diff --git a/core/grayscale_test.go b/core/grayscale_test.go
new file mode 100644
index 0000000..76ef2c2
--- /dev/null
+++ b/core/grayscale_test.go
@@ -0,0 +1,32 @@
+package pigo
+
+import (
+	"image"
+	"image/color"
+	"testing"
+)
+
+const ImgWidth = 10
+const ImgHeight = 10
+
+func TestGrayscale(t *testing.T) {
+	img := image.NewRGBA(image.Rect(0, 0, ImgWidth, ImgHeight))
+	for i := 0; i < img.Bounds().Dx(); i++ {
+		for j := 0; j < img.Bounds().Dy(); j++ {
+			img.Set(i, j, color.RGBA{177, 177, 177, 255})
+		}
+	}
+
+	for i := 0; i < img.Bounds().Dx(); i++ {
+		for j := 0; j < img.Bounds().Dy(); j++ {
+			r, g, b, _ := img.At(i, j).RGBA()
+			r = r >> 8
+			g = g >> 8
+			b = b >> 8
+
+			if r != g || r != b || g != b {
+				t.Errorf("R, G, B value expected to be equal. Got %v, %v, %v", r, g, b)
+			}
+		}
+	}
+}
diff --git a/core/image_test.go b/core/image_test.go
new file mode 100644
index 0000000..6d4eac6
--- /dev/null
+++ b/core/image_test.go
@@ -0,0 +1,230 @@
+package pigo
+
+import (
+	"fmt"
+	"image"
+	"image/color"
+	"image/color/palette"
+	"image/draw"
+	"testing"
+)
+
+func TestImgToNRGBA(t *testing.T) {
+	rect := image.Rect(-1, -1, 15, 15)
+	colors := palette.Plan9
+	testCases := []struct {
+		name string
+		img  image.Image
+	}{
+		{
+			name: "NRGBA",
+			img:  makeNRGBAImage(rect, colors),
+		},
+		{
+			name: "YCbCr-444",
+			img:  makeYCbCrImage(rect, colors, image.YCbCrSubsampleRatio444),
+		},
+		{
+			name: "YCbCr-422",
+			img:  makeYCbCrImage(rect, colors, image.YCbCrSubsampleRatio422),
+		},
+		{
+			name: "YCbCr-420",
+			img:  makeYCbCrImage(rect, colors, image.YCbCrSubsampleRatio420),
+		},
+		{
+			name: "YCbCr-440",
+			img:  makeYCbCrImage(rect, colors, image.YCbCrSubsampleRatio440),
+		},
+		{
+			name: "YCbCr-410",
+			img:  makeYCbCrImage(rect, colors, image.YCbCrSubsampleRatio410),
+		},
+		{
+			name: "YCbCr-411",
+			img:  makeYCbCrImage(rect, colors, image.YCbCrSubsampleRatio411),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			r := tc.img.Bounds()
+			for y := r.Min.Y; y < r.Max.Y; y++ {
+				buf := make([]byte, r.Dx()*4)
+				scan(tc.img, 0, y-r.Min.Y, r.Dx(), y+1-r.Min.Y, buf)
+				wantBuf := readRow(tc.img, y)
+				if !compareBytes(buf, wantBuf, 1) {
+					fmt.Println(tc.img)
+					t.Fatalf("scan horizontal line (y=%d): got %v want %v", y, buf, wantBuf)
+				}
+			}
+			for x := r.Min.X; x < r.Max.X; x++ {
+				buf := make([]byte, r.Dy()*4)
+				scan(tc.img, x-r.Min.X, 0, x+1-r.Min.X, r.Dy(), buf)
+				wantBuf := readColumn(tc.img, x)
+				if !compareBytes(buf, wantBuf, 1) {
+					t.Fatalf("scan vertical line (x=%d): got %v want %v", x, buf, wantBuf)
+				}
+			}
+		})
+	}
+}
+
+
+func scan(img image.Image, x1, y1, x2, y2 int, dst []uint8) {
+	switch img := img.(type) {
+	case *image.NRGBA:
+		size := (x2 - x1) * 4
+		j := 0
+		i := y1*img.Stride + x1*4
+		for y := y1; y < y2; y++ {
+			copy(dst[j:j+size], img.Pix[i:i+size])
+			j += size
+			i += img.Stride
+		}
+	case *image.YCbCr:
+		j := 0
+		x1 += img.Rect.Min.X
+		x2 += img.Rect.Min.X
+		y1 += img.Rect.Min.Y
+		y2 += img.Rect.Min.Y
+		for y := y1; y < y2; y++ {
+			iy := (y-img.Rect.Min.Y)*img.YStride + (x1 - img.Rect.Min.X)
+			for x := x1; x < x2; x++ {
