diff --git a/graphicscontext.go b/graphicscontext.go
index 5f47f2322..ed95f3117 100644
--- a/graphicscontext.go
+++ b/graphicscontext.go
@@ -72,7 +72,5 @@ func (c *graphicsContext) postUpdate() error {
 	if err := c.defaultR.drawImage(c.screen, options); err != nil {
 		return err
 	}
-
-	opengl.Flush()
 	return nil
 }
diff --git a/image.go b/image.go
index 10d622765..060a870c7 100644
--- a/image.go
+++ b/image.go
@@ -17,7 +17,6 @@ package ebiten
 import (
 	"github.com/hajimehoshi/ebiten/internal"
 	"github.com/hajimehoshi/ebiten/internal/opengl"
-	"github.com/hajimehoshi/ebiten/internal/opengl/internal/shader"
 	"image"
 	"image/color"
 )
@@ -44,12 +43,8 @@ func (i *innerImage) Clear() error {
 }
 
 func (i *innerImage) Fill(clr color.Color) error {
-	if err := i.framebuffer.SetAsViewport(); err != nil {
-		return err
-	}
 	r, g, b, a := internal.RGBA(clr)
-	opengl.Clear(r, g, b, a)
-	return nil
+	return i.framebuffer.Fill(r, g, b, a)
 }
 
 func (i *innerImage) drawImage(img *innerImage, options *DrawImageOptions) error {
@@ -68,14 +63,9 @@ func (i *innerImage) drawImage(img *innerImage, options *DrawImageOptions) error
 	}
 	geo := options.GeoM
 	clr := options.ColorM
-
-	if err := i.framebuffer.SetAsViewport(); err != nil {
-		return err
-	}
-	w, h := img.texture.Size()
-	quads := textureQuads(parts, w, h)
-	projectionMatrix := i.framebuffer.ProjectionMatrix()
-	return shader.DrawTexture(img.texture.Native(), projectionMatrix, quads, geo, clr)
+	w, h := img.size()
+	quads := &textureQuads{parts, w, h}
+	return i.framebuffer.DrawTexture(img.texture, quads, geo, clr)
 }
 
 func u(x float64, width int) float32 {
@@ -86,22 +76,25 @@ func v(y float64, height int) float32 {
 	return float32(y) / float32(internal.NextPowerOf2Int(height))
 }
 
-func textureQuads(parts []ImagePart, width, height int) []shader.TextureQuad {
-	quads := make([]shader.TextureQuad, 0, len(parts))
-	for _, part := range parts {
-		dst, src := part.Dst, part.Src
-		x1 := float32(dst.Min.X)
-		x2 := float32(dst.Max.X)
-		y1 := float32(dst.Min.Y)
-		y2 := float32(dst.Max.Y)
-		u1 := u(float64(src.Min.X), width)
-		u2 := u(float64(src.Max.X), width)
-		v1 := v(float64(src.Min.Y), height)
-		v2 := v(float64(src.Max.Y), height)
-		quad := shader.TextureQuad{x1, x2, y1, y2, u1, u2, v1, v2}
-		quads = append(quads, quad)
-	}
-	return quads
+type textureQuads struct {
+	parts  []ImagePart
+	width  int
+	height int
+}
+
+func (t *textureQuads) Len() int {
+	return len(t.parts)
+}
+
+func (t *textureQuads) Vertex(i int) (x0, y0, x1, y1 float32) {
+	dst := t.parts[i].Dst
+	return float32(dst.Min.X), float32(dst.Min.Y), float32(dst.Max.X), float32(dst.Max.Y)
+}
+
+func (t *textureQuads) Texture(i int) (u0, v0, u1, v1 float32) {
+	src := t.parts[i].Src
+	w, h := t.width, t.height
+	return u(float64(src.Min.X), w), v(float64(src.Min.Y), h), u(float64(src.Max.X), w), v(float64(src.Max.Y), h)
 }
 
 type syncer interface {
diff --git a/internal/opengl/framebuffer.go b/internal/opengl/framebuffer.go
index 82877d63b..c29886e5b 100644
--- a/internal/opengl/framebuffer.go
+++ b/internal/opengl/framebuffer.go
@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"github.com/go-gl/gl"
 	"github.com/hajimehoshi/ebiten/internal"
+	"github.com/hajimehoshi/ebiten/internal/opengl/internal/shader"
 )
 
 func orthoProjectionMatrix(left, right, bottom, top int) [4][4]float64 {
@@ -84,7 +85,7 @@ func createFramebuffer(nativeTexture gl.Texture) (gl.Framebuffer, error) {
 	return framebuffer, nil
 }
 
-func (f *Framebuffer) SetAsViewport() error {
+func (f *Framebuffer) setAsViewport() error {
 	gl.Flush()
 	f.framebuffer.Bind()
 	err := gl.CheckFramebufferStatus(gl.FRAMEBUFFER)
@@ -101,7 +102,7 @@ func (f *Framebuffer) SetAsViewport() error {
 	return nil
 }
 
