graphics: Add TestImageMipmapNegativeDet

image_test.go

@@ -1047,6 +1047,60 @@ func TestImageMipmap(t *testing.T) {
 	}
 }
 
+func TestImageMipmapNegativeDet(t *testing.T) {
+	src, _, err := openEbitenImage()
+	if err != nil {
+		t.Fatal(err)
+		return
+	}
+	w, h := src.Size()
+
+	l1, _ := NewImage(w/2, h/2, FilterDefault)
+	op := &DrawImageOptions{}
+	op.GeoM.Scale(1/2.0, 1/2.0)
+	op.Filter = FilterLinear
+	l1.DrawImage(src, op)
+
+	l1w, l1h := l1.Size()
+	l2, _ := NewImage(l1w/2, l1h/2, FilterDefault)
+	op = &DrawImageOptions{}
+	op.GeoM.Scale(1/2.0, 1/2.0)
+	op.Filter = FilterLinear
+	l2.DrawImage(l1, op)
+
+	gotDst, _ := NewImage(w, h, FilterDefault)
+	op = &DrawImageOptions{}
+	op.GeoM.Scale(-1/5.0, -1/5.0)
+	op.GeoM.Translate(float64(w), float64(h))
+	op.Filter = FilterLinear
+	gotDst.DrawImage(src, op)
+
+	wantDst, _ := NewImage(w, h, FilterDefault)
+	op = &DrawImageOptions{}
+	op.GeoM.Scale(-4.0/5.0, -4.0/5.0)
+	op.GeoM.Translate(float64(w), float64(h))
+	op.Filter = FilterLinear
+	wantDst.DrawImage(l2, op)
+
+	allZero := true
+	for j := 0; j < h; j++ {
+		for i := 0; i < h; i++ {
+			got := gotDst.At(i, j).(color.RGBA)
+			want := wantDst.At(i, j).(color.RGBA)
+			if !sameColors(got, want, 1) {
+				t.Errorf("At(%d, %d): got: %#v, want: %#v", i, j, got, want)
+			}
+			if got.A > 0 {
+				allZero = false
+			}
+		}
+	}
+
+	if allZero {
+		t.Errorf("the image must include non-zero values but not")
+	}
+}
+
 // Issue #710
 func TestImageMipmapColor(t *testing.T) {
 	img0, _ := NewImage(256, 256, FilterDefault)