diff --git a/affine.go b/affine.go
index 295132082..6ec14472f 100644
--- a/affine.go
+++ b/affine.go
@@ -18,7 +18,7 @@ package ebiten
 
 type affine interface {
 	dim() int
-	element(i, j int) float64
+	Element(i, j int) float64
 	setElement(i, j int, element float64)
 }
 
@@ -26,7 +26,7 @@ func isIdentity(ebiten affine) bool {
 	dim := ebiten.dim()
 	for i := 0; i < dim-1; i++ {
 		for j := 0; j < dim; j++ {
-			element := ebiten.element(i, j)
+			element := ebiten.Element(i, j)
 			if i == j && element != 1 {
 				return false
 			} else if i != j && element != 0 {
@@ -47,11 +47,11 @@ func mul(lhs, rhs, result affine) {
 		for j := 0; j < dim; j++ {
 			element := float64(0)
 			for k := 0; k < dim-1; k++ {
-				element += lhs.element(i, k) *
-					rhs.element(k, j)
+				element += lhs.Element(i, k) *
+					rhs.Element(k, j)
 			}
 			if j == dim-1 {
-				element += lhs.element(i, j)
+				element += lhs.Element(i, j)
 			}
 			result.setElement(i, j, element)
 		}
diff --git a/colormatrix.go b/colormatrix.go
index 213ac95f3..d2aec220e 100644
--- a/colormatrix.go
+++ b/colormatrix.go
@@ -42,6 +42,10 @@ func (c *ColorMatrix) dim() int {
 	return ColorMatrixDim
 }
 
+func (c *ColorMatrix) Element(i, j int) float64 {
+	return c.Elements[i][j]
+}
+
 func (c *ColorMatrix) Concat(other ColorMatrix) {
 	result := ColorMatrix{}
 	mul(&other, c, &result)
@@ -52,10 +56,6 @@ func (c *ColorMatrix) IsIdentity() bool {
 	return isIdentity(c)
 }
 
-func (c *ColorMatrix) element(i, j int) float64 {
-	return c.Elements[i][j]
-}
-
 func (c *ColorMatrix) setElement(i, j int, element float64) {
 	c.Elements[i][j] = element
 }
diff --git a/geometrymatrix.go b/geometrymatrix.go
index 1316d6070..970a62a28 100644
--- a/geometrymatrix.go
+++ b/geometrymatrix.go
@@ -39,6 +39,10 @@ func (g *GeometryMatrix) dim() int {
 	return GeometryMatrixDim
 }
 
+func (g *GeometryMatrix) Element(i, j int) float64 {
+	return g.Elements[i][j]
+}
+
 func (g *GeometryMatrix) Concat(other GeometryMatrix) {
 	result := GeometryMatrix{}
 	mul(&other, g, &result)
@@ -49,10 +53,6 @@ func (g *GeometryMatrix) IsIdentity() bool {
 	return isIdentity(g)
 }
 
-func (g *GeometryMatrix) element(i, j int) float64 {
-	return g.Elements[i][j]
-}
-
 func (g *GeometryMatrix) setElement(i, j int, element float64) {
 	g.Elements[i][j] = element
 }
diff --git a/graphics.go b/graphics.go
index 11f3016cc..cb0bece64 100644
--- a/graphics.go
+++ b/graphics.go
@@ -50,6 +50,7 @@ type GraphicsContext interface {
 	Fill(r, g, b uint8)
 	Texture(id TextureID) Drawer
 	RenderTarget(id RenderTargetID) Drawer
+	// TODO: ScreenRenderTarget() Drawer
 	PushRenderTarget(id RenderTargetID)
 	PopRenderTarget()
 }
diff --git a/internal/glfw/canvas.go b/internal/glfw/canvas.go
index 19450ca77..2523c3ad7 100644
--- a/internal/glfw/canvas.go
+++ b/internal/glfw/canvas.go
@@ -17,6 +17,7 @@ limitations under the License.
 package glfw
 