-func (f *Framebuffer) ProjectionMatrix() [4][4]float64 {
+func (f *Framebuffer) projectionMatrix() [4][4]float64 {
 	width := internal.NextPowerOf2Int(f.width)
 	height := internal.NextPowerOf2Int(f.height)
 	m := orthoProjectionMatrix(0, width, 0, height)
@@ -111,3 +112,31 @@ func (f *Framebuffer) ProjectionMatrix() [4][4]float64 {
 	}
 	return m
 }
+
+type Matrix interface {
+	Element(i, j int) float64
+}
+
+type TextureQuads interface {
+	Len() int
+	Vertex(i int) (x0, y0, x1, y1 float32)
+	Texture(i int) (u0, v0, u1, v1 float32)
+}
+
+func (f *Framebuffer) Fill(r, g, b, a float64) error {
+	if err := f.setAsViewport(); err != nil {
+		return err
+	}
+	gl.ClearColor(gl.GLclampf(r), gl.GLclampf(g), gl.GLclampf(b), gl.GLclampf(a))
+	gl.Clear(gl.COLOR_BUFFER_BIT)
+	return nil
+}
+
+func (f *Framebuffer) DrawTexture(t *Texture, quads TextureQuads, geo, clr Matrix) error {
+	if err := f.setAsViewport(); err != nil {
+		return err
+	}
+	projectionMatrix := f.projectionMatrix()
+	// TODO: Define texture.Draw()
+	return shader.DrawTexture(t.native, projectionMatrix, quads, geo, clr)
+}
diff --git a/internal/opengl/gl.go b/internal/opengl/gl.go
index a96d6a6cb..3cd92010e 100644
--- a/internal/opengl/gl.go
+++ b/internal/opengl/gl.go
@@ -32,12 +32,3 @@ func Init() {
 	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/internal/shader/drawtexture.go b/internal/opengl/internal/shader/drawtexture.go
index 89c1c7ef2..8ae6ff85b 100644
--- a/internal/opengl/internal/shader/drawtexture.go
+++ b/internal/opengl/internal/shader/drawtexture.go
@@ -32,6 +32,12 @@ type Matrix interface {
 	Element(i, j int) float64
 }
 
+type TextureQuads interface {
+	Len() int
+	Vertex(i int) (x0, y0, x1, y1 float32)
+	Texture(i int) (u0, v0, u1, v1 float32)
+}
+
 var initialized = false
 
 const size = 10000
@@ -40,7 +46,7 @@ const size = 10000
 const uint16Size = 2
 const short32Size = 4
 
-func DrawTexture(native gl.Texture, projectionMatrix [4][4]float64, quads []TextureQuad, geo Matrix, color Matrix) error {
+func DrawTexture(native gl.Texture, projectionMatrix [4][4]float64, quads TextureQuads, geo Matrix, color Matrix) error {
 	// TODO: Check len(quads) and gl.MAX_ELEMENTS_INDICES?
 	const stride = 4 * 4
 	if !initialized {
@@ -69,7 +75,7 @@ func DrawTexture(native gl.Texture, projectionMatrix [4][4]float64, quads []Text
 		initialized = true
 	}
 
-	if len(quads) == 0 {
+	if quads.Len() == 0 {
 		return nil
 	}
 	// TODO: Check performance
@@ -92,15 +98,9 @@ func DrawTexture(native gl.Texture, projectionMatrix [4][4]float64, quads []Text
 	texCoordAttrLocation.AttribPointer(2, gl.FLOAT, false, stride, uintptr(short32Size*2))
 
 	vertices := []float32{}
-	for _, quad := range quads {
-		x0 := quad.VertexX0
-		x1 := quad.VertexX1
-		y0 := quad.VertexY0
-		y1 := quad.VertexY1
-		u0 := quad.TextureCoordU0
-		u1 := quad.TextureCoordU1
-		v0 := quad.TextureCoordV0
-		v1 := quad.TextureCoordV1
+	for i := 0; i < quads.Len(); i++ {
+		x0, y0, x1, y1 := quads.Vertex(i)
+		u0, v0, u1, v1 := quads.Texture(i)
 		vertices = append(vertices,
 			x0, y0, u0, v0,
 			x1, y0, u1, v0,
@@ -109,7 +109,7 @@ func DrawTexture(native gl.Texture, projectionMatrix [4][4]float64, quads []Text
 		)
 	}
 	gl.BufferSubData(gl.ARRAY_BUFFER, 0, short32Size*len(vertices), vertices)
-	gl.DrawElements(gl.TRIANGLES, 6*len(quads), gl.UNSIGNED_SHORT, uintptr(0))
+	gl.DrawElements(gl.TRIANGLES, 6*quads.Len(), gl.UNSIGNED_SHORT, uintptr(0))
 
 	gl.Flush()
 	return nil
diff --git a/internal/opengl/internal/shader/texturequad.go b/internal/opengl/internal/shader/texturequad.go
deleted file mode 100644
index eb07e8585..000000000
--- a/internal/opengl/internal/shader/texturequad.go
+++ /dev/null
@@ -1,26 +0,0 @@
-// 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 shader
-
-type TextureQuad struct {
-	VertexX0       float32
-	VertexX1       float32
-	VertexY0       float32
-	VertexY1       float32
-	TextureCoordU0 float32
-	TextureCoordU1 float32
-	TextureCoordV0 float32
-	TextureCoordV1 float32
-}