diff --git a/graphics/opengl/context.go b/graphics/opengl/context.go
index e48aa741f..791a423da 100644
--- a/graphics/opengl/context.go
+++ b/graphics/opengl/context.go
@@ -9,6 +9,7 @@ import (
 	"github.com/hajimehoshi/go-ebiten/graphics"
 	"github.com/hajimehoshi/go-ebiten/graphics/matrix"
 	"github.com/hajimehoshi/go-ebiten/graphics/opengl/offscreen"
+	"github.com/hajimehoshi/go-ebiten/graphics/opengl/rendertarget"
 	"math"
 )
 
@@ -20,9 +21,12 @@ type Context struct {
 }
 
 func newContext(ids *ids, screenWidth, screenHeight, screenScale int) *Context {
+	mainFramebuffer := rendertarget.NewWithCurrentFramebuffer(
+		screenWidth*screenScale,
+		screenHeight*screenScale)
 	context := &Context{
 		ids:         ids,
-		offscreen:   offscreen.New(screenWidth, screenHeight, screenScale),
+		offscreen:   offscreen.New(mainFramebuffer),
 		screenScale: screenScale,
 	}
 
diff --git a/graphics/opengl/offscreen/offscreen.go b/graphics/opengl/offscreen/offscreen.go
index 1a455d433..962ee99db 100644
--- a/graphics/opengl/offscreen/offscreen.go
+++ b/graphics/opengl/offscreen/offscreen.go
@@ -3,67 +3,49 @@ package offscreen
 import (
 	"github.com/hajimehoshi/go-ebiten/graphics"
 	"github.com/hajimehoshi/go-ebiten/graphics/matrix"
-	"github.com/hajimehoshi/go-ebiten/graphics/opengl/rendertarget"
-	"github.com/hajimehoshi/go-ebiten/graphics/opengl/texture"
 )
 
+type Texture interface {
+	Draw(projectionMatrix [4][4]float64,
+		geometryMatrix matrix.Geometry, colorMatrix matrix.Color)
+	DrawParts(parts []graphics.TexturePart, projectionMatrix [4][4]float64,
+		geometryMatrix matrix.Geometry, colorMatrix matrix.Color)
+}
+
+type RenderTarget interface {
+	SetAsViewport()
+	ProjectionMatrix() [4][4]float64
+}
+
 type Offscreen struct {
-	screenHeight           int
-	screenScale            int
-	currentRenderTarget    *rendertarget.RenderTarget
-	mainFramebufferTexture *rendertarget.RenderTarget
+	currentRenderTarget RenderTarget
+	mainFramebuffer     RenderTarget
 }
 
-func New(screenWidth, screenHeight, screenScale int) *Offscreen {
-	offscreen := &Offscreen{
-		screenHeight: screenHeight,
-		screenScale:  screenScale,
+func New(mainFramebuffer RenderTarget) *Offscreen {
+	return &Offscreen{
+		mainFramebuffer: mainFramebuffer,
 	}
-
-	offscreen.mainFramebufferTexture = rendertarget.NewWithCurrentFramebuffer(
-		screenWidth*screenScale,
-		screenHeight*screenScale)
-
-	return offscreen
 }
 
-func (o *Offscreen) Set(rt *rendertarget.RenderTarget) {
+func (o *Offscreen) Set(rt RenderTarget) {
 	o.currentRenderTarget = rt
 	rt.SetAsViewport()
 }
 
 func (o *Offscreen) SetMainFramebuffer() {
-	o.Set(o.mainFramebufferTexture)
+	o.Set(o.mainFramebuffer)
 }
 
-func (o *Offscreen) DrawTexture(texture *texture.Texture,
+func (o *Offscreen) DrawTexture(texture Texture,
 	geometryMatrix matrix.Geometry, colorMatrix matrix.Color) {
-	projectionMatrix := o.projectionMatrix()
+	projectionMatrix := o.currentRenderTarget.ProjectionMatrix()
 	texture.Draw(projectionMatrix, geometryMatrix, colorMatrix)
 }
 
-func (o *Offscreen) DrawTextureParts(texture *texture.Texture,
+func (o *Offscreen) DrawTextureParts(texture Texture,
 	parts []graphics.TexturePart,
 	geometryMatrix matrix.Geometry, colorMatrix matrix.Color) {
-	projectionMatrix := o.projectionMatrix()
+	projectionMatrix := o.currentRenderTarget.ProjectionMatrix()
 	texture.DrawParts(parts, projectionMatrix, geometryMatrix, colorMatrix)
 }
-
-func (o *Offscreen) projectionMatrix() [16]float32 {
-	matrix := o.currentRenderTarget.ProjectionMatrix()
-	if o.currentRenderTarget == o.mainFramebufferTexture {
-		actualScreenHeight := o.screenHeight * o.screenScale
-		// Flip Y and move to fit with the top of the window.
-		matrix[1][1] *= -1
-		matrix[1][3] += float64(actualScreenHeight) /
-			float64(graphics.AdjustSizeForTexture(actualScreenHeight)) * 2
-	}
-
-	projectionMatrix := [16]float32{}
-	for j := 0; j < 4; j++ {
-		for i := 0; i < 4; i++ {
-			projectionMatrix[i+j*4] = float32(matrix[i][j])
-		}
-	}
-	return projectionMatrix
-}
diff --git a/graphics/opengl/rendertarget/render_target.go b/graphics/opengl/rendertarget/render_target.go
index 7fc33d41e..80a11ae33 100644
--- a/graphics/opengl/rendertarget/render_target.go
+++ b/graphics/opengl/rendertarget/render_target.go
@@ -15,12 +15,15 @@ type RenderTarget struct {
 	framebuffer C.GLuint
 	width       int
 	height      int
+	flipY       bool
 }
 
