func animateDeadGopher(a *f32.Affine, t clock.Time) {
	dt := float32(t)
	a.Scale(a, 1+dt/20, 1+dt/20)
	a.Translate(a, 0.5, 0.5)
	a.Rotate(a, dt/math.Pi/-8)
	a.Translate(a, -0.5, -0.5)
}

func (a *arrangerFunc) Arrange(e s.Engine, n *s.Node, t clock.Time) {
	sprite, _ := a.eng.sprites[n]
	frameTime := float32(t - a.eng.lastUpdate)
	updatePosition(sprite, frameTime)

	screenWidthScaler, screenHeightScaler := a.eng.GetScreenScalers()
	actualScaleX := screenWidthScaler * sprite.ScaleX
	actualScaleY := screenHeightScaler * sprite.ScaleY
	actualPositionX := screenWidthScaler * sprite.X
	actualPositionY := screenHeightScaler * sprite.Y

	e.SetSubTex(n, *sprite.GetCurrentFrame().Texture)

	r := sprite.Rotation * math.Pi / 180
	matrix := f32.Affine{
		{1, 0, 0},
		{0, 1, 0},
	}

	matrix.Translate(&matrix, actualPositionX, actualPositionY)
	//matrix.Translate(&matrix, sprite.X, sprite.Y)
	matrix.Rotate(&matrix, r)
	matrix.Scale(&matrix, actualScaleX, actualScaleY)
	e.SetTransform(n, matrix)

	a.eng.lastUpdate = t
}

func (e *engine) render(n *sprite.Node, t clock.Time, sz size.Event) {
	if n.EngineFields.Index == 0 {
		panic("glsprite: sprite.Node not registered")
	}
	if n.Arranger != nil {
		n.Arranger.Arrange(e, n, t)
	}

	// Push absTransforms.
	// TODO: cache absolute transforms and use EngineFields.Dirty?
	rel := &e.nodes[n.EngineFields.Index].relTransform
	m := f32.Affine{}
	m.Mul(&e.absTransforms[len(e.absTransforms)-1], rel)
	e.absTransforms = append(e.absTransforms, m)

	if x := n.EngineFields.SubTex; x.T != nil {
		x.T.(*texture).glImage.Draw(
			sz,
			geom.Point{
				geom.Pt(m[0][2]),
				geom.Pt(m[1][2]),
			},
			geom.Point{
				geom.Pt(m[0][2] + m[0][0]),
				geom.Pt(m[1][2] + m[1][0]),
			},
			geom.Point{
				geom.Pt(m[0][2] + m[0][1]),
				geom.Pt(m[1][2] + m[1][1]),
			},
			x.R,
		)
	}

	for c := n.FirstChild; c != nil; c = c.NextSibling {
		e.render(c, t, sz)
	}

	// Pop absTransforms.
	e.absTransforms = e.absTransforms[:len(e.absTransforms)-1]
}

func TestAffineMask(t *testing.T) {
	f, err := os.Open("../../../testdata/testpattern.png")
	if err != nil {
		t.Fatal(err)
	}
	defer f.Close()
	srcOrig, _, err := image.Decode(f)
	if err != nil {
		t.Fatal(err)
	}
	b := srcOrig.Bounds()
	src := image.NewRGBA(b)
	draw.Draw(src, src.Rect, srcOrig, b.Min, draw.Src)
	mask := image.NewAlpha(b)
	for y := b.Min.Y; y < b.Max.Y; y++ {
		for x := b.Min.X; x < b.Max.X; x++ {
			mask.Set(x, y, color.Alpha{A: uint8(x - b.Min.X)})
		}
	}
	want := image.NewRGBA(b)
	draw.DrawMask(want, want.Rect, src, b.Min, mask, b.Min, draw.Src)

	a := new(f32.Affine)
	a.Identity()
	got := image.NewRGBA(b)
	affine(got, src, b, mask, a, draw.Src)

	if !imageEq(got, want) {
		gotPath, err := writeTempPNG("testpattern-mask-got", got)
		if err != nil {
			t.Fatal(err)
		}
		wantPath, err := writeTempPNG("testpattern-mask-want", want)
		if err != nil {
			t.Fatal(err)
		}
		t.Errorf("got\n%s\nwant\n%s", gotPath, wantPath)
	}
}

func (e *engine) render(n *sprite.Node, t clock.Time) {
	if n.EngineFields.Index == 0 {
		panic("portable: sprite.Node not registered")
	}
	if n.Arranger != nil {
		n.Arranger.Arrange(e, n, t)
	}

