diff --git a/image_test.go b/image_test.go
index d24e16df9..38fefaae9 100644
--- a/image_test.go
+++ b/image_test.go
@@ -94,7 +94,7 @@ func TestImagePixels(t *testing.T) {
 
 	w, h := img0.Bounds().Size().X, img0.Bounds().Size().Y
 	// Check out of range part
-	w2, h2 := graphics.NextPowerOf2Int(w), graphics.NextPowerOf2Int(h)
+	w2, h2 := graphics.InternalImageSize(w), graphics.InternalImageSize(h)
 	for j := -100; j < h2+100; j++ {
 		for i := -100; i < w2+100; i++ {
 			got := img0.At(i, j)
diff --git a/internal/graphics/math.go b/internal/graphics/math.go
index ad52c8630..d222aed35 100644
--- a/internal/graphics/math.go
+++ b/internal/graphics/math.go
@@ -14,10 +14,19 @@
 
 package graphics
 
-// NextPowerOf2Int returns a nearest power of 2 to x.
-func NextPowerOf2Int(x int) int {
+// minInternalImageSize is the minimum size of internal images (texture/framebuffer).
+//
+// For example, the image size less than 15 is not supported on some iOS devices.
+// See also: https://stackoverflow.com/questions/15935651/certain-framebuffer-sizes-fail-on-ios-devices-gl-framebuffer-unsupported
+const minInternalImageSize = 16
+
+// InternalImageSize returns a nearest appropriate size as an internal image.
+func InternalImageSize(x int) int {
 	if x <= 0 {
-		panic("x must be positive")
+		panic("graphics: x must be positive")
+	}
+	if x < minInternalImageSize {
+		return minInternalImageSize
 	}
 	r := 1
 	for r < x {
@@ -25,3 +34,13 @@ func NextPowerOf2Int(x int) int {
 	}
 	return r
 }
+
+func isInternalImageSize(x int) bool {
+	if x <= 0 {
+		return false
+	}
+	if x < minInternalImageSize {
+		return false
+	}
+	return (x & (x - 1)) == 0
+}
diff --git a/internal/graphics/math_test.go b/internal/graphics/math_test.go
index 39b665d55..65abe8424 100644
--- a/internal/graphics/math_test.go
+++ b/internal/graphics/math_test.go
@@ -20,7 +20,7 @@ import (
 	. "github.com/hajimehoshi/ebiten/internal/graphics"
 )
 
-func TestNextPowerOf2(t *testing.T) {
+func TestInternalImageSize(t *testing.T) {
 	testCases := []struct {
 		expected int
 		arg      int
@@ -31,7 +31,7 @@ func TestNextPowerOf2(t *testing.T) {
 	}
 
 	for _, testCase := range testCases {
-		got := NextPowerOf2Int(testCase.arg)
+		got := InternalImageSize(testCase.arg)
 		wanted := testCase.expected
 		if wanted != got {
 			t.Errorf("Clp(%d) = %d, wanted %d", testCase.arg, got, wanted)
diff --git a/internal/graphics/vertices.go b/internal/graphics/vertices.go
index d26a2abb9..a76260cdd 100644
--- a/internal/graphics/vertices.go
+++ b/internal/graphics/vertices.go
@@ -59,19 +59,14 @@ func (v *verticesBackend) slice(n int) []float32 {
 	return s
 }
 
