diff --git a/examples/glut/main.go b/examples/glut/main.go
index 1c321f86e..9aaa7be84 100644
--- a/examples/glut/main.go
+++ b/examples/glut/main.go
@@ -92,6 +92,7 @@ func (ui *GlutUI) Run(device *graphics.Device) {
 
 type DemoGame struct {
 	ebitenTexture *graphics.Texture
+	x int
 }
 
 func (game *DemoGame) Update() {
@@ -109,6 +110,7 @@ func (game *DemoGame) Update() {
 
 		game.ebitenTexture = currentUI.device.NewTextureFromImage(img)
 	}
+	game.x++
 }
 
 func (game *DemoGame) Draw(g *graphics.GraphicsContext, offscreen *graphics.Texture) {
@@ -116,9 +118,12 @@ func (game *DemoGame) Draw(g *graphics.GraphicsContext, offscreen *graphics.Text
 	if game.ebitenTexture == nil {
 		return
 	}
+	geometryMatrix := graphics.IdentityGeometryMatrix()
+	geometryMatrix.SetTx(graphics.AffineMatrixElement(game.x))
+	geometryMatrix.SetTy(graphics.AffineMatrixElement(game.x))
 	g.DrawTexture(game.ebitenTexture,
 		0, 0, game.ebitenTexture.Width, game.ebitenTexture.Height,
-		graphics.IdentityGeometryMatrix(),
+		geometryMatrix,
 		graphics.IdentityColorMatrix())
 }
 
diff --git a/graphics/graphics_context.go b/graphics/graphics_context.go
index b6baaef3b..2185632d6 100644
--- a/graphics/graphics_context.go
+++ b/graphics/graphics_context.go
@@ -8,7 +8,6 @@ import "C"
 import (
 	"fmt"
 	"image/color"
-	"math"
 	"unsafe"
 )
 
@@ -110,10 +109,20 @@ func (context *GraphicsContext) DrawTexture(texture *Texture,
 	C.glDisableClientState(C.GL_VERTEX_ARRAY)
 }
 
+func abs(x int) int {
+	if x < 0 {
+		return -x
+	}
+	return x
+}
+
 func (context *GraphicsContext) SetOffscreen(texture *Texture) {
 	framebuffer := C.GLuint(0)
 	if texture != nil {
 		framebuffer = context.getFramebuffer(texture)
+		if framebuffer == context.mainFramebuffer {
+			panic("invalid framebuffer")
+		}
 	} else {
 		framebuffer = context.mainFramebuffer
 	}
@@ -136,9 +145,7 @@ func (context *GraphicsContext) SetOffscreen(texture *Texture) {
 		tx     = -1
 		ty     = 1
 	}
-	C.glViewport(0, 0,
-		C.GLsizei(math.Abs(float64(width))),
-		C.GLsizei(math.Abs(float64(height))))
+	C.glViewport(0, 0, C.GLsizei(abs(width)), C.GLsizei(abs(height)))
 	e11 := float32(2.0) / float32(width)
 	e22 := float32(2.0) / float32(height)
 	e41 := float32(tx)
diff --git a/graphics/texture.go b/graphics/texture.go
index 9601ca183..8840ad0bc 100644
--- a/graphics/texture.go
+++ b/graphics/texture.go
@@ -28,7 +28,7 @@ type Texture struct {
 }
 
 func createTexture(device *Device, width, height int, pixels []uint8) *Texture{
-	textureWidth := int(Clp2(uint64(width)))
+	textureWidth  := int(Clp2(uint64(width)))
 	textureHeight := int(Clp2(uint64(height)))
 	if pixels != nil {
 		if width != textureWidth {
diff --git a/ui/ui.go b/ui/ui.go
index c3447577b..a0e1b56f6 100644
--- a/ui/ui.go
+++ b/ui/ui.go
@@ -10,8 +10,3 @@ type UI interface {
 	ScreenScale() int
 	Run(device *graphics.Device)
 }
-
-func ExecuteOnUIThread(f func()) {
-	// TODO: implement!
-	f()
-}