pixel: add Grayscale2bit color

pixel/image.go

@@ -149,6 +149,20 @@ func (img Image[T]) setPixel(index int, c T) {
 		}
 
 		return
+	case zeroColor.BitsPerPixel() == 2:
+		// Grayscale2bit.
+		offset := index / 4      // 4 pixels per byte
+		shift := 6 - (index%4)*2 // bits: 6, 4, 2, 0
+
+		ptr := (*byte)(unsafe.Add(img.data, offset))
+
+		raw := *(*uint8)(unsafe.Pointer(&c))
+		gray := raw & 0b11
+
+		mask := byte(0b11 << shift)
+		*ptr = (*ptr &^ mask) | (gray << shift)
+		return
+
 	case zeroColor.BitsPerPixel()%8 == 0:
 		// Each color starts at a whole byte offset.
 		// This is the easy case.
@@ -206,6 +220,13 @@ func (img Image[T]) Get(x, y int) T {
 		ptr := (*byte)(unsafe.Add(img.data, offset))
 		c = ((*ptr >> (7 - uint8(bits))) & 0x1) > 0
 		return any(c).(T)
+	case zeroColor.BitsPerPixel() == 2:
+		// Grayscale2bit.
+		offset := index / 4      // 4 pixels per byte
+		shift := 6 - (index%4)*2 // bits: 6, 4, 2, 0
+		ptr := (*byte)(unsafe.Add(img.data, offset))
+		value := ((*ptr) >> shift) & 0b11
+		return any(Grayscale2bit(value)).(T)
 	case zeroColor.BitsPerPixel()%8 == 0:
 		// Colors like RGB565, RGB888, etc.
 		offset := index * int(unsafe.Sizeof(zeroColor))

pixel/image_test.go

@@ -107,6 +107,66 @@ func TestImageRGB444BE(t *testing.T) {
 	}
 }
 
+func TestImageGrayscale2bit(t *testing.T) {
+	image := pixel.NewImage[pixel.Grayscale2bit](128, 64)
+
+	if width, height := image.Size(); width != 128 || height != 64 {
+		t.Errorf("image.Size(): expected 128, 64 but got %d, %d", width, height)
+	}
+
+	// Define test colors representing 4 Grayscale levels.
+	testColors := []color.RGBA{
+		{R: 0x00, G: 0x00, B: 0x00, A: 0xff}, // black
+		{R: 0x55, G: 0x55, B: 0x55, A: 0xff}, // dark gray
+		{R: 0xaa, G: 0xaa, B: 0xaa, A: 0xff}, // light gray
+		{R: 0xff, G: 0xff, B: 0xff, A: 0xff}, // white
+	}
+
+	// Single pixel roundtrip test at a fixed coordinate.
+	for _, c := range testColors {
+		encoded := pixel.NewColor[pixel.Grayscale2bit](c.R, c.G, c.B)
+		image.Set(5, 3, encoded)
+		actual := image.Get(5, 3).RGBA()
+		if actual != c {
+			t.Errorf("failed to roundtrip color: expected %v but got %v", c, actual)
+		}
+	}
+
+	// Multi-coordinate test across the image.
+	for x := 0; x < 8; x++ {
+		for y, c := range testColors {
+			encoded := pixel.NewColor[pixel.Grayscale2bit](c.R, c.G, c.B)
+			image.Set(x, y, encoded)
+			actual := image.Get(x, y).RGBA()
+			if actual != c {
+				t.Errorf("Set/Get mismatch at (%d,%d): expected %v but got %v", x, y, c, actual)
+			}
+		}
+	}
+}
+
+func TestNewGrayscale2bitMapping(t *testing.T) {
+	testCases := []struct {
+		input  color.RGBA
+		expect pixel.Grayscale2bit
+	}{
+		{color.RGBA{R: 0x00, G: 0x00, B: 0x00}, 0}, // 0
+		{color.RGBA{R: 0x3F, G: 0x3F, B: 0x3F}, 0}, // 63
+		{color.RGBA{R: 0x40, G: 0x40, B: 0x40}, 1}, // 64
+		{color.RGBA{R: 0x7F, G: 0x7F, B: 0x7F}, 1}, // 127
+		{color.RGBA{R: 0x80, G: 0x80, B: 0x80}, 2}, // 128
+		{color.RGBA{R: 0xBF, G: 0xBF, B: 0xBF}, 2}, // 191
+		{color.RGBA{R: 0xC0, G: 0xC0, B: 0xC0}, 3}, // 192
+		{color.RGBA{R: 0xFF, G: 0xFF, B: 0xFF}, 3}, // 255
+	}
+	for _, tc := range testCases {
+		actual := pixel.NewColor[pixel.Grayscale2bit](tc.input.R, tc.input.G, tc.input.B)
+		if actual != tc.expect {
+			t.Errorf("NewGrayscale2bit(%#v) = %d, want %d", tc.input, actual, tc.expect)
+		}
+	}
+}
+
 func TestImageMonochrome(t *testing.T) {
 	image := pixel.NewImage[pixel.Monochrome](128, 64)
 	if width, height := image.Size(); width != 128 || height != 64 {
@@ -236,6 +296,9 @@ func TestImageNoise(t *testing.T) {
 	t.Run("RGB444BE", func(t *testing.T) {
 		testImageNoiseN[pixel.RGB444BE](t)
 	})
+	t.Run("Grayscale2bit", func(t *testing.T) {
+		testImageNoiseN[pixel.Grayscale2bit](t)
+	})
 	t.Run("Monochrome", func(t *testing.T) {
 		testImageNoiseN[pixel.Monochrome](t)
 	})