-func isPowerOf2(x int) bool {
-	if x <= 0 {
-		return false
-	}
-	return (x & (x - 1)) == 0
-}
-
 func QuadVertices(width, height int, sx0, sy0, sx1, sy1 int, a, b, c, d, tx, ty float32, cr, cg, cb, ca float32) []float32 {
-	if !isPowerOf2(width) {
-		panic(fmt.Sprintf("graphics: width must be power of 2 but not at QuadVertices: %d", width))
+	// For performance reason, graphics.InternalImageSize is not applied to width/height here.
+
+	if !isInternalImageSize(width) {
+		panic(fmt.Sprintf("graphics: width must be an internal image size at QuadVertices: %d", width))
 	}
-	if !isPowerOf2(height) {
-		panic(fmt.Sprintf("graphics: height must be power of 2 but not at QuadVertices: %d", height))
+	if !isInternalImageSize(height) {
+		panic(fmt.Sprintf("graphics: height must be an internal image size at QuadVertices: %d", height))
 	}
 
 	if sx0 >= sx1 || sy0 >= sy1 {
@@ -166,11 +161,11 @@ func QuadIndices() []uint16 {
 }
 
 func PutVertex(vs []float32, width, height int, dx, dy, su, sv float32, u0, v0, u1, v1 float32, cr, cg, cb, ca float32) {
-	if !isPowerOf2(width) {
-		panic(fmt.Sprintf("graphics: width must be power of 2 but not at PutVertices: %d", width))
+	if !isInternalImageSize(width) {
+		panic(fmt.Sprintf("graphics: width must be an internal image size at PutVertices: %d", width))
 	}
-	if !isPowerOf2(height) {
-		panic(fmt.Sprintf("graphics: height must be power of 2 but not at PutVertices: %d", height))
+	if !isInternalImageSize(height) {
+		panic(fmt.Sprintf("graphics: height must be an internal image size at PutVertices: %d", height))
 	}
 
 	vs[0] = dx
diff --git a/internal/graphicsdriver/metal/driver.go b/internal/graphicsdriver/metal/driver.go
index a3a1df79f..51e24b7a1 100644
--- a/internal/graphicsdriver/metal/driver.go
+++ b/internal/graphicsdriver/metal/driver.go
@@ -340,10 +340,10 @@ func (d *Driver) checkSize(width, height int) {
 	})
 
 	if width < 1 {
-		panic(fmt.Sprintf("metal: width (%d) must be equal or more than 1", width))
+		panic(fmt.Sprintf("metal: width (%d) must be equal or more than %d", width, 1))
 	}
 	if height < 1 {
-		panic(fmt.Sprintf("metal: height (%d) must be equal or more than 1", height))
+		panic(fmt.Sprintf("metal: height (%d) must be equal or more than %d", height, 1))
 	}
 	if width > m {
 		panic(fmt.Sprintf("metal: width (%d) must be less than or equal to %d", width, m))
@@ -357,8 +357,8 @@ func (d *Driver) NewImage(width, height int) (graphicsdriver.Image, error) {
 	d.checkSize(width, height)
 	td := mtl.TextureDescriptor{
 		PixelFormat: mtl.PixelFormatRGBA8UNorm,
-		Width:       graphics.NextPowerOf2Int(width),
-		Height:      graphics.NextPowerOf2Int(height),
+		Width:       graphics.InternalImageSize(width),
+		Height:      graphics.InternalImageSize(height),
 		StorageMode: mtl.StorageModeManaged,
 
 		// MTLTextureUsageRenderTarget might cause a problematic render result. Not sure the reason.
@@ -581,8 +581,8 @@ func (d *Driver) Draw(indexLen int, indexOffset int, mode graphics.CompositeMode
 		rce.SetVertexBytes(unsafe.Pointer(&viewportSize[0]), unsafe.Sizeof(viewportSize), 1)
 
 		sourceSize := [...]float32{
-			float32(graphics.NextPowerOf2Int(d.src.width)),
-			float32(graphics.NextPowerOf2Int(d.src.height)),
+			float32(graphics.InternalImageSize(d.src.width)),
+			float32(graphics.InternalImageSize(d.src.height)),
 		}
 		rce.SetFragmentBytes(unsafe.Pointer(&sourceSize[0]), unsafe.Sizeof(sourceSize), 2)
 