 import (
+	"github.com/go-gl/gl"
 	glfw "github.com/go-gl/glfw3"
 	"github.com/hajimehoshi/ebiten"
 	"github.com/hajimehoshi/ebiten/internal/opengl"
@@ -55,7 +56,16 @@ func (c *canvas) NewTextureID(img image.Image, filter ebiten.Filter) (ebiten.Tex
 	var id ebiten.TextureID
 	var err error
 	c.use(func() {
-		id, err = opengl.NewTextureID(img, filter)
+		glFilter := 0
+		switch filter {
+		case ebiten.FilterNearest:
+			glFilter = gl.NEAREST
+		case ebiten.FilterLinear:
+			glFilter = gl.LINEAR
+		default:
+			panic("not reached")
+		}
+		id, err = opengl.NewTextureID(img, glFilter)
 	})
 	return id, err
 }
@@ -64,7 +74,16 @@ func (c *canvas) NewRenderTargetID(width, height int, filter ebiten.Filter) (ebi
 	var id ebiten.RenderTargetID
 	var err error
 	c.use(func() {
-		id, err = opengl.NewRenderTargetID(width, height, filter)
+		glFilter := 0
+		switch filter {
+		case ebiten.FilterNearest:
+			glFilter = gl.NEAREST
+		case ebiten.FilterLinear:
+			glFilter = gl.LINEAR
+		default:
+			panic("not reached")
+		}
+		id, err = opengl.NewRenderTargetID(width, height, glFilter)
 	})
 	return id, err
 }
diff --git a/internal/opengl/context.go b/internal/opengl/context.go
index 5c79dd8c5..0f8455055 100644
--- a/internal/opengl/context.go
+++ b/internal/opengl/context.go
@@ -40,7 +40,7 @@ func Initialize(screenWidth, screenHeight, screenScale int) (*GraphicsContext, e
 	})
 
 	var err error
-	c.screenID, err = idsInstance.createRenderTarget(screenWidth, screenHeight, ebiten.FilterNearest)
+	c.screenID, err = idsInstance.createRenderTarget(screenWidth, screenHeight, gl.NEAREST)
 	if err != nil {
 		return nil, err
 	}
diff --git a/internal/opengl/ids.go b/internal/opengl/ids.go
index 6c5189cd6..2cbe728ac 100644
--- a/internal/opengl/ids.go
+++ b/internal/opengl/ids.go
@@ -41,11 +41,11 @@ var idsInstance = &ids{
 	currentRenderTargetId: -1,
 }
 
-func NewRenderTargetID(width, height int, filter ebiten.Filter) (ebiten.RenderTargetID, error) {
+func NewRenderTargetID(width, height int, filter int) (ebiten.RenderTargetID, error) {
 	return idsInstance.createRenderTarget(width, height, filter)
 }
 
-func NewTextureID(img image.Image, filter ebiten.Filter) (ebiten.TextureID, error) {
+func NewTextureID(img image.Image, filter int) (ebiten.TextureID, error) {
 	return idsInstance.createTexture(img, filter)
 }
 
@@ -67,7 +67,7 @@ func (i *ids) toTexture(id ebiten.RenderTargetID) ebiten.TextureID {
 	return i.renderTargetToTexture[id]
 }
 
-func (i *ids) createTexture(img image.Image, filter ebiten.Filter) (ebiten.TextureID, error) {
+func (i *ids) createTexture(img image.Image, filter int) (ebiten.TextureID, error) {
 	texture, err := createTextureFromImage(img, filter)
 	if err != nil {
 		return 0, err
@@ -81,7 +81,7 @@ func (i *ids) createTexture(img image.Image, filter ebiten.Filter) (ebiten.Textu
 	return textureId, nil
 }
 
-func (i *ids) createRenderTarget(width, height int, filter ebiten.Filter) (ebiten.RenderTargetID, error) {
+func (i *ids) createRenderTarget(width, height int, filter int) (ebiten.RenderTargetID, error) {
 	texture, err := createTexture(width, height, filter)
 	if err != nil {
 		return 0, err
@@ -148,7 +148,8 @@ func (i *ids) drawTexture(target ebiten.RenderTargetID, id ebiten.TextureID, par
 	i.setViewportIfNeeded(target)
 	r := i.renderTargetAt(target)
 	projectionMatrix := r.projectionMatrix()
-	shader.DrawTexture(texture.native, texture.width, texture.height, projectionMatrix, parts, geo, color)
+	quads := textureQuads(parts, texture.width, texture.height)
+	shader.DrawTexture(texture.native, projectionMatrix, quads, &geo, &color)
 }
 
 func (i *ids) setViewportIfNeeded(id ebiten.RenderTargetID) {
diff --git a/internal/opengl/internal/shader/drawtexture.go b/internal/opengl/internal/shader/drawtexture.go
index 5392a11ad..280f6d04a 100644
--- a/internal/opengl/internal/shader/drawtexture.go
+++ b/internal/opengl/internal/shader/drawtexture.go
@@ -18,15 +18,14 @@ package shader
 
 import (
 	"github.com/go-gl/gl"
-	"github.com/hajimehoshi/ebiten"
 	"sync"
 )
 
