diff --git a/gamecontext.go b/gamecontext.go
index 4f904b79d..943d88b1e 100644
--- a/gamecontext.go
+++ b/gamecontext.go
@@ -17,7 +17,6 @@ limitations under the License.
 package ebiten
 
 import (
-	"github.com/go-gl/gl"
 	"image"
 )
 
@@ -36,23 +35,12 @@ func IsMouseButtonPressed(mouseButton MouseButton) bool {
 	return currentUI.input.isMouseButtonPressed(mouseButton)
 }
 
-func glFilter(f Filter) int {
-	switch f {
-	case FilterNearest:
-		return gl.NEAREST
-	case FilterLinear:
-		return gl.LINEAR
-	default:
-		panic("not reached")
-	}
-}
-
 // NewImage returns an empty image.
 func NewImage(width, height int, filter Filter) (*Image, error) {
-	return currentUI.newImage(width, height, glFilter(filter))
+	return currentUI.newImage(width, height, filter)
 }
 
 // NewImageFromImage creates a new image with the given image (img).
 func NewImageFromImage(img image.Image, filter Filter) (*Image, error) {
-	return currentUI.newImageFromImage(img, glFilter(filter))
+	return currentUI.newImageFromImage(img, filter)
 }
diff --git a/graphics.go b/graphics.go
index b924a6311..e3ab0f63d 100644
--- a/graphics.go
+++ b/graphics.go
@@ -16,6 +16,10 @@ limitations under the License.
 
 package ebiten
 
+import (
+	"github.com/hajimehoshi/ebiten/internal/opengl"
+)
+
 // A Rect represents a rectangle.
 type Rect struct {
 	X      float64
@@ -40,10 +44,10 @@ func DrawWholeImage(target *Image, image *Image, geo GeometryMatrix, color Color
 }
 
 // Filter represents the type of filter to be used when an image is maginified or minified.
-type Filter int
+type Filter opengl.Filter
 
 // Filters
 const (
-	FilterNearest Filter = iota
-	FilterLinear
+	FilterNearest = Filter(opengl.FilterNearest)
+	FilterLinear  = Filter(opengl.FilterLinear)
 )
diff --git a/graphicscontext.go b/graphicscontext.go
index 208fac797..353a2e267 100644
--- a/graphicscontext.go
+++ b/graphicscontext.go
@@ -17,7 +17,6 @@ limitations under the License.
 package ebiten
 
 import (
-	"github.com/go-gl/gl"
 	"github.com/hajimehoshi/ebiten/internal/opengl"
 )
 
@@ -27,11 +26,11 @@ func newGraphicsContext(screenWidth, screenHeight, screenScale int) (*graphicsCo
 		return nil, err
 	}
 
-	texture, err := opengl.NewTexture(screenWidth, screenHeight, gl.NEAREST)
+	texture, err := opengl.NewTexture(screenWidth, screenHeight, opengl.Filter(FilterNearest))
 	if err != nil {
 		return nil, err
 	}
-	screen, err := newInnerImage(texture, gl.NEAREST)
+	screen, err := newInnerImage(texture)
 	if err != nil {
 		return nil, err
 	}
@@ -79,6 +78,6 @@ func (c *graphicsContext) postUpdate() error {
 		return err
 	}
 
-	gl.Flush()
+	opengl.Flush()
 	return nil
 }
diff --git a/image.go b/image.go
index 56cb1cbd5..4453ff0c1 100644
--- a/image.go
+++ b/image.go
@@ -17,7 +17,6 @@ limitations under the License.
 package ebiten
 
 import (
-	"github.com/go-gl/gl"
 	"github.com/hajimehoshi/ebiten/internal"
 	"github.com/hajimehoshi/ebiten/internal/opengl"
 	"github.com/hajimehoshi/ebiten/internal/opengl/internal/shader"
@@ -30,7 +29,7 @@ type innerImage struct {
 	texture     *opengl.Texture
 }
 
-func newInnerImage(texture *opengl.Texture, filter int) (*innerImage, error) {
+func newInnerImage(texture *opengl.Texture) (*innerImage, error) {
 	framebuffer, err := opengl.NewFramebufferFromTexture(texture)
 	if err != nil {
 		return nil, err
@@ -50,9 +49,8 @@ func (i *innerImage) Fill(clr color.Color) error {
 	if err := i.framebuffer.SetAsViewport(); err != nil {
 		return err
 	}
-	rf, gf, bf, af := internal.RGBA(clr)
-	gl.ClearColor(gl.GLclampf(rf), gl.GLclampf(gf), gl.GLclampf(bf), gl.GLclampf(af))
-	gl.Clear(gl.COLOR_BUFFER_BIT)
+	r, g, b, a := internal.RGBA(clr)
+	opengl.Clear(r, g, b, a)
 	return nil
 }
 