@@ -644,7 +644,7 @@ func (i *Image) viewportSize() (int, int) {
 	if i.screen {
 		return i.width, i.height
 	}
-	return graphics.NextPowerOf2Int(i.width), graphics.NextPowerOf2Int(i.height)
+	return graphics.InternalImageSize(i.width), graphics.InternalImageSize(i.height)
 }
 
 func (i *Image) Dispose() {
diff --git a/internal/graphicsdriver/opengl/driver.go b/internal/graphicsdriver/opengl/driver.go
index a9eb2f4ac..25ddaf506 100644
--- a/internal/graphicsdriver/opengl/driver.go
+++ b/internal/graphicsdriver/opengl/driver.go
@@ -39,10 +39,10 @@ func (d *Driver) SetWindow(window uintptr) {
 
 func (d *Driver) checkSize(width, height int) {
 	if width < 1 {
-		panic(fmt.Sprintf("opengl: width (%d) must be equal or more than 1", width))
+		panic(fmt.Sprintf("opengl: width (%d) must be equal or more than %d", width, 1))
 	}
 	if height < 1 {
-		panic(fmt.Sprintf("opengl: height (%d) must be equal or more than 1", height))
+		panic(fmt.Sprintf("opengl: height (%d) must be equal or more than %d", height, 1))
 	}
 	m := d.context.getMaxTextureSize()
 	if width > m {
@@ -59,8 +59,8 @@ func (d *Driver) NewImage(width, height int) (graphicsdriver.Image, error) {
 		width:  width,
 		height: height,
 	}
-	w := graphics.NextPowerOf2Int(width)
-	h := graphics.NextPowerOf2Int(height)
+	w := graphics.InternalImageSize(width)
+	h := graphics.InternalImageSize(height)
 	d.checkSize(w, h)
 	t, err := d.context.newTexture(w, h)
 	if err != nil {
diff --git a/internal/graphicsdriver/opengl/image.go b/internal/graphicsdriver/opengl/image.go
index 0bb5e58a2..216f285c4 100644
--- a/internal/graphicsdriver/opengl/image.go
+++ b/internal/graphicsdriver/opengl/image.go
@@ -76,7 +76,7 @@ func (i *Image) ensureFramebuffer() error {
 		return nil
 	}
 
-	w, h := graphics.NextPowerOf2Int(i.width), graphics.NextPowerOf2Int(i.height)
+	w, h := graphics.InternalImageSize(i.width), graphics.InternalImageSize(i.height)
 	f, err := newFramebufferFromTexture(&i.driver.context, i.textureNative, w, h)
 	if err != nil {
 		return err
diff --git a/internal/graphicsdriver/opengl/program.go b/internal/graphicsdriver/opengl/program.go
index 99be1717c..9255ea37b 100644
--- a/internal/graphicsdriver/opengl/program.go
+++ b/internal/graphicsdriver/opengl/program.go
@@ -310,8 +310,8 @@ func (d *Driver) useProgram(mode graphics.CompositeMode, colorM *affine.ColorM,
 		d.state.lastColorMatrixTranslation = esTranslate
 	}
 
-	sw := graphics.NextPowerOf2Int(srcW)
-	sh := graphics.NextPowerOf2Int(srcH)
+	sw := graphics.InternalImageSize(srcW)
+	sh := graphics.InternalImageSize(srcH)
 
 	if d.state.lastSourceWidth != sw || d.state.lastSourceHeight != sh {
 		d.context.uniformFloats(program, "source_size", []float32{float32(sw), float32(sh)})
diff --git a/internal/restorable/image.go b/internal/restorable/image.go
index 210d84edc..2bab16cd2 100644
--- a/internal/restorable/image.go
+++ b/internal/restorable/image.go
@@ -230,8 +230,8 @@ func (i *Image) Size() (int, int) {
 func (i *Image) InternalSize() (int, int) {
 	if i.w2 == 0 || i.h2 == 0 {
 		w, h := i.image.Size()
-		i.w2 = graphics.NextPowerOf2Int(w)
-		i.h2 = graphics.NextPowerOf2Int(h)
+		i.w2 = graphics.InternalImageSize(w)
+		i.h2 = graphics.InternalImageSize(h)
 	}
 	return i.w2, i.h2
 }
diff --git a/internal/restorable/images_test.go b/internal/restorable/images_test.go
index 80c8b4ddb..1cf4e6a40 100644
--- a/internal/restorable/images_test.go
+++ b/internal/restorable/images_test.go
@@ -105,6 +105,14 @@ func TestRestoreWithoutDraw(t *testing.T) {
 	}
 }
 