-func glMatrix(ebiten [4][4]float64) [16]float32 {
+func glMatrix(m [4][4]float64) [16]float32 {
 	result := [16]float32{}
 	for j := 0; j < 4; j++ {
 		for i := 0; i < 4; i++ {
-			result[i+j*4] = float32(ebiten[i][j])
+			result[i+j*4] = float32(m[i][j])
 		}
 	}
 	return result
@@ -34,16 +33,19 @@ func glMatrix(ebiten [4][4]float64) [16]float32 {
 
 var once sync.Once
 
+type Matrix interface {
+	Element(i, j int) float64
+}
+
 // TODO: Use VBO
-func DrawTexture(native gl.Texture, width, height int, projectionMatrix [4][4]float64, parts []ebiten.TexturePart, geo ebiten.GeometryMatrix, color ebiten.ColorMatrix) {
+func DrawTexture(native gl.Texture, projectionMatrix [4][4]float64, quads []TextureQuad, geo Matrix, color Matrix) {
 	once.Do(func() {
 		initialize()
 	})
 
-	if len(parts) == 0 {
+	if len(quads) == 0 {
 		return
 	}
-	quads := textureQuads(parts, width, height)
 	// TODO: Check performance
 	shaderProgram := use(glMatrix(projectionMatrix), geo, color)
 
@@ -67,7 +69,7 @@ func DrawTexture(native gl.Texture, width, height int, projectionMatrix [4][4]fl
 	vertices := []float32{}
 	texCoords := []float32{}
 	indicies := []uint32{}
-	// TODO: Check len(parts) and gl.MAX_ELEMENTS_INDICES?
+	// TODO: Check len(quads) and gl.MAX_ELEMENTS_INDICES?
 	for i, quad := range quads {
 		x1 := quad.VertexX1
 		x2 := quad.VertexX2
diff --git a/internal/opengl/internal/shader/program.go b/internal/opengl/internal/shader/program.go
index a0d3b785c..7385b5844 100644
--- a/internal/opengl/internal/shader/program.go
+++ b/internal/opengl/internal/shader/program.go
@@ -18,7 +18,6 @@ package shader
 
 import (
 	"github.com/go-gl/gl"
-	"github.com/hajimehoshi/ebiten"
 )
 
 type program struct {
@@ -81,18 +80,18 @@ func getUniformLocation(program gl.Program, name string) gl.UniformLocation {
 	return location
 }
 
-func use(projectionMatrix [16]float32, geo ebiten.GeometryMatrix, color ebiten.ColorMatrix) gl.Program {
+func use(projectionMatrix [16]float32, geo Matrix, color Matrix) gl.Program {
 	// TODO: Check the performance.
 	program := programColorMatrix
 
 	getUniformLocation(program.native, "projection_matrix").UniformMatrix4fv(false, projectionMatrix)
 
-	a := float32(geo.Elements[0][0])
-	b := float32(geo.Elements[0][1])
-	c := float32(geo.Elements[1][0])
-	d := float32(geo.Elements[1][1])
-	tx := float32(geo.Elements[0][2])
-	ty := float32(geo.Elements[1][2])
+	a := float32(geo.Element(0, 0))
+	b := float32(geo.Element(0, 1))
+	c := float32(geo.Element(1, 0))
+	d := float32(geo.Element(1, 1))
+	tx := float32(geo.Element(0, 2))
+	ty := float32(geo.Element(1, 2))
 	glModelviewMatrix := [...]float32{
 		a, c, 0, 0,
 		b, d, 0, 0,
@@ -106,7 +105,7 @@ func use(projectionMatrix [16]float32, geo ebiten.GeometryMatrix, color ebiten.C
 	e := [4][5]float32{}
 	for i := 0; i < 4; i++ {
 		for j := 0; j < 5; j++ {
-			e[i][j] = float32(color.Elements[i][j])
+			e[i][j] = float32(color.Element(i, j))
 		}
 	}
 
diff --git a/internal/opengl/internal/shader/texturequad.go b/internal/opengl/internal/shader/texturequad.go
index 3f9245d7d..4a3758da9 100644
--- a/internal/opengl/internal/shader/texturequad.go
+++ b/internal/opengl/internal/shader/texturequad.go
@@ -16,11 +16,7 @@ limitations under the License.
 