+				var ic int
+				switch img.SubsampleRatio {
+				case image.YCbCrSubsampleRatio444:
+					ic = (y-img.Rect.Min.Y)*img.CStride + (x - img.Rect.Min.X)
+				case image.YCbCrSubsampleRatio422:
+					ic = (y-img.Rect.Min.Y)*img.CStride + (x/2 - img.Rect.Min.X/2)
+				case image.YCbCrSubsampleRatio420:
+					ic = (y/2-img.Rect.Min.Y/2)*img.CStride + (x/2 - img.Rect.Min.X/2)
+				case image.YCbCrSubsampleRatio440:
+					ic = (y/2-img.Rect.Min.Y/2)*img.CStride + (x - img.Rect.Min.X)
+				default:
+					ic = img.COffset(x, y)
+				}
+
+				yy := int(img.Y[iy])
+				cb := int(img.Cb[ic]) - 128
+				cr := int(img.Cr[ic]) - 128
+
+				r := (yy<<16 + 91881*cr + 1<<15) >> 16
+				if r > 0xff {
+					r = 0xff
+				} else if r < 0 {
+					r = 0
+				}
+
+				g := (yy<<16 - 22554*cb - 46802*cr + 1<<15) >> 16
+				if g > 0xff {
+					g = 0xff
+				} else if g < 0 {
+					g = 0
+				}
+
+				b := (yy<<16 + 116130*cb + 1<<15) >> 16
+				if b > 0xff {
+					b = 0xff
+				} else if b < 0 {
+					b = 0
+				}
+
+				dst[j+0] = uint8(r)
+				dst[j+1] = uint8(g)
+				dst[j+2] = uint8(b)
+				dst[j+3] = 0xff
+
+				iy++
+				j += 4
+			}
+		}
+	}
+}
+
+func makeYCbCrImage(rect image.Rectangle, colors []color.Color, sr image.YCbCrSubsampleRatio) *image.YCbCr {
+	img := image.NewYCbCr(rect, sr)
+	j := 0
+	for y := rect.Min.Y; y < rect.Max.Y; y++ {
+		for x := rect.Min.X; x < rect.Max.X; x++ {
+			iy := img.YOffset(x, y)
+			ic := img.COffset(x, y)
+			c := color.NRGBAModel.Convert(colors[j]).(color.NRGBA)
+			img.Y[iy], img.Cb[ic], img.Cr[ic] = color.RGBToYCbCr(c.R, c.G, c.B)
+			j++
+		}
+	}
+	return img
+}
+
+func makeNRGBAImage(rect image.Rectangle, colors []color.Color) *image.NRGBA {
+	img := image.NewNRGBA(rect)
+	fillDrawImage(img, colors)
+	return img
+}
+
+func fillDrawImage(img draw.Image, colors []color.Color) {
+	colorsNRGBA := make([]color.NRGBA, len(colors))
+	for i, c := range colors {
+		nrgba := color.NRGBAModel.Convert(c).(color.NRGBA)
+		nrgba.A = uint8(i % 256)
+		colorsNRGBA[i] = nrgba
+	}
+	rect := img.Bounds()
+	i := 0
+	for y := rect.Min.Y; y < rect.Max.Y; y++ {
+		for x := rect.Min.X; x < rect.Max.X; x++ {
+			img.Set(x, y, colorsNRGBA[i])
+			i++
+		}
+	}
+}
+
+func readRow(img image.Image, y int) []uint8 {
+	row := make([]byte, img.Bounds().Dx()*4)
+	i := 0
+	for x := img.Bounds().Min.X; x < img.Bounds().Max.X; x++ {
+		c := color.NRGBAModel.Convert(img.At(x, y)).(color.NRGBA)
+		row[i+0] = c.R
+		row[i+1] = c.G
+		row[i+2] = c.B
+		row[i+3] = c.A
+		i += 4
+	}
+	return row
+}
+
+func readColumn(img image.Image, x int) []uint8 {
+	column := make([]byte, img.Bounds().Dy()*4)
+	i := 0
+	for y := img.Bounds().Min.Y; y < img.Bounds().Max.Y; y++ {
+		c := color.NRGBAModel.Convert(img.At(x, y)).(color.NRGBA)
+		column[i+0] = c.R
+		column[i+1] = c.G
+		column[i+2] = c.B
+		column[i+3] = c.A
+		i += 4
+	}
+	return column
+}
+
+func compareBytes(a, b []uint8, delta int) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := 0; i < len(a); i++ {
+		if absint(int(a[i])-int(b[i])) > delta {
+			return false
+		}
+	}
+	return true
+}
+
+
+// absint returns the absolute value of i.
+func absint(i int) int {
+	if i < 0 {
+		return -i
+	}
+	return i
+}
\ No newline at end of file