image/gif: allow encoding a single-frame image whose top-left corner

isn't (0, 0).

Also fix a s/b.Min.X/b.Max.X/ typo in bounds checking.

Fixes #10676

Change-Id: Ie5ff7ec20ca30367a8e65d32061959a2d8e089e9
Reviewed-on: https://go-review.googlesource.com/9712Reviewed-by: Rob Pike <r@golang.org>

src/image/gif/writer.go

@@ -190,7 +190,7 @@ func (e *encoder) writeImageBlock(pm *image.Paletted, delay int, disposal byte)
 	}
 
 	b := pm.Bounds()
-	if b.Dx() >= 1<<16 || b.Dy() >= 1<<16 || b.Min.X < 0 || b.Min.X >= 1<<16 || b.Min.Y < 0 || b.Min.Y >= 1<<16 {
+	if b.Min.X < 0 || b.Max.X >= 1<<16 || b.Min.Y < 0 || b.Max.Y >= 1<<16 {
 		e.err = errors.New("gif: image block is too large to encode")
 		return
 	}
@@ -365,6 +365,15 @@ func Encode(w io.Writer, m image.Image, o *Options) error {
 		opts.Drawer.Draw(pm, b, m, image.ZP)
 	}
 
+	// 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.
+	if pm.Rect.Min != (image.Point{}) {
+		dup := *pm
+		dup.Rect = dup.Rect.Sub(dup.Rect.Min)
+		pm = &dup
+	}
+
 	return EncodeAll(w, &GIF{
 		Image: []*image.Paletted{pm},
 		Delay: []int{0},

src/image/gif/writer_test.go

@@ -297,7 +297,7 @@ func TestEncodeZeroGIF(t *testing.T) {
 	}
 }
 
-func TestEncodeFrameOutOfBounds(t *testing.T) {
+func TestEncodeAllFramesOutOfBounds(t *testing.T) {
 	images := []*image.Paletted{
 		image.NewPaletted(image.Rect(0, 0, 5, 5), palette.Plan9),
 		image.NewPaletted(image.Rect(2, 2, 8, 8), palette.Plan9),
@@ -326,6 +326,32 @@ func TestEncodeFrameOutOfBounds(t *testing.T) {
 	}
 }
 
+func TestEncodeNonZeroMinPoint(t *testing.T) {
+	points := []image.Point{
+		image.Point{-8, -9},
+		image.Point{-4, -4},
+		image.Point{-3, +3},
+		image.Point{+0, +0},
+		image.Point{+2, +2},
+	}
+	for _, p := range points {
+		src := image.NewPaletted(image.Rectangle{Min: p, Max: p.Add(image.Point{6, 6})}, palette.Plan9)
+		var buf bytes.Buffer
+		if err := Encode(&buf, src, nil); err != nil {
+			t.Errorf("p=%v: Encode: %v", p, err)
+			continue
+		}
+		m, err := Decode(&buf)
+		if err != nil {
+			t.Errorf("p=%v: Decode: %v", p, err)
+			continue
+		}
+		if got, want := m.Bounds(), image.Rect(0, 0, 6, 6); got != want {
+			t.Errorf("p=%v: got %v, want %v", p, got, want)
+		}
+	}
+}
+
 func TestEncodeImplicitConfigSize(t *testing.T) {
 	// For backwards compatibility for Go 1.4 and earlier code, the Config
 	// field is optional, and if zero, the width and height is implied by the