 func NewWithCurrentFramebuffer(width, height int) *RenderTarget {
 	framebuffer := C.GLint(0)
 	C.glGetIntegerv(C.GL_FRAMEBUFFER_BINDING, &framebuffer)
-	return &RenderTarget{C.GLuint(framebuffer), width, height}
+	rt := &RenderTarget{C.GLuint(framebuffer), width, height, true}
+	rt.setAsViewport()
+	return rt
 }
 
 func createFramebuffer(nativeTexture C.GLuint) C.GLuint {
@@ -50,10 +53,10 @@ func createFramebuffer(nativeTexture C.GLuint) C.GLuint {
 
 func CreateFromTexture(native NativeTexture, width, height int) *RenderTarget {
 	framebuffer := createFramebuffer(C.GLuint(native))
-	return &RenderTarget{framebuffer, width, height}
+	return &RenderTarget{framebuffer, width, height, false}
 }
 
-func (r *RenderTarget) SetAsViewport() {
+func (r *RenderTarget) setAsViewport() {
 	C.glFlush()
 
 	C.glBindFramebuffer(C.GL_FRAMEBUFFER, C.GLuint(r.framebuffer))
@@ -70,10 +73,26 @@ func (r *RenderTarget) SetAsViewport() {
 	C.glViewport(0, 0, C.GLsizei(width), C.GLsizei(height))
 }
 
+func (r *RenderTarget) SetAsViewport() {
+	current := C.GLint(0)
+	C.glGetIntegerv(C.GL_FRAMEBUFFER_BINDING, &current)
+	if C.GLuint(current) == r.framebuffer {
+		return
+	}
+	r.setAsViewport()
+}
+
 func (r *RenderTarget) ProjectionMatrix() [4][4]float64 {
 	width := graphics.AdjustSizeForTexture(r.width)
 	height := graphics.AdjustSizeForTexture(r.height)
-	return graphics.OrthoProjectionMatrix(0, width, 0, height)
+	matrix := graphics.OrthoProjectionMatrix(0, width, 0, height)
+	if r.flipY {
+		// Flip Y and move to fit with the top of the window.
+		matrix[1][1] *= -1
+		matrix[1][3] += float64(r.height) /
+			float64(graphics.AdjustSizeForTexture(r.height)) * 2
+	}
+	return matrix
 }
 
 func (r *RenderTarget) Dispose() {
diff --git a/graphics/opengl/texture/texture.go b/graphics/opengl/texture/texture.go
index 90a6dd5b5..52c02432c 100644
--- a/graphics/opengl/texture/texture.go
+++ b/graphics/opengl/texture/texture.go
@@ -19,6 +19,16 @@ type Texture struct {
 	height int
 }
 
+func glMatrix(matrix [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(matrix[i][j])
+		}
+	}
+	return result
+}
+
 func createNativeTexture(textureWidth, textureHeight int, pixels []uint8,
 	filter graphics.Filter) C.GLuint {
 	nativeTexture := C.GLuint(0)
@@ -73,18 +83,18 @@ func (t *Texture) CreateRenderTarget() *rendertarget.RenderTarget {
 		rendertarget.NativeTexture(t.native), t.width, t.height)
 }
 
-func (t *Texture) Draw(projectionMatrix [16]float32, geometryMatrix matrix.Geometry, colorMatrix matrix.Color) {
+func (t *Texture) Draw(projectionMatrix [4][4]float64, geometryMatrix matrix.Geometry, colorMatrix matrix.Color) {
 	quad := graphics.TextureQuadForTexture(t.width, t.height)
 	shader.DrawTexture(shader.NativeTexture(t.native),
-		projectionMatrix, []graphics.TextureQuad{quad},
+		glMatrix(projectionMatrix), []graphics.TextureQuad{quad},
 		geometryMatrix, colorMatrix)
 }
 
-func (t *Texture) DrawParts(parts []graphics.TexturePart, projectionMatrix [16]float32,
+func (t *Texture) DrawParts(parts []graphics.TexturePart, projectionMatrix [4][4]float64,
 	geometryMatrix matrix.Geometry, colorMatrix matrix.Color) {
 	quads := graphics.TextureQuadsForTextureParts(parts, t.width, t.height)
 	shader.DrawTexture(shader.NativeTexture(t.native),
-		projectionMatrix, quads,
+		glMatrix(projectionMatrix), quads,
 		geometryMatrix, colorMatrix)
 }