graphics: Use 'negative' mipmap when enlarging a too small image

This is a hack to render edges correctly.

This works only when the filter is nearest.

Fixes #611

export_test.go

@@ -15,6 +15,6 @@
 package ebiten
 
 var (
-	CopyImage   = copyImage
-	MipmapLevel = mipmapLevel
+	CopyImage               = copyImage
+	MipmapLevelForDownscale = mipmapLevelForDownscale
 )

image.go

@@ -249,22 +249,15 @@ func (i *Image) DrawImage(img *Image, options *DrawImageOptions) error {
 		filter = driver.Filter(img.filter)
 	}
 
-	a, b, c, d, tx, ty := geom.elements()
+	if det := geom.det(); det == 0 {
+		return nil
+	} else if math.IsNaN(float64(det)) {
+		return nil
+	}
 
-	level := 0
-	if filter == driver.FilterLinear && !img.mipmap.original().IsVolatile() {
-		det := geom.det()
-		if det == 0 {
-			return nil
-		}
-		if math.IsNaN(float64(det)) {
-			return nil
-		}
-		level = mipmapLevel(det)
-		if level < 0 {
-			panic(fmt.Sprintf("ebiten: level must be >= 0 but %d", level))
-		}
+	level := img.mipmap.mipmapLevel(geom, bounds.Dx(), bounds.Dy(), filter)
 
+	if level > 0 {
 		// If the image can be scaled into 0 size, adjust the level. (#839)
 		w, h := bounds.Dx(), bounds.Dy()
 		for level >= 0 {
@@ -281,9 +274,13 @@ func (i *Image) DrawImage(img *Image, options *DrawImageOptions) error {
 			return nil
 		}
 	}
+
 	if level > 6 {
 		level = 6
 	}
+	if level < -6 {
+		level = -6
+	}
 
 	// TODO: Add (*mipmap).drawImage and move the below code.
 	colorm := options.ColorM.impl
@@ -297,6 +294,7 @@ func (i *Image) DrawImage(img *Image, options *DrawImageOptions) error {
 		colorm = nil
 	}
 
+	a, b, c, d, tx, ty := geom.elements()
 	if level == 0 {
 		src := img.mipmap.original()
 		vs := vertexSlice(4)
@@ -305,11 +303,11 @@ func (i *Image) DrawImage(img *Image, options *DrawImageOptions) error {
 		i.mipmap.original().DrawTriangles(src, vs, is, colorm, mode, filter, driver.AddressClampToZero)
 	} else if src := img.mipmap.level(bounds, level); src != nil {
 		w, h := src.Size()
-		s := 1 << uint(level)
-		a *= float32(s)
-		b *= float32(s)
-		c *= float32(s)
-		d *= float32(s)
+		s := pow2(level)
+		a *= s
+		b *= s
+		c *= s
+		d *= s
 		vs := vertexSlice(4)
 		graphics.PutQuadVertices(vs, src, 0, 0, w, h, a, b, c, d, tx, ty, cr, cg, cb, ca)
 		is := graphics.QuadIndices()

image_test.go

@@ -940,36 +940,42 @@ func TestImageCopy(t *testing.T) {
 	img1.Fill(color.Transparent)
 }
 
+// Issue #611, #907
 func TestImageStretch(t *testing.T) {
-	img0, _ := NewImage(16, 17, FilterDefault)
+	const w = 16
 
-	pix := make([]byte, 4*16*17)
-	for i := 0; i < 16*16; i++ {
-		pix[4*i] = 0xff
-		pix[4*i+3] = 0xff
-	}
-	for i := 0; i < 16; i++ {
-		pix[4*(16*16+i)+1] = 0xff
-		pix[4*(16*16+i)+3] = 0xff
-	}
-	img0.ReplacePixels(pix)
+	dst, _ := NewImage(w, 4096, FilterDefault)
+loop:
+	for h := 1; h <= 32; h++ {
+		src, _ := NewImage(w, h+1, FilterDefault)
 
