1: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 2: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 3: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 4: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 5: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 6: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 7: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 8: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 9: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 10: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 11: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 12: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 13: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 14: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 15: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 16: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 17: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 18: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 19: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 20: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 21: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 22: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 23: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 24: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 25: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 26: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 27: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 28: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 29: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 30: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 31: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 32: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 33: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 34: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
image 35: palette index 255: color is not equal: current=color.RGBA{R:0x0, G:0x0, B:0x0, A:0x0}, global=color.RGBA{R:0xff, G:0xff, B:0xff, A:0xff}
background index: 255

At index 255, the color is transparent in all frames, and white in the global palette.

If the ColorModel is defined in GIF, and it's a non empty Palette, I expect to have the same Palette in all frames.

Is it related to the background color index?

Thank you.

[rsc]

Yes, that's probably why. The background color index says that that color is "transparent", meaning R=G=B=A=0. White would be R=G=B=A=0xFF, and black would be R=G=B=0, A=0xFF. So the global palette says white, but the per-image palette correctly says transparent. But I still don't understand why the global palette doesn't report transparent too.

/cc @nigeltao @robpike

[nigeltao]

It's to do with the Transparent Color Index values, not the Background Color Index value. As per the https://www.w3.org/Graphics/GIF/spec-gif89a.txt spec:

Background Color Index (a property of the Global Color Table) is defined in section 18.

Transparent Color Index (a property of a Local Color Table) is defined in section 23. Transparency is important when composing successive frames, using the "Do not dispose" disposal method, but a limitation of the GIF format is that none of the Global Color Table's entries can be transparent.

Note that, even without a Transparent Color Index set, a frame of an animated GIF can have a different ColorModel (i.e. palette) than the overall global ColorModel. See section 21 re Local Color Tables.