 package shader
 
-import (
-	"github.com/hajimehoshi/ebiten"
-)
-
-type textureQuad struct {
+type TextureQuad struct {
 	VertexX1       float32
 	VertexX2       float32
 	VertexY1       float32
@@ -30,43 +26,3 @@ type textureQuad struct {
 	TextureCoordV1 float32
 	TextureCoordV2 float32
 }
-
-func NextPowerOf2(x uint64) uint64 {
-	x -= 1
-	x |= (x >> 1)
-	x |= (x >> 2)
-	x |= (x >> 4)
-	x |= (x >> 8)
-	x |= (x >> 16)
-	x |= (x >> 32)
-	return x + 1
-}
-
-func AdjustSizeForTexture(size int) int {
-	return int(NextPowerOf2(uint64(size)))
-}
-
-func u(x int, width int) float32 {
-	return float32(x) / float32(AdjustSizeForTexture(width))
-}
-
-func v(y int, height int) float32 {
-	return float32(y) / float32(AdjustSizeForTexture(height))
-}
-
-func textureQuads(parts []ebiten.TexturePart, width, height int) []textureQuad {
-	quads := []textureQuad{}
-	for _, part := range parts {
-		x1 := float32(part.LocationX)
-		x2 := float32(part.LocationX + part.Source.Width)
-		y1 := float32(part.LocationY)
-		y2 := float32(part.LocationY + part.Source.Height)
-		u1 := u(part.Source.X, width)
-		u2 := u(part.Source.X+part.Source.Width, width)
-		v1 := v(part.Source.Y, height)
-		v2 := v(part.Source.Y+part.Source.Height, height)
-		quad := textureQuad{x1, x2, y1, y2, u1, u2, v1, v2}
-		quads = append(quads, quad)
-	}
-	return quads
-}
diff --git a/internal/opengl/rendertarget.go b/internal/opengl/rendertarget.go
index 4ffd7e633..510866af2 100644
--- a/internal/opengl/rendertarget.go
+++ b/internal/opengl/rendertarget.go
@@ -19,7 +19,6 @@ package opengl
 import (
 	"fmt"
 	"github.com/go-gl/gl"
-	"github.com/hajimehoshi/ebiten/internal/opengl/internal/shader"
 )
 
 func orthoProjectionMatrix(left, right, bottom, top int) [4][4]float64 {
@@ -70,18 +69,18 @@ func (r *renderTarget) setAsViewport() {
 
 	gl.BlendFuncSeparate(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA, gl.ZERO, gl.ONE)
 
-	width := shader.AdjustSizeForTexture(r.width)
-	height := shader.AdjustSizeForTexture(r.height)
+	width := adjustSizeForTexture(r.width)
+	height := adjustSizeForTexture(r.height)
 	gl.Viewport(0, 0, width, height)
 }
 
 func (r *renderTarget) projectionMatrix() [4][4]float64 {
-	width := shader.AdjustSizeForTexture(r.width)
-	height := shader.AdjustSizeForTexture(r.height)
+	width := adjustSizeForTexture(r.width)
+	height := adjustSizeForTexture(r.height)
 	ebiten := orthoProjectionMatrix(0, width, 0, height)
 	if r.flipY {
 		ebiten[1][1] *= -1
-		ebiten[1][3] += float64(r.height) / float64(shader.AdjustSizeForTexture(r.height)) * 2
+		ebiten[1][3] += float64(r.height) / float64(adjustSizeForTexture(r.height)) * 2
 	}
 	return ebiten
 }
diff --git a/internal/opengl/texture.go b/internal/opengl/texture.go
index 05d6cc20a..ae0af2c1b 100644
--- a/internal/opengl/texture.go
+++ b/internal/opengl/texture.go
@@ -18,8 +18,6 @@ package opengl
 
 import (
 	"github.com/go-gl/gl"
-	"github.com/hajimehoshi/ebiten"
-	"github.com/hajimehoshi/ebiten/internal/opengl/internal/shader"
 	"image"
 	"image/draw"
 )
@@ -29,8 +27,8 @@ func adjustImageForTexture(img image.Image) *image.NRGBA {
 	adjustedImageBounds := image.Rectangle{
 		image.ZP,
 		image.Point{
-			shader.AdjustSizeForTexture(width),
-			shader.AdjustSizeForTexture(height),
+			adjustSizeForTexture(width),
+			adjustSizeForTexture(height),
 		},
 	}
 	if nrgba, ok := img.(*image.NRGBA); ok && img.Bounds() == adjustedImageBounds {
@@ -52,7 +50,7 @@ type texture struct {
 	height int
 }
 
