diff --git a/examples/shader/main.go b/examples/shader/main.go
index 00ad3625d..15b5003b1 100644
--- a/examples/shader/main.go
+++ b/examples/shader/main.go
@@ -29,17 +29,20 @@ const (
 
 const shaderSrc = `package main
 
+// Internal_ViewportSize is a predefined uniform variable.
+// TODO: Hide this and create a function for the projection matrix?
+
 func Vertex(position vec2, texCoord vec2, color vec4) vec4 {
 	return mat4(
-		2.0/640, 0, 0, 0,
-		0, 2.0/480, 0, 0,
+		2.0/Internal_ViewportSize.x, 0, 0, 0,
+		0, 2.0/Internal_ViewportSize.y, 0, 0,
 		0, 0, 1, 0,
 		-1, -1, 0, 1,
 	) * vec4(position, 0, 1)
 }
 
 func Fragment(position vec4) vec4 {
-	return vec4(position.x/640, position.y/480, 0, 1)
+	return vec4(position.x/Internal_ViewportSize.x, position.y/Internal_ViewportSize.y, 0, 1)
 }`
 
 type Game struct {
diff --git a/image.go b/image.go
index 4a359a10f..2ff60589c 100644
--- a/image.go
+++ b/image.go
@@ -384,6 +384,10 @@ func (i *Image) DrawTrianglesWithShader(vertices []Vertex, indices []uint16, sha
 		}
 	}
 
+	// The last uniform variable is Internal_ViewportSize.
+	// The actual value is set at graphicscommand package.
+	us = append(us, []float32{0, 0})
+
 	var bx0, by0, bx1, by1 float32
 	if firstImage != nil {
 		b := firstImage.Bounds()
diff --git a/internal/graphicscommand/command.go b/internal/graphicscommand/command.go
index 60c834193..6bba313d2 100644
--- a/internal/graphicscommand/command.go
+++ b/internal/graphicscommand/command.go
@@ -453,6 +453,13 @@ func (c *drawTrianglesCommand) Exec(indexOffset int) error {
 				us[i] = v
 			}
 		}
+
+		// The last uniform variables are added at /shader.go and represents a viewport size.
+		w, h := c.dst.InternalSize()
+		viewport := us[len(us)-1].([]float32)
+		viewport[0] = float32(w)
+		viewport[1] = float32(h)
+
 		return theGraphicsDriver.DrawShader(c.dst.image.ID(), c.shader.shader.ID(), c.nindices, indexOffset, c.mode, us)
 	}
 	return theGraphicsDriver.Draw(c.dst.image.ID(), c.src.image.ID(), c.nindices, indexOffset, c.mode, c.color, c.filter, c.address)
diff --git a/shader.go b/shader.go
index d9d1fa724..cacd631d7 100644
--- a/shader.go
+++ b/shader.go
@@ -15,16 +15,25 @@
 package ebiten
 
 import (
+	"bytes"
+
 	"github.com/hajimehoshi/ebiten/internal/buffered"
 	"github.com/hajimehoshi/ebiten/internal/shader"
 )
 
+const shaderSuffix = `
+var Internal_ViewportSize vec2`
+
 type Shader struct {
 	shader *buffered.Shader
 }
 
 func NewShader(src []byte) (*Shader, error) {
-	s, err := shader.Compile(src, "Vertex", "Fragment")
+	var b bytes.Buffer
+	b.Write(src)
+	b.Write([]byte(shaderSuffix))
+
+	s, err := shader.Compile(b.Bytes(), "Vertex", "Fragment")
 	if err != nil {
 		return nil, err
 	}