diff --git a/graphics/opengl/context.go b/graphics/opengl/context.go
index c524948d2..e46a039d1 100644
--- a/graphics/opengl/context.go
+++ b/graphics/opengl/context.go
@@ -6,7 +6,6 @@ package opengl
 // #include <OpenGL/gl.h>
 import "C"
 import (
-	"fmt"
 	"github.com/hajimehoshi/go-ebiten/graphics"
 	"github.com/hajimehoshi/go-ebiten/graphics/matrix"
 	"github.com/hajimehoshi/go-ebiten/graphics/opengl/rendertarget"
@@ -115,46 +114,16 @@ func (context *Context) SetOffscreen(renderTargetId graphics.RenderTargetId) {
 	context.setOffscreen(renderTarget)
 }
 
-func (context *Context) doSetOffscreen(
-	usingMainFramebuffer bool,
-	framebuffer interface{}, x, y, width, height int) {
-	f := framebuffer.(rendertarget.Framebuffer)
-	C.glBindFramebuffer(C.GL_FRAMEBUFFER, C.GLuint(f))
-	err := C.glCheckFramebufferStatus(C.GL_FRAMEBUFFER)
-	if err != C.GL_FRAMEBUFFER_COMPLETE {
-		panic(fmt.Sprintf("glBindFramebuffer failed: %d", err))
-	}
-
-	C.glBlendFuncSeparate(C.GL_SRC_ALPHA, C.GL_ONE_MINUS_SRC_ALPHA,
-		C.GL_ZERO, C.GL_ONE)
-
-	C.glViewport(C.GLint(x), C.GLint(y),
-		C.GLsizei(width), C.GLsizei(height))
-
-	matrix := graphics.OrthoProjectionMatrix(x, width, y, height)
-	if usingMainFramebuffer {
-		actualScreenHeight := context.screenHeight * context.screenScale
-		// Flip Y and move to fit with the top of the window.
-		matrix[1][1] *= -1
-		matrix[1][3] += float64(actualScreenHeight) / float64(height) * 2
-	}
-
-	for j := 0; j < 4; j++ {
-		for i := 0; i < 4; i++ {
-			context.projectionMatrix[i+j*4] = float32(matrix[i][j])
-		}
-	}
-}
-
 func (context *Context) setOffscreen(rt *grendertarget.RenderTarget) {
 	C.glFlush()
 
-	usingMainFramebuffer := rt == context.mainFramebufferTexture
-	rt.SetAsOffscreen(func(framebuffer interface{},
-		x, y, width, height int) {
-		context.doSetOffscreen(usingMainFramebuffer, framebuffer,
-			x, y, width, height)
-	})
+	setter := &OffscreenSetter{
+		context.screenHeight,
+		context.screenScale,
+		rt == context.mainFramebufferTexture,
+		&context.projectionMatrix,
+	}
+	rt.SetAsOffscreen(setter)
 }
 
 func (context *Context) setMainFramebufferOffscreen() {
diff --git a/graphics/opengl/offscreen.go b/graphics/opengl/offscreen.go
new file mode 100644
index 000000000..7672ab36c
--- /dev/null
+++ b/graphics/opengl/offscreen.go
@@ -0,0 +1,48 @@
+package opengl
+
+// #cgo LDFLAGS: -framework OpenGL
+//
+// #include <stdlib.h>
+// #include <OpenGL/gl.h>
+import "C"
+import (
+	"fmt"
+	"github.com/hajimehoshi/go-ebiten/graphics"
+	"github.com/hajimehoshi/go-ebiten/graphics/opengl/rendertarget"
+)
+
+type OffscreenSetter struct {
+	screenHeight int
+	screenScale int
+	usingMainFramebuffer bool
+	projectionMatrix *[16]float32
+}
+
+func (s *OffscreenSetter) Set(framebuffer interface{}, x, y, width, height int) {
+	f := framebuffer.(rendertarget.Framebuffer)
+	C.glBindFramebuffer(C.GL_FRAMEBUFFER, C.GLuint(f))
+	err := C.glCheckFramebufferStatus(C.GL_FRAMEBUFFER)
+	if err != C.GL_FRAMEBUFFER_COMPLETE {
+		panic(fmt.Sprintf("glBindFramebuffer failed: %d", err))
+	}
+
+	C.glBlendFuncSeparate(C.GL_SRC_ALPHA, C.GL_ONE_MINUS_SRC_ALPHA,
+		C.GL_ZERO, C.GL_ONE)
+
+	C.glViewport(C.GLint(x), C.GLint(y),
+		C.GLsizei(width), C.GLsizei(height))
+
+	matrix := graphics.OrthoProjectionMatrix(x, width, y, height)
+	if s.usingMainFramebuffer {
+		actualScreenHeight := s.screenHeight * s.screenScale
+		// Flip Y and move to fit with the top of the window.
+		matrix[1][1] *= -1
+		matrix[1][3] += float64(actualScreenHeight) / float64(height) * 2
+	}
+
+	for j := 0; j < 4; j++ {
+		for i := 0; i < 4; i++ {
+			s.projectionMatrix[i+j*4] = float32(matrix[i][j])
+		}
+	}
+}
diff --git a/graphics/opengl/texture/texture.go b/graphics/opengl/texture/texture.go
index 2de71ab69..1fd8e7902 100644
--- a/graphics/opengl/texture/texture.go
+++ b/graphics/opengl/texture/texture.go
@@ -56,8 +56,7 @@ func createNativeTexture(textureWidth, textureHeight int, pixels []uint8,
 
 func create(textureWidth, textureHeight int, filter Filter) (
 	interface{}, error) {
-	return createNativeTexture(textureWidth, textureHeight,
-		nil, filter), nil
+	return createNativeTexture(textureWidth, textureHeight, nil, filter), nil
 }
 
 func createFromImage(img *image.NRGBA) (interface{}, error) {
diff --git a/graphics/rendertarget/render_target.go b/graphics/rendertarget/render_target.go
index d3c02e054..105106464 100644
--- a/graphics/rendertarget/render_target.go
+++ b/graphics/rendertarget/render_target.go
@@ -4,6 +4,10 @@ import (
 	"github.com/hajimehoshi/go-ebiten/graphics/texture"
 )
 
+type OffscreenSetter interface {
+	Set(framebuffer interface{}, x, y, width, height int)
+}
+
 type RenderTarget struct {
 	texture     *texture.Texture
 	framebuffer interface{}
@@ -16,9 +20,8 @@ func NewWithFramebuffer(texture *texture.Texture, framebuffer interface{}) *Rend
 	}
 }
 
-func (renderTarget *RenderTarget) SetAsOffscreen(
-	setter func(framebuffer interface{}, x, y, width, height int)) {
+func (renderTarget *RenderTarget) SetAsOffscreen(setter OffscreenSetter) {
 	renderTarget.texture.SetAsViewport(func(x, y, width, height int) {
-		setter(renderTarget.framebuffer, x, y, width, height)
+		setter.Set(renderTarget.framebuffer, x, y, width, height)
 	})
 }