pixel/pixel.go

@@ -16,7 +16,7 @@ import (
 // particular display. Each pixel is at least 1 byte in size.
 // The color format is sRGB (or close to it) in all cases except for 1-bit.
 type Color interface {
-	RGB888 | RGB565BE | RGB555 | RGB444BE | Monochrome
+	RGB888 | RGB565BE | RGB555 | RGB444BE | Grayscale2bit | Monochrome
 
 	BaseColor
 }
@@ -50,6 +50,8 @@ func NewColor[T Color](r, g, b uint8) T {
 		return any(NewRGB555(r, g, b)).(T)
 	case RGB444BE:
 		return any(NewRGB444BE(r, g, b)).(T)
+	case Grayscale2bit:
+		return any(NewGrayscale2bit(r, g, b)).(T)
 	case Monochrome:
 		return any(NewMonochrome(r, g, b)).(T)
 	default:
@@ -204,6 +206,34 @@ func (c RGB444BE) RGBA() color.RGBA {
 	return color
 }
 
+// Grayscale2bit represents a 2-bit Grayscale value (4 levels: black, dark gray, light gray, white).
+type Grayscale2bit uint8
+
+func NewGrayscale2bit(r, g, b uint8) Grayscale2bit {
+	// Convert RGB to luminance using standard weights (approximation of human perception)
+	// Use shift-based operations to reduce processing time.
+	// luminance := (299*uint32(r) + 587*uint32(g) + 114*uint32(b)) / 1000
+	luminance := (77*uint32(r) + 150*uint32(g) + 29*uint32(b)) >> 8
+	// Map to 2-bit value: 0–63 => 0, 64–127 => 1, 128–191 => 2, 192–255 => 3
+	return Grayscale2bit((luminance >> 6) & 0b11)
+}
+
+func (c Grayscale2bit) BitsPerPixel() int {
+	return 2
+}
+
+func (c Grayscale2bit) RGBA() color.RGBA {
+	// Expand 2-bit Grayscale back to 8-bit (0–255) using multiplication
+	// 0 → 0x00, 1 → 0x55, 2 → 0xAA, 3 → 0xFF (i.e., multiply by 85)
+	gray := uint8(c&0b11) * 85
+	return color.RGBA{
+		R: gray,
+		G: gray,
+		B: gray,
+		A: 255,
+	}
+}
+
 type Monochrome bool
 
 func NewMonochrome(r, g, b uint8) Monochrome {