diff --git a/webp/rgb_image_test.go b/webp/rgb_image_test.go
new file mode 100644
index 0000000..5cad78d
--- /dev/null
+++ b/webp/rgb_image_test.go
@@ -0,0 +1,64 @@
+package webp
+
+import (
+	"image"
+	"image/color"
+	"testing"
+)
+
+func TestConvertFromRGBA(t *testing.T) {
+	rgba := color.RGBA{0x11, 0x22, 0x33, 0xFF}
+	expect := RGB{0x11, 0x22, 0x33}
+	if got := RGBModel.Convert(rgba); got != expect {
+		t.Errorf("got: %v, expect: %v", got, expect)
+	}
+}
+
+func TestConvertFromRGB(t *testing.T) {
+	c := RGB{0x11, 0x22, 0x33}
+	if got := RGBModel.Convert(c); got != c {
+		t.Errorf("got: %v, expect: %v", got, c)
+	}
+}
+
+func TestColorRGBA(t *testing.T) {
+	c := RGB{0x11, 0x22, 0x33}
+	r, g, b, a := uint32(0x1111), uint32(0x2222), uint32(0x3333), uint32(0xFFFF)
+
+	gotR, gotG, gotB, gotA := c.RGBA()
+	if gotR != r {
+		t.Errorf("got R: %v, expect R: %v", gotR, r)
+	}
+	if gotG != g {
+		t.Errorf("got G: %v, expect G: %v", gotG, g)
+	}
+	if gotB != b {
+		t.Errorf("got B: %v, expect B: %v", gotB, b)
+	}
+	if gotA != a {
+		t.Errorf("got A: %v, expect A: %v", gotA, a)
+	}
+}
+
+func TestImageInterface(t *testing.T) {
+	rect := image.Rect(0, 0, 100, 100)
+	img := NewRGBImage(rect)
+
+	if got := img.ColorModel(); got != RGBModel {
+		t.Errorf("ColorModel() should return rgb.ColorModel, got: %v", got)
+	}
+
+	if got := img.Bounds(); got != rect {
+		t.Errorf("Bounds() should return %v, got: %v", rect, got)
+	}
+
+	black := color.RGBA{0x00, 0x00, 0x00, 0xFF}
+	if got := img.At(0, 0); got != black {
+		t.Errorf("At(0, 0) should return %v, got: %v", black, got)
+	}
+
+	blank := color.RGBA{}
+	if got := img.At(-1, -1); got != blank {
+		t.Errorf("At(0, 0) should return %v, got: %v", blank, got)
+	}
+}
diff --git a/webp/webp_test.go b/webp/webp_test.go
index 103c0ce..15915df 100644
--- a/webp/webp_test.go
+++ b/webp/webp_test.go
@@ -9,7 +9,6 @@ import (
 
 	"github.com/harukasan/go-libwebp/test/util"
 	"github.com/harukasan/go-libwebp/webp"
-	"image/color"
 )
 
 func TestMain(m *testing.M) {
@@ -221,60 +220,3 @@ func TestEncodeYUVA(t *testing.T) {
 		return
 	}
 }
-
-func TestImageInterface(t *testing.T) {
-	rect := image.Rect(0, 0, 100, 100)
-	img := webp.NewRGBImage(rect)
-
-	if got := img.ColorModel(); got != webp.RGBModel {
-		t.Errorf("ColorModel() should return rgb.ColorModel, got: %v", got)
-	}
-
-	if got := img.Bounds(); got != rect {
-		t.Errorf("Bounds() should return %v, got: %v", rect, got)
-	}
-
-	black := color.RGBA{0x00, 0x00, 0x00, 0xFF}
-	if got := img.At(0, 0); got != black {
-		t.Errorf("At(0, 0) should return %v, got: %v", black, got)
-	}
-
-	blank := color.RGBA{}
-	if got := img.At(-1, -1); got != blank {
-		t.Errorf("At(0, 0) should return %v, got: %v", blank, got)
-	}
-}
-
-func TestConvertFromRGBA(t *testing.T) {
-	rgba := color.RGBA{0x11, 0x22, 0x33, 0xFF}
-	expect := webp.RGB{0x11, 0x22, 0x33}
-	if got := webp.RGBModel.Convert(rgba); got != expect {
-		t.Errorf("got: %v, expect: %v", got, expect)
-	}
-}
-
-func TestConvertFromRGB(t *testing.T) {
-	c := webp.RGB{0x11, 0x22, 0x33}
-	if got := webp.RGBModel.Convert(c); got != c {
-		t.Errorf("got: %v, expect: %v", got, c)
-	}
-}
-
-func TestColorRGBA(t *testing.T) {
-	c := webp.RGB{0x11, 0x22, 0x33}
-	r, g, b, a := uint32(0x1111), uint32(0x2222), uint32(0x3333), uint32(0xFFFF)
-
-	gotR, gotG, gotB, gotA := c.RGBA()
-	if gotR != r {
-		t.Errorf("got R: %v, expect R: %v", gotR, r)
-	}
-	if gotG != g {
-		t.Errorf("got G: %v, expect G: %v", gotG, g)
-	}
-	if gotB != b {
-		t.Errorf("got B: %v, expect B: %v", gotB, b)
-	}
-	if gotA != a {
-		t.Errorf("got A: %v, expect A: %v", gotA, a)
-	}
-}