	// Push absTransforms.
	// TODO: cache absolute transforms and use EngineFields.Dirty?
	rel := &e.nodes[n.EngineFields.Index].relTransform
	m := f32.Affine{}
	m.Mul(&e.absTransforms[len(e.absTransforms)-1], rel)
	e.absTransforms = append(e.absTransforms, m)

	if x := n.EngineFields.SubTex; x.T != nil {
		// Affine transforms work in geom.Pt, which is entirely
		// independent of the number of pixels in a texture. A texture
		// of any image.Rectangle bounds rendered with
		//
		//	Affine{{1, 0, 0}, {0, 1, 0}}
		//
		// should have the dimensions (1pt, 1pt). To do this we divide
		// by the pixel width and height, reducing the texture to
		// (1px, 1px) of the destination image. Multiplying by
		// cfg.PixelsPerPt, done in Render above, makes it (1pt, 1pt).
		dx, dy := x.R.Dx(), x.R.Dy()
		if dx > 0 && dy > 0 {
			m.Scale(&m, 1/float32(dx), 1/float32(dy))
			// TODO(nigeltao): delete the double-inverse: one here and one
			// inside func affine.
			m.Inverse(&m) // See the documentation on the affine function.
			affine(e.dst, x.T.(*texture).m, x.R, nil, &m, draw.Over)
		}
	}

	for c := n.FirstChild; c != nil; c = c.NextSibling {
		e.render(c, t)
	}

	// Pop absTransforms.
	e.absTransforms = e.absTransforms[:len(e.absTransforms)-1]
}

func TestAffine(t *testing.T) {
	f, err := os.Open("../../../testdata/testpattern.png")
	if err != nil {
		t.Fatal(err)
	}
	defer f.Close()
	srcOrig, _, err := image.Decode(f)
	if err != nil {
		t.Fatal(err)
	}
	src := image.NewRGBA(srcOrig.Bounds())
	draw.Draw(src, src.Rect, srcOrig, srcOrig.Bounds().Min, draw.Src)

	const (
		pixW = 100
		pixH = 100
		ptW  = geom.Pt(50)
		ptH  = geom.Pt(50)
	)
	cfg := event.Config{
		Width:       ptW,
		Height:      ptH,
		PixelsPerPt: float32(pixW) / float32(ptW),
	}

	got := image.NewRGBA(image.Rect(0, 0, pixW, pixH))
	blue := image.NewUniform(color.RGBA{B: 0xff, A: 0xff})
	draw.Draw(got, got.Bounds(), blue, image.Point{}, draw.Src)

	b := src.Bounds()
	b.Min.X += 10
	b.Max.Y /= 2

	var a f32.Affine
	a.Identity()
	a.Scale(&a, cfg.PixelsPerPt, cfg.PixelsPerPt)
	a.Translate(&a, 0, 24)
	a.Rotate(&a, float32(math.Asin(12./20)))
	// See commentary in the render method defined in portable.go.
	a.Scale(&a, 40/float32(b.Dx()), 20/float32(b.Dy()))
	a.Inverse(&a)

	affine(got, src, b, nil, &a, draw.Over)

	ptTopLeft := geom.Point{0, 24}
	ptBottomRight := geom.Point{12 + 32, 16}

	drawCross(got, 0, 0)
	drawCross(got, int(ptTopLeft.X.Px(cfg.PixelsPerPt)), int(ptTopLeft.Y.Px(cfg.PixelsPerPt)))
	drawCross(got, int(ptBottomRight.X.Px(cfg.PixelsPerPt)), int(ptBottomRight.Y.Px(cfg.PixelsPerPt)))
	drawCross(got, pixW-1, pixH-1)

	const wantPath = "../../../testdata/testpattern-window.png"
	f, err = os.Open(wantPath)
	if err != nil {
		t.Fatal(err)
	}
	defer f.Close()
	wantSrc, _, err := image.Decode(f)
	if err != nil {
		t.Fatal(err)
	}
	want, ok := wantSrc.(*image.RGBA)
	if !ok {
		b := wantSrc.Bounds()
		want = image.NewRGBA(b)
		draw.Draw(want, b, wantSrc, b.Min, draw.Src)
	}

	if !imageEq(got, want) {
		gotPath, err := writeTempPNG("testpattern-window-got", got)
		if err != nil {
			t.Fatal(err)
		}
		t.Errorf("got\n%s\nwant\n%s", gotPath, wantPath)
	}
}