diff --git a/graphics/opengl/context.go b/graphics/opengl/context.go
index 791a423da..2dddac242 100644
--- a/graphics/opengl/context.go
+++ b/graphics/opengl/context.go
@@ -10,7 +10,6 @@ import (
 	"github.com/hajimehoshi/go-ebiten/graphics/matrix"
 	"github.com/hajimehoshi/go-ebiten/graphics/opengl/offscreen"
 	"github.com/hajimehoshi/go-ebiten/graphics/opengl/rendertarget"
-	"math"
 )
 
 type Context struct {
@@ -36,12 +35,14 @@ func newContext(ids *ids, screenWidth, screenHeight, screenScale int) *Context {
 	if err != nil {
 		panic("initializing the offscreen failed: " + err.Error())
 	}
+	context.ResetOffscreen()
 	context.Clear()
 
 	return context
 }
 
 func (context *Context) Dispose() {
+	// TODO: remove main framebuffer?
 	context.ids.DeleteRenderTarget(context.screenId)
 }
 
@@ -71,13 +72,7 @@ func (context *Context) Clear() {
 }
 
 func (context *Context) Fill(r, g, b uint8) {
-	const max = float64(math.MaxUint8)
-	C.glClearColor(
-		C.GLclampf(float64(r)/max),
-		C.GLclampf(float64(g)/max),
-		C.GLclampf(float64(b)/max),
-		1)
-	C.glClear(C.GL_COLOR_BUFFER_BIT)
+	context.offscreen.Fill(r, g, b)
 }
 
 func (context *Context) DrawTexture(
diff --git a/graphics/opengl/offscreen/offscreen.go b/graphics/opengl/offscreen/offscreen.go
index 962ee99db..f68d32643 100644
--- a/graphics/opengl/offscreen/offscreen.go
+++ b/graphics/opengl/offscreen/offscreen.go
@@ -15,6 +15,8 @@ type Texture interface {
 type RenderTarget interface {
 	SetAsViewport()
 	ProjectionMatrix() [4][4]float64
+	Clear()
+	Fill(r, g, b uint8)
 }
 
 type Offscreen struct {
@@ -30,7 +32,6 @@ func New(mainFramebuffer RenderTarget) *Offscreen {
 
 func (o *Offscreen) Set(rt RenderTarget) {
 	o.currentRenderTarget = rt
-	rt.SetAsViewport()
 }
 
 func (o *Offscreen) SetMainFramebuffer() {
@@ -39,6 +40,7 @@ func (o *Offscreen) SetMainFramebuffer() {
 
 func (o *Offscreen) DrawTexture(texture Texture,
 	geometryMatrix matrix.Geometry, colorMatrix matrix.Color) {
+	o.currentRenderTarget.SetAsViewport()
 	projectionMatrix := o.currentRenderTarget.ProjectionMatrix()
 	texture.Draw(projectionMatrix, geometryMatrix, colorMatrix)
 }
@@ -46,6 +48,15 @@ func (o *Offscreen) DrawTexture(texture Texture,
 func (o *Offscreen) DrawTextureParts(texture Texture,
 	parts []graphics.TexturePart,
 	geometryMatrix matrix.Geometry, colorMatrix matrix.Color) {
+	o.currentRenderTarget.SetAsViewport()
 	projectionMatrix := o.currentRenderTarget.ProjectionMatrix()
 	texture.DrawParts(parts, projectionMatrix, geometryMatrix, colorMatrix)
 }
+
+func (o *Offscreen) Clear() {
+	o.currentRenderTarget.Clear()
+}
+
+func (o *Offscreen) Fill(r, g, b uint8) {
+	o.currentRenderTarget.Fill(r, g, b)
+}
diff --git a/graphics/opengl/rendertarget/render_target.go b/graphics/opengl/rendertarget/render_target.go
index 80a11ae33..990a447e6 100644
--- a/graphics/opengl/rendertarget/render_target.go
+++ b/graphics/opengl/rendertarget/render_target.go
@@ -7,6 +7,7 @@ import "C"
 import (
 	"fmt"
 	"github.com/hajimehoshi/go-ebiten/graphics"
+	"math"
 )
 
 type NativeTexture C.GLuint
@@ -21,9 +22,7 @@ type RenderTarget struct {
 func NewWithCurrentFramebuffer(width, height int) *RenderTarget {
 	framebuffer := C.GLint(0)
 	C.glGetIntegerv(C.GL_FRAMEBUFFER_BINDING, &framebuffer)
-	rt := &RenderTarget{C.GLuint(framebuffer), width, height, true}
-	rt.setAsViewport()
-	return rt
+	return &RenderTarget{C.GLuint(framebuffer), width, height, true}
 }
 
 func createFramebuffer(nativeTexture C.GLuint) C.GLuint {
@@ -87,7 +86,6 @@ func (r *RenderTarget) ProjectionMatrix() [4][4]float64 {
 	height := graphics.AdjustSizeForTexture(r.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
@@ -98,3 +96,18 @@ func (r *RenderTarget) ProjectionMatrix() [4][4]float64 {
 func (r *RenderTarget) Dispose() {
 	C.glDeleteFramebuffers(1, &r.framebuffer)
 }
+
+func (r *RenderTarget) Clear() {
+	r.Fill(0, 0, 0)
+}
+
+func (r *RenderTarget) Fill(red, green, blue uint8) {
+	r.SetAsViewport()
+	const max = float64(math.MaxUint8)
+	C.glClearColor(
+		C.GLclampf(float64(red)/max),
+		C.GLclampf(float64(green)/max),
+		C.GLclampf(float64(blue)/max),
+		1)
+	C.glClear(C.GL_COLOR_BUFFER_BIT)
+}