diff --git a/internal/graphicscommand/framebuffer.go b/internal/graphicscommand/framebuffer.go
index 3cdf237a8..8814737d1 100644
--- a/internal/graphicscommand/framebuffer.go
+++ b/internal/graphicscommand/framebuffer.go
@@ -18,23 +18,6 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/opengl"
 )
 
-// orthoProjectionMatrix returns an orthogonal projection matrix for OpenGL.
-//
-// The matrix converts the coodinates (left, bottom) - (right, top) to the normalized device coodinates (-1, -1) - (1, 1).
-func orthoProjectionMatrix(left, right, bottom, top int) []float32 {
-	e11 := 2 / float32(right-left)
-	e22 := 2 / float32(top-bottom)
-	e14 := -1 * float32(right+left) / float32(right-left)
-	e24 := -1 * float32(top+bottom) / float32(top-bottom)
-
-	return []float32{
-		e11, 0, 0, 0,
-		0, e22, 0, 0,
-		0, 0, 1, 0,
-		e14, e24, 0, 1,
-	}
-}
-
 // framebuffer is a wrapper of OpenGL's framebuffer.
 type framebuffer struct {
 	native    opengl.Framebuffer
@@ -89,6 +72,6 @@ func (f *framebuffer) projectionMatrix() []float32 {
 		return f.proMatrix
 	}
 	w, h := f.viewportSize()
-	f.proMatrix = orthoProjectionMatrix(0, w, 0, h)
+	f.proMatrix = opengl.OrthoProjectionMatrix(0, w, 0, h)
 	return f.proMatrix
 }
diff --git a/internal/opengl/matrix.go b/internal/opengl/matrix.go
new file mode 100644
index 000000000..b3dc73853
--- /dev/null
+++ b/internal/opengl/matrix.go
@@ -0,0 +1,32 @@
+// Copyright 2018 The Ebiten Authors
+//
+// 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 opengl
+
+// OrthoProjectionMatrix returns an orthogonal projection matrix for OpenGL.
+//
+// The matrix converts the coodinates (left, bottom) - (right, top) to the normalized device coodinates (-1, -1) - (1, 1).
+func OrthoProjectionMatrix(left, right, bottom, top int) []float32 {
+	e11 := 2 / float32(right-left)
+	e22 := 2 / float32(top-bottom)
+	e14 := -1 * float32(right+left) / float32(right-left)
+	e24 := -1 * float32(top+bottom) / float32(top-bottom)
+
+	return []float32{
+		e11, 0, 0, 0,
+		0, e22, 0, 0,
+		0, 0, 1, 0,
+		e14, e24, 0, 1,
+	}
+}