+func quadVertices(dw, dh, sw, sh, x, y int) []float32 {
+	// dw/dh must be internal image sizes.
+	return graphics.QuadVertices(dw, dh,
+		0, 0, sw, sh,
+		1, 0, 0, 1, float32(x), float32(y),
+		1, 1, 1, 1)
+}
+
 func TestRestoreChain(t *testing.T) {
 	const num = 10
 	imgs := []*Image{}
@@ -120,8 +128,8 @@ func TestRestoreChain(t *testing.T) {
 	clr := color.RGBA{0x00, 0x00, 0x00, 0xff}
 	imgs[0].Fill(clr.R, clr.G, clr.B, clr.A)
 	for i := 0; i < num-1; i++ {
-		w, h := imgs[i].Size()
-		vs := graphics.QuadVertices(w, h, 0, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+		w, h := imgs[i].InternalSize()
+		vs := quadVertices(w, h, 1, 1, 0, 0)
 		is := graphics.QuadIndices()
 		imgs[i+1].DrawImage(imgs[i], vs, is, nil, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)
 	}
@@ -162,7 +170,7 @@ func TestRestoreChain2(t *testing.T) {
 	clr8 := color.RGBA{0x00, 0x00, 0xff, 0xff}
 	imgs[8].Fill(clr8.R, clr8.G, clr8.B, clr8.A)
 
-	vs := graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	vs := quadVertices(graphics.InternalImageSize(w), graphics.InternalImageSize(h), w, h, 0, 0)
 	is := graphics.QuadIndices()
 	imgs[8].DrawImage(imgs[7], vs, is, nil, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)
 	imgs[9].DrawImage(imgs[8], vs, is, nil, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)
@@ -204,7 +212,7 @@ func TestRestoreOverrideSource(t *testing.T) {
 	clr0 := color.RGBA{0x00, 0x00, 0x00, 0xff}
 	clr1 := color.RGBA{0x00, 0x00, 0x01, 0xff}
 	img1.Fill(clr0.R, clr0.G, clr0.B, clr0.A)
-	vs := graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	vs := quadVertices(graphics.InternalImageSize(w), graphics.InternalImageSize(h), w, h, 0, 0)
 	is := graphics.QuadIndices()
 	img2.DrawImage(img1, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
 	img3.DrawImage(img2, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
@@ -284,24 +292,26 @@ func TestRestoreComplexGraph(t *testing.T) {
 		img1.Dispose()
 		img0.Dispose()
 	}()
-	vs := graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	dw := graphics.InternalImageSize(w)
+	dh := graphics.InternalImageSize(h)
+	vs := quadVertices(dw, dh, w, h, 0, 0)
 	is := graphics.QuadIndices()
 	img3.DrawImage(img0, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
-	vs = graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 1, 0, 1, 1, 1, 1)
+	vs = quadVertices(dw, dh, w, h, 1, 0)
 	img3.DrawImage(img1, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
-	vs = graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 1, 0, 1, 1, 1, 1)
+	vs = quadVertices(dw, dh, w, h, 1, 0)
 	img4.DrawImage(img1, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
-	vs = graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 2, 0, 1, 1, 1, 1)
+	vs = quadVertices(dw, dh, w, h, 2, 0)
 	img4.DrawImage(img2, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
-	vs = graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	vs = quadVertices(dw, dh, w, h, 0, 0)
 	img5.DrawImage(img3, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
-	vs = graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	vs = quadVertices(dw, dh, w, h, 0, 0)
 	img6.DrawImage(img3, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
-	vs = graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 1, 0, 1, 1, 1, 1)
+	vs = quadVertices(dw, dh, w, h, 1, 0)
 	img6.DrawImage(img4, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
-	vs = graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	vs = quadVertices(dw, dh, w, h, 0, 0)
 	img7.DrawImage(img2, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
-	vs = graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 2, 0, 1, 1, 1, 1)
+	vs = quadVertices(dw, dh, w, h, 2, 0)
 	img7.DrawImage(img3, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
 	ResolveStaleImages()
 	if err := Restore(); err != nil {
@@ -391,7 +401,7 @@ func TestRestoreRecursive(t *testing.T) {
 		img1.Dispose()
 		img0.Dispose()
 	}()
-	vs := graphics.QuadVertices(w, h, 0, 0, w, h, 1, 0, 0, 1, 1, 0, 1, 1, 1, 1)
+	vs := quadVertices(graphics.InternalImageSize(w), graphics.InternalImageSize(h), w, h, 1, 0)
 	is := graphics.QuadIndices()
 	img1.DrawImage(img0, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
 	img0.DrawImage(img1, vs, is, nil, graphics.CompositeModeSourceOver, graphics.FilterNearest, graphics.AddressClampToZero)
@@ -481,7 +491,7 @@ func TestDrawImageAndReplacePixels(t *testing.T) {
 	img1 := NewImage(2, 1)
 	defer img1.Dispose()
 