-	// TODO: 4096 doesn't pass on MacBook Pro (#611).
-	const h = 4000
-	img1, _ := NewImage(16, h, FilterDefault)
-	for i := 1; i < h; i++ {
-		img1.Clear()
-		op := &DrawImageOptions{}
-		op.GeoM.Scale(1, float64(i)/16)
-		img1.DrawImage(img0.SubImage(image.Rect(0, 0, 16, 16)).(*Image), op)
-		for j := -1; j <= 1; j++ {
-			got := img1.At(0, i+j).(color.RGBA)
-			want := color.RGBA{}
-			if j < 0 {
-				want = color.RGBA{0xff, 0, 0, 0xff}
-			}
-			if got != want {
-				t.Fatalf("At(%d, %d) (i=%d): got: %#v, want: %#v", 0, i+j, i, got, want)
+		pix := make([]byte, 4*w*(h+1))
+		for i := 0; i < w*h; i++ {
+			pix[4*i] = 0xff
+			pix[4*i+3] = 0xff
+		}
+		for i := 0; i < w; i++ {
+			pix[4*(w*h+i)+1] = 0xff
+			pix[4*(w*h+i)+3] = 0xff
+		}
+		src.ReplacePixels(pix)
+
+		_, dh := dst.Size()
+		for i := 32; i < dh; i += 32 {
+			dst.Clear()
+			op := &DrawImageOptions{}
+			op.GeoM.Scale(1, float64(i)/float64(h))
+			dst.DrawImage(src.SubImage(image.Rect(0, 0, w, h)).(*Image), op)
+			for j := -2; j <= 2; j++ {
+				got := dst.At(0, i+j).(color.RGBA)
+				want := color.RGBA{}
+				if j < 0 {
+					want = color.RGBA{0xff, 0, 0, 0xff}
+				}
+				if got != want {
+					t.Errorf("At(%d, %d) (height=%d, scale=%d/%d): got: %#v, want: %#v", 0, i+j, h, i, h, got, want)
+					continue loop
+				}
 			}
 		}
 	}

mipmap.go

@@ -15,6 +15,7 @@
 package ebiten
 
 import (
+	"fmt"
 	"image"
 	"math"
 
@@ -42,8 +43,8 @@ func (m *mipmap) original() *shareable.Image {
 }
 
 func (m *mipmap) level(r image.Rectangle, level int) *shareable.Image {
-	if level <= 0 {
-		panic("ebiten: level must be positive at level")
+	if level == 0 {
+		panic("ebiten: level must be non-zero at level")
 	}
 
 	if m.orig.IsVolatile() {
@@ -63,30 +64,50 @@ func (m *mipmap) level(r image.Rectangle, level int) *shareable.Image {
 	w, h := size.X, size.Y
 
 	w2, h2 := w, h
-	for i := 0; i < level; i++ {
-		w2 /= 2
-		h2 /= 2
-		if w == 0 || h == 0 {
-			imgs[level] = nil
-			return nil
+	if level > 0 {
+		for i := 0; i < level; i++ {
+			w2 /= 2
+			h2 /= 2
+			if w == 0 || h == 0 {
+				imgs[level] = nil
+				return nil
+			}
+		}
+	} else {
+		for i := 0; i < -level; i++ {
+			w2 *= 2
+			h2 *= 2
 		}
 	}
-	s := shareable.NewImage(w2, h2)
+
 	var src *shareable.Image
 	vs := vertexSlice(4)
-	if level == 1 {
+	var filter driver.Filter
+	switch {
+	case level == 1:
 		src = m.orig
 		graphics.PutQuadVertices(vs, src, r.Min.X, r.Min.Y, r.Max.X, r.Max.Y, 0.5, 0, 0, 0.5, 0, 0, 1, 1, 1, 1)
-	} else {
+		filter = driver.FilterLinear
+	case level > 1:
 		src = m.level(r, level-1)
 		if src == nil {
 			imgs[level] = nil
 			return nil
 		}
 		graphics.PutQuadVertices(vs, src, 0, 0, w, h, 0.5, 0, 0, 0.5, 0, 0, 1, 1, 1, 1)
+		filter = driver.FilterLinear
+	case level < 0:
+		src = m.orig
+		s := pow2(-level)
+		graphics.PutQuadVertices(vs, src, r.Min.X, r.Min.Y, r.Max.X, r.Max.Y, s, 0, 0, s, 0, 0, 1, 1, 1, 1)
+		filter = driver.FilterNearest
+	default:
+		panic(fmt.Sprintf("ebiten: invalid level: %d", level))
 	}
 	is := graphics.QuadIndices()
-	s.DrawTriangles(src, vs, is, nil, driver.CompositeModeCopy, driver.FilterLinear, driver.AddressClampToZero)
+
+	s := shareable.NewImage(w2, h2)
+	s.DrawTriangles(src, vs, is, nil, driver.CompositeModeCopy, filter, driver.AddressClampToZero)
 	imgs[level] = s
 
 	return imgs[level]
@@ -123,17 +144,65 @@ func (m *mipmap) resetRestoringState() {
 