diff --git a/internal/opengl/gl.go b/internal/opengl/gl.go
new file mode 100644
index 000000000..f8f0c4d3a
--- /dev/null
+++ b/internal/opengl/gl.go
@@ -0,0 +1,45 @@
+/*
+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/go-gl/gl"
+)
+
+type Filter int
+
+const (
+	FilterNearest Filter = gl.NEAREST
+	FilterLinear         = gl.LINEAR
+)
+
+func Init() {
+	gl.Init()
+	gl.Enable(gl.TEXTURE_2D)
+	// Textures' pixel formats are alpha premultiplied.
+	gl.Enable(gl.BLEND)
+	gl.BlendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA)
+}
+
+func Clear(r, g, b, a float64) {
+	gl.ClearColor(gl.GLclampf(r), gl.GLclampf(g), gl.GLclampf(b), gl.GLclampf(a))
+	gl.Clear(gl.COLOR_BUFFER_BIT)
+}
+
+func Flush() {
+	gl.Flush()
+}
diff --git a/internal/opengl/texture.go b/internal/opengl/texture.go
index 1890fbf4f..433d7c1e9 100644
--- a/internal/opengl/texture.go
+++ b/internal/opengl/texture.go
@@ -61,7 +61,7 @@ func (t *Texture) Size() (width, height int) {
 	return t.width, t.height
 }
 
-func createNativeTexture(textureWidth, textureHeight int, pixels []uint8, filter int) gl.Texture {
+func createNativeTexture(textureWidth, textureHeight int, pixels []uint8, filter Filter) gl.Texture {
 	nativeTexture := gl.GenTexture()
 	if nativeTexture < 0 {
 		panic("glGenTexture failed")
@@ -70,15 +70,15 @@ func createNativeTexture(textureWidth, textureHeight int, pixels []uint8, filter
 	nativeTexture.Bind(gl.TEXTURE_2D)
 	defer gl.Texture(0).Bind(gl.TEXTURE_2D)
 
-	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, filter)
-	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, filter)
+	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, int(filter))
+	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, int(filter))
 
 	gl.TexImage2D(gl.TEXTURE_2D, 0, gl.RGBA, textureWidth, textureHeight, 0, gl.RGBA, gl.UNSIGNED_BYTE, pixels)
 
 	return nativeTexture
 }
 
-func NewTexture(width, height int, filter int) (*Texture, error) {
+func NewTexture(width, height int, filter Filter) (*Texture, error) {
 	w := internal.NextPowerOf2Int(width)
 	h := internal.NextPowerOf2Int(height)
 	if w < 4 {
@@ -91,7 +91,7 @@ func NewTexture(width, height int, filter int) (*Texture, error) {
 	return &Texture{native, width, height}, nil
 }
 
-func NewTextureFromImage(img image.Image, filter int) (*Texture, error) {
+func NewTextureFromImage(img image.Image, filter Filter) (*Texture, error) {
 	origSize := img.Bounds().Size()
 	if origSize.X < 4 {
 		return nil, errors.New("width must be equal or more than 4.")
diff --git a/ui.go b/ui.go
index 43ab86fb9..6e575c24f 100644
--- a/ui.go
+++ b/ui.go
@@ -18,7 +18,6 @@ package ebiten
 
 import (
 	"fmt"
-	"github.com/go-gl/gl"
 	glfw "github.com/go-gl/glfw3"
 	"github.com/hajimehoshi/ebiten/internal/opengl"
 	"image"
@@ -50,11 +49,7 @@ func init() {
 	}
 	currentUI.run()
 	currentUI.use(func() {
-		gl.Init()
-		gl.Enable(gl.TEXTURE_2D)
-		// Textures' pixel formats are alpha premultiplied.
-		gl.Enable(gl.BLEND)
-		gl.BlendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA)
+		opengl.Init()
 	})
 }
 
@@ -134,16 +129,16 @@ func (u *ui) draw(f func(*Image) error) (err error) {
 	return
 }
 
-func (u *ui) newImageFromImage(img image.Image, filter int) (*Image, error) {
+func (u *ui) newImageFromImage(img image.Image, filter Filter) (*Image, error) {
 	var innerImage *innerImage
 	var err error
 	u.use(func() {
 		var texture *opengl.Texture
-		texture, err = opengl.NewTextureFromImage(img, filter)
+		texture, err = opengl.NewTextureFromImage(img, opengl.Filter(filter))
 		if err != nil {
 			return
 		}
-		innerImage, err = newInnerImage(texture, filter)
+		innerImage, err = newInnerImage(texture)
 	})
 	if err != nil {
 		return nil, err
@@ -151,16 +146,16 @@ func (u *ui) newImageFromImage(img image.Image, filter int) (*Image, error) {
 	return &Image{syncer: u, inner: innerImage}, nil
 }
 
-func (u *ui) newImage(width, height int, filter int) (*Image, error) {
+func (u *ui) newImage(width, height int, filter Filter) (*Image, error) {
 	var innerImage *innerImage
 	var err error
 	u.use(func() {
 		var texture *opengl.Texture
-		texture, err = opengl.NewTexture(width, height, filter)
+		texture, err = opengl.NewTexture(width, height, opengl.Filter(filter))
 		if err != nil {
 			return
 		}
-		innerImage, err = newInnerImage(texture, filter)
+		innerImage, err = newInnerImage(texture)
 		innerImage.Clear()
 	})
 	if err != nil {