colors: add threshold filter

README.md

@@ -125,6 +125,7 @@ gift.New().DrawAt(dstImage, fgImage, image.Pt(100, 100), gift.OverOperator)
     - Sepia(percentage float32)
     - Sigmoid(midpoint, factor float32)
     - Sobel()
+    - Threshold(percentage float32)
     - UnsharpMask(sigma, amount, threshold float32)
 
 

colors.go

@@ -466,6 +466,21 @@ func ColorBalance(percentageRed, percentageGreen, percentageBlue float32) Filter
 	}
 }
 
+// Threshold creates a filter that applies black/white thresholding to the image.
+// The percentage parameter must be in range (0, 100).
+func Threshold(percentage float32) Filter {
+	p := minf32(maxf32(percentage, 0), 100) / 100
+	return &colorFilter{
+		fn: func(px pixel) pixel {
+			y := 0.299*px.r + 0.587*px.g + 0.114*px.b
+			if y > p {
+				return pixel{1, 1, 1, px.a}
+			}
+			return pixel{0, 0, 0, px.a}
+		},
+	}
+}
+
 // ColorFunc creates a filter that changes the colors of an image using custom function.
 // The fn parameter specifies a function that takes red, green, blue and alpha channels of a pixel
 // as float32 values in range (0, 1) and returns the modified channel values.

colors_test.go

@@ -1007,6 +1007,85 @@ func TestColorBalance(t *testing.T) {
 	}
 }
 
+func TestThreshold(t *testing.T) {
+	testData := []struct {
+		desc           string
+		percentage     float32
+		srcb, dstb     image.Rectangle
+		srcPix, dstPix []uint8
+	}{
+		{
+			"threshold -1",
+			-1,
+			image.Rect(-1, -1, 5, 0),
+			image.Rect(0, 0, 6, 1),
+			[]uint8{0x00, 0x33, 0x66, 0x99, 0xcc, 0xff},
+			[]uint8{0x00, 0xff, 0xff, 0xff, 0xff, 0xff},
+		},
+		{
+			"threshold 0",
+			0,
+			image.Rect(-1, -1, 5, 0),
+			image.Rect(0, 0, 6, 1),
+			[]uint8{0x00, 0x33, 0x66, 0x99, 0xcc, 0xff},
+			[]uint8{0x00, 0xff, 0xff, 0xff, 0xff, 0xff},
+		},
+		{
+			"threshold 30",
+			30,
+			image.Rect(-1, -1, 5, 0),
+			image.Rect(0, 0, 6, 1),
+			[]uint8{0x00, 0x33, 0x66, 0x99, 0xcc, 0xff},
+			[]uint8{0x00, 0x00, 0xff, 0xff, 0xff, 0xff},
+		},
+		{
+			"threshold 50",
+			50,
+			image.Rect(-1, -1, 5, 0),
+			image.Rect(0, 0, 6, 1),
+			[]uint8{0x00, 0x33, 0x66, 0x99, 0xcc, 0xff},
+			[]uint8{0x00, 0x00, 0x00, 0xff, 0xff, 0xff},
+		},
+		{
+			"threshold 90",
+			90,
+			image.Rect(-1, -1, 5, 0),
+			image.Rect(0, 0, 6, 1),
+			[]uint8{0x00, 0x33, 0x66, 0x99, 0xcc, 0xff},
+			[]uint8{0x00, 0x00, 0x00, 0x00, 0x00, 0xff},
+		},
+		{
+			"threshold 100",
+			100,
+			image.Rect(-1, -1, 5, 0),
+			image.Rect(0, 0, 6, 1),
+			[]uint8{0x00, 0x33, 0x66, 0x99, 0xcc, 0xff},
+			[]uint8{0x00, 0x00, 0x00, 0x00, 0x00, 0x00},
+		},
+		{
+			"threshold 101",
+			101,
+			image.Rect(-1, -1, 5, 0),
+			image.Rect(0, 0, 6, 1),
+			[]uint8{0x00, 0x33, 0x66, 0x99, 0xcc, 0xff},
+			[]uint8{0x00, 0x00, 0x00, 0x00, 0x00, 0x00},
+		},
+	}
+
+	for _, d := range testData {
+		src := image.NewGray(d.srcb)
+		src.Pix = d.srcPix
+
+		f := Threshold(d.percentage)
+		dst := image.NewGray(f.Bounds(src.Bounds()))
+		f.Draw(dst, src, nil)
+
+		if !checkBoundsAndPix(dst.Bounds(), d.dstb, dst.Pix, d.dstPix) {
+			t.Errorf("test [%s] failed: %#v, %#v", d.desc, dst.Bounds(), dst.Pix)
+		}
+	}
+}
+
 func TestColorFunc(t *testing.T) {
 	testData := []struct {
 		desc           string