Added MitchellNetravali, changed Lanczos a bit

filters.go

@@ -115,16 +115,30 @@ func Bicubic(img image.Image) Filter {
 	}, make([]rgba16, 4), make([]rgba16, 4)}
 }
 
+func MitchellNetravali(img image.Image) Filter {
+	return &filterModel{img, 4, func(x float32) (y float32) {
+		absX := float32(math.Abs(float64(x)))
+		if absX <= 1 {
+			y = absX*absX*(7*absX-12) + 16.0/3
+		} else {
+			y = -(absX - 2) * (absX - 2) / 3 * (7*absX - 8)
+		}
+		return
+	}, make([]rgba16, 4), make([]rgba16, 4)}
+}
+
+func lanczosKernel(a uint) func(float32) float32 {
+	return func(x float32) float32 {
+		return float32(Sinc(float64(x))) * float32(Sinc(float64(x/float32(a))))
+	}
+}
+
 // Lanczos interpolation (a=2).
 func Lanczos2(img image.Image) Filter {
-	return &filterModel{img, 4, func(x float32) float32 {
-		return float32(Sinc(float64(x))) * float32(Sinc(float64((x)/float32(2))))
-	}, make([]rgba16, 4), make([]rgba16, 4)}
+	return &filterModel{img, 4, lanczosKernel(2), make([]rgba16, 4), make([]rgba16, 4)}
 }
 
 // Lanczos interpolation (a=3).
 func Lanczos3(img image.Image) Filter {
-	return &filterModel{img, 6, func(x float32) float32 {
-		return float32(Sinc(float64(x))) * float32(Sinc(float64((x)/float32(3))))
-	}, make([]rgba16, 6), make([]rgba16, 6)}
+	return &filterModel{img, 6, lanczosKernel(3), make([]rgba16, 6), make([]rgba16, 6)}
 }

resize_test.go

@@ -3,12 +3,14 @@ package resize
 import (
 	"image"
 	"image/color"
+	"runtime"
 	"testing"
 )
 
 var img = image.NewGray16(image.Rect(0, 0, 3, 3))
 
 func init() {
+	runtime.GOMAXPROCS(runtime.NumCPU())
 	img.Set(1, 1, color.White)
 }
 
@@ -43,6 +45,9 @@ func Test_ZeroImg(t *testing.T) {
 }
 
 func Benchmark_BigResize(b *testing.B) {
-	m := Resize(1000, 1000, img, Lanczos3)
+	var m image.Image
+	for i := 0; i < b.N; i++ {
+		m = Resize(1000, 1000, img, Lanczos3)
+	}
 	m.At(0, 0)
 }