-func createNativeTexture(textureWidth, textureHeight int, pixels []uint8, filter ebiten.Filter) gl.Texture {
+func createNativeTexture(textureWidth, textureHeight int, pixels []uint8, filter int) gl.Texture {
 	nativeTexture := gl.GenTexture()
 	if nativeTexture < 0 {
 		panic("glGenTexture failed")
@@ -61,31 +59,22 @@ func createNativeTexture(textureWidth, textureHeight int, pixels []uint8, filter
 	nativeTexture.Bind(gl.TEXTURE_2D)
 	defer gl.Texture(0).Bind(gl.TEXTURE_2D)
 
-	glFilter := 0
-	switch filter {
-	case ebiten.FilterLinear:
-		glFilter = gl.LINEAR
-	case ebiten.FilterNearest:
-		glFilter = gl.NEAREST
-	default:
-		panic("not reached")
-	}
-	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, glFilter)
-	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, glFilter)
+	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, filter)
+	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, filter)
 
 	gl.TexImage2D(gl.TEXTURE_2D, 0, gl.RGBA, textureWidth, textureHeight, 0, gl.RGBA, gl.UNSIGNED_BYTE, pixels)
 
 	return nativeTexture
 }
 
-func createTexture(width, height int, filter ebiten.Filter) (*texture, error) {
-	w := shader.AdjustSizeForTexture(width)
-	h := shader.AdjustSizeForTexture(height)
+func createTexture(width, height int, filter int) (*texture, error) {
+	w := adjustSizeForTexture(width)
+	h := adjustSizeForTexture(height)
 	native := createNativeTexture(w, h, nil, filter)
 	return &texture{native, width, height}, nil
 }
 
-func createTextureFromImage(img image.Image, filter ebiten.Filter) (*texture, error) {
+func createTextureFromImage(img image.Image, filter int) (*texture, error) {
 	adjustedImage := adjustImageForTexture(img)
 	size := adjustedImage.Bounds().Size()
 	native := createNativeTexture(size.X, size.Y, adjustedImage.Pix, filter)
diff --git a/internal/opengl/texturequad.go b/internal/opengl/texturequad.go
new file mode 100644
index 000000000..a2a66ed68
--- /dev/null
+++ b/internal/opengl/texturequad.go
@@ -0,0 +1,62 @@
+/*
+Copyright 2014 Hajime Hoshi
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package opengl
+
+import (
+	"github.com/hajimehoshi/ebiten"
+	"github.com/hajimehoshi/ebiten/internal/opengl/internal/shader"
+)
+
+func NextPowerOf2(x uint64) uint64 {
+	x -= 1
+	x |= (x >> 1)
+	x |= (x >> 2)
+	x |= (x >> 4)
+	x |= (x >> 8)
+	x |= (x >> 16)
+	x |= (x >> 32)
+	return x + 1
+}
+
+func adjustSizeForTexture(size int) int {
+	return int(NextPowerOf2(uint64(size)))
+}
+
+func u(x int, width int) float32 {
+	return float32(x) / float32(adjustSizeForTexture(width))
+}
+
+func v(y int, height int) float32 {
+	return float32(y) / float32(adjustSizeForTexture(height))
+}
+
+func textureQuads(parts []ebiten.TexturePart, width, height int) []shader.TextureQuad {
+	quads := make([]shader.TextureQuad, 0, len(parts))
+	for _, part := range parts {
+		x1 := float32(part.LocationX)
+		x2 := float32(part.LocationX + part.Source.Width)
+		y1 := float32(part.LocationY)
+		y2 := float32(part.LocationY + part.Source.Height)
+		u1 := u(part.Source.X, width)
+		u2 := u(part.Source.X+part.Source.Width, width)
+		v1 := v(part.Source.Y, height)
+		v2 := v(part.Source.Y+part.Source.Height, height)
+		quad := shader.TextureQuad{x1, x2, y1, y2, u1, u2, v1, v2}
+		quads = append(quads, quad)
+	}
+	return quads
+}
diff --git a/internal/opengl/internal/shader/texturequad_test.go b/internal/opengl/texturequad_test.go
similarity index 97%
rename from internal/opengl/internal/shader/texturequad_test.go
rename to internal/opengl/texturequad_test.go
index 2d465c690..f25a73a2d 100644
--- a/internal/opengl/internal/shader/texturequad_test.go
+++ b/internal/opengl/texturequad_test.go
@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-package shader_test
+package opengl_test
 
 import (
 	. "."