image/gif: Encode does not preserve Bounds.Min

[dvyukov]

The following program fails:

package main

import (
    "bytes"
    "fmt"
    "image/gif"
    "reflect"
)

func main() {
    data := []byte("GIF89a\n\x0000000,\x00\x000\x00\n\x00\n\x00\x80000000\x02\b\xf01u\xb9\xfdal\x05\x00;")
    img, err := gif.Decode(bytes.NewReader(data))
    if err != nil {
        panic(err)
    }
    var w bytes.Buffer
    err = gif.Encode(&w, img, nil)
    if err != nil {
        panic(err)
    }
    img1, err := gif.Decode(&w)
    if err != nil {
        panic(err)
    }
    if !reflect.DeepEqual(img.Bounds(), img1.Bounds()) {
        fmt.Printf("img0: %#v\n", img.Bounds())
        fmt.Printf("img1: %#v\n", img1.Bounds())
        panic("bounds changed")
    }
}

img0: image.Rectangle{Min:image.Point{X:0, Y:48}, Max:image.Point{X:10, Y:58}}
img1: image.Rectangle{Min:image.Point{X:0, Y:0}, Max:image.Point{X:10, Y:10}}
panic: bounds changed

Bounds.Min should be preserved by Encode.

go version devel +b0532a9 Mon Jun 8 05:13:15 2015 +0000 linux/amd64

[jeffallen]

Works as designed:

// When calling Encode instead of EncodeAll, the single-frame image is
// translated such that its top-left corner is (0, 0), so that the single
// frame completely fills the overall GIF's bounds.

This test of a round trip through DecodeAll/EncodeAll passes:

func TestRoundtrip(t *testing.T) {
data := []byte("GIF89a\n\x0000000,\x00\x000\x00\n\x00\n\x00\x80000000\x02\b\xf01u\xb9\xfdal\x05\x00;")
gif, err := DecodeAll(bytes.NewReader(data))
if err != nil {
t.Fatal(err)
}
var w bytes.Buffer
err = EncodeAll(&w, gif)
if err != nil {
t.Fatal(err)
}
gif1, err := DecodeAll(&w)
if err != nil {
t.Fatal(err)
}
if len(gif.Image) != len(gif1.Image) {
t.Fatal("wrong number of images")
}

if !reflect.DeepEqual(gif.Image[0].Bounds(), gif1.Image[0].Bounds()) {
    t.Fatal("bounds changed: %v -> %v", gif.Image[0].Bounds(), gif1.Image[0].Bounds())
}

}