-	vs := graphics.QuadVertices(1, 1, 0, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	vs := quadVertices(graphics.InternalImageSize(1), graphics.InternalImageSize(1), 1, 1, 0, 0)
 	is := graphics.QuadIndices()
 	img1.DrawImage(img0, vs, is, nil, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)
 	img1.ReplacePixels([]byte{0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}, 0, 0, 2, 1)
@@ -514,7 +524,7 @@ func TestDispose(t *testing.T) {
 	img2 := newImageFromImage(base2)
 	defer img2.Dispose()
 
-	vs := graphics.QuadVertices(1, 1, 0, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	vs := quadVertices(graphics.InternalImageSize(1), graphics.InternalImageSize(1), 1, 1, 0, 0)
 	is := graphics.QuadIndices()
 	img1.DrawImage(img2, vs, is, nil, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)
 	img0.DrawImage(img1, vs, is, nil, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)
@@ -608,7 +618,7 @@ func TestReplacePixelsOnly(t *testing.T) {
 		img0.ReplacePixels([]byte{1, 2, 3, 4}, i%w, i/w, 1, 1)
 	}
 
-	vs := graphics.QuadVertices(w, h, 0, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	vs := quadVertices(graphics.InternalImageSize(w), graphics.InternalImageSize(h), 1, 1, 0, 0)
 	is := graphics.QuadIndices()
 	img1.DrawImage(img0, vs, is, nil, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)
 	img0.ReplacePixels([]byte{5, 6, 7, 8}, 0, 0, 1, 1)
@@ -653,7 +663,7 @@ func TestReadPixelsFromVolatileImage(t *testing.T) {
 
 	// Second, draw src to dst. If the implementation is correct, dst becomes stale.
 	src.Fill(0xff, 0xff, 0xff, 0xff)
-	vs := graphics.QuadVertices(w, h, 0, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 1, 1, 1)
+	vs := quadVertices(graphics.InternalImageSize(w), graphics.InternalImageSize(h), 1, 1, 0, 0)
 	is := graphics.QuadIndices()
 	dst.DrawImage(src, vs, is, nil, graphics.CompositeModeCopy, graphics.FilterNearest, graphics.AddressClampToZero)