if d < 0 {
		return -d
	}
	return d
}

func readPng(filename string) (image.Image, error) {
	f, err := os.Open(filename)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	return png.Decode(f)
}

func TestWriter(t *testing.T) {
	for _, tc := range testCase {
		// Read the image.
		m0, err := readPng(tc.filename)
		if err != nil {
			t.Error(tc.filename, err)
			continue

		names = filenamesShort
	}
	for _, fn := range names {
		qfn := "testdata/pngsuite/" + fn + ".png"
		// Read the image.
		m0, err := readPNG(qfn)
		if err != nil {
			t.Error(fn, err)
			continue
		}
		// Read the image again, encode it, and decode it.
		m1, err := readPNG(qfn)
		if err != nil {
			t.Error(fn, err)
			continue
		}
		m2, err := encodeDecode(m1)
		if err != nil {
			t.Error(fn, err)

		}
		if pal == nil {
			t.Errorf("%s: palette not initialized", fn)
			continue
		}
	}
}

func TestInterlaced(t *testing.T) {
	a, err := readPNG("testdata/gray-gradient.png")
	if err != nil {
		t.Fatal(err)
	}
	b, err := readPNG("testdata/gray-gradient.interlaced.png")
	if err != nil {
		t.Fatal(err)
	}
	if !reflect.DeepEqual(a, b) {