 // mipmapLevel returns an appropriate mipmap level for the given determinant of a geometry matrix.
 //
-// mipmapLevel returns -1 if det is 0.
-//
-// mipmapLevel panics if det is NaN.
-func mipmapLevel(det float32) int {
+// mipmapLevel panics if det is NaN or 0.
+func (m *mipmap) mipmapLevel(geom *GeoM, width, height int, filter driver.Filter) int {
+	det := geom.det()
 	if math.IsNaN(float64(det)) {
-		panic("graphicsutil: det must be finite")
+		panic("ebiten: det must be finite at mipmapLevel")
 	}
 	if det == 0 {
-		return -1
+		panic("ebiten: dst must be non zero at mipmapLevel")
 	}
 
+	// Use 'negative' mipmap to render edges correctly (#611, #907).
+	// It looks like 256 is the enlargement factor that causes edge missings to pass the test TestImageStretch.
+	const tooBigScale = 256
+	if sx, sy := geomScaleSize(geom); sx >= tooBigScale || sy >= tooBigScale {
+		// If the filter is not nearest, the target needs to be rendered with gradiation. Don't use mipmaps.
+		if filter != driver.FilterNearest {
+			return 0
+		}
+
+		const mipmapMaxSize = 1024
+		w, h := width, height
+		if w >= mipmapMaxSize || h >= mipmapMaxSize {
+			return 0
+		}
+
+		level := 0
+		for sx >= tooBigScale || sy >= tooBigScale {
+			level--
+			sx /= 2
+			sy /= 2
+			w *= 2
+			h *= 2
+			if w >= mipmapMaxSize || h >= mipmapMaxSize {
+				break
+			}
+		}
+		return level
+	}
+
+	if filter != driver.FilterLinear {
+		return 0
+	}
+	if m.original().IsVolatile() {
+		return 0
+	}
+
+	// This is a separate function for testing.
+	return mipmapLevelForDownscale(det)
+}
+
+func mipmapLevelForDownscale(det float32) int {
+	if math.IsNaN(float64(det)) {
+		panic("ebiten: det must be finite at mipmapLevelForDownscale")
+	}
+	if det == 0 {
+		panic("ebiten: dst must be non zero at mipmapLevelForDownscale")
+	}
+
+	// TODO: Should this be determined by x/y scales instead of det?
 	d := math.Abs(float64(det))
 	level := 0
 	for d < 0.25 {
@@ -142,3 +211,58 @@ func mipmapLevel(det float32) int {
 	}
 	return level
 }
+
+func pow2(power int) float32 {
+	if power >= 0 {
+		x := 1
+		return float32(x << uint(power))
+	}
+
+	x := float32(1)
+	for i := 0; i < -power; i++ {
+		x /= 2
+	}
+	return x
+}
+
+func maxf32(values ...float32) float32 {
+	max := float32(math.Inf(-1))
+	for _, v := range values {
+		if max < v {
+			max = v
+		}
+	}
+	return max
+}
+
+func minf32(values ...float32) float32 {
+	min := float32(math.Inf(1))
+	for _, v := range values {
+		if min > v {
+			min = v
+		}
+	}
+	return min
+}
+
+func geomScaleSize(geom *GeoM) (sx, sy float32) {
+	a, b, c, d, _, _ := geom.elements()
+	// (0, 1)
+	x0 := 0*a + 1*b
+	y0 := 0*c + 1*d
+
+	// (1, 0)
+	x1 := 1*a + 0*b
+	y1 := 1*c + 0*d
+
+	// (1, 1)
+	x2 := 1*a + 1*b
+	y2 := 1*c + 1*d
+
+	maxx := maxf32(0, x0, x1, x2)
+	maxy := maxf32(0, y0, y1, y2)
+	minx := minf32(0, x0, x1, x2)
+	miny := minf32(0, y0, y1, y2)
+
+	return maxx - minx, maxy - miny
+}

mipmap_test.go

@@ -21,13 +21,12 @@ import (
 	. "github.com/hajimehoshi/ebiten"
 )
 
-func TestMipmapLevel(t *testing.T) {
+func TestMipmapLevelForDownscale(t *testing.T) {
 	inf := float32(math.Inf(1))
 	cases := []struct {
 		In  float32
 		Out int
 	}{
-		{0, -1},
 		{1, 0},
 		{-1, 0},
 		{2, 0},
@@ -55,7 +54,7 @@ func TestMipmapLevel(t *testing.T) {
 	}
 
 	for _, c := range cases {
-		got := MipmapLevel(c.In)
+		got := MipmapLevelForDownscale(c.In)
 		want := c.Out
 		if got != want {
 			t.Errorf("MipmapLevel(%v): got %v, want %v", c.In, got, want)