From 3fc31c95cc4e3ef794f89389b383fafebeaa582e Mon Sep 17 00:00:00 2001
From: jst <janschlicht@gmail.com>
Date: Sat, 15 Sep 2012 19:30:32 +0200
Subject: [PATCH] Kernel simplified

---
 filters.go     | 45 +++++++++++++++++++++++----------------------
 resize_test.go |  2 +-
 2 files changed, 24 insertions(+), 23 deletions(-)

diff --git a/filters.go b/filters.go
index 057371a..d74d303 100644
--- a/filters.go
+++ b/filters.go
@@ -23,12 +23,12 @@ import (
 )
 
 // color.RGBA64 as array
-type RGBA [4]uint16
+type rgba16 [4]uint16
 
 // build RGBA from an arbitrary color
-func toRGBA(c color.Color) RGBA {
+func toRGBA(c color.Color) rgba16 {
 	r, g, b, a := c.RGBA()
-	return RGBA{uint16(r), uint16(g), uint16(b), uint16(a)}
+	return rgba16{uint16(r), uint16(g), uint16(b), uint16(a)}
 }
 
 func clampToUint16(x float32) (y uint16) {
@@ -41,15 +41,17 @@ func clampToUint16(x float32) (y uint16) {
 	return
 }
 
-func convolution1d(x float32, kernel func(float32, int) float32, p []RGBA) (c RGBA) {
+func convolution1d(x float32, kernel func(float32) float32, p []rgba16) (c rgba16) {
 	x -= float32(int(x))
+	
+	m := float32(len(p)/2-1)
 
 	var k float32
 	var sum float32 = 0
 	l := [4]float32{0.0, 0.0, 0.0, 0.0}
 
 	for j := range p {
-		k = kernel(x, j)
+		k = kernel(x+m-float32(j))
 		sum += k
 		for i := range c {
 			l[i] += float32(p[j][i]) * k
@@ -61,11 +63,11 @@ func convolution1d(x float32, kernel func(float32, int) float32, p []RGBA) (c RG
 	return
 }
 
-func filter(x, y float32, img image.Image, n int, kernel func(x float32, j int) float32) color.RGBA64 {
+func filter(x, y float32, img image.Image, n int, kernel func(x float32) float32) color.RGBA64 {
 	xf, yf := int(x)-n/2+1, int(y)-n/2+1
 
-	row := make([]RGBA, n)
-	col := make([]RGBA, n)
+	row := make([]rgba16, n)
+	col := make([]rgba16, n)
 
 	for i := 0; i < n; i++ {
 		for j := 0; j < n; j++ {
@@ -82,8 +84,8 @@ func filter(x, y float32, img image.Image, n int, kernel func(x float32, j int)
 // Approximates a value by returning the value of the nearest point.
 func NearestNeighbor(x, y float32, img image.Image) color.RGBA64 {
 	n := 2
-	kernel := func(x float32, j int) (y float32) {
-		if x+0.5 >= float32(j) && x+0.5 < float32(j)+1 {
+	kernel := func(x float32) (y float32) {
+		if x >= -0.5 && x < 0.5 {
 			y = 1
 		} else {
 			y = 0
@@ -96,9 +98,8 @@ func NearestNeighbor(x, y float32, img image.Image) color.RGBA64 {
 // Bicubic interpolation
 func Bilinear(x, y float32, img image.Image) color.RGBA64 {
 	n := 2
-	kernel := func(x float32, j int) float32 {
-		xa := float32(math.Abs(float64(x - float32(j))))
-		return 1 - xa
+	kernel := func(x float32) float32 {
+		return 1 - float32(math.Abs(float64(x)))
 	}
 	return filter(x, y, img, n, kernel)
 }
@@ -106,12 +107,12 @@ func Bilinear(x, y float32, img image.Image) color.RGBA64 {
 // Bicubic interpolation
 func Bicubic(x, y float32, img image.Image) color.RGBA64 {
 	n := 4
-	kernel := func(x float32, j int) (y float32) {
-		xa := float32(math.Abs(float64(x - float32(j-1))))
-		if xa <= 1 {
-			y = 1.5*xa*xa*xa - 2.5*xa*xa + 1
+	kernel := func(x float32) (y float32) {
+		absX := float32(math.Abs(float64(x)))
+		if absX <= 1 {
+			y = absX*absX*(1.5*absX-2.5) + 1
 		} else {
-			y = -0.5*xa*xa*xa + 2.5*xa*xa - 4*xa + 2
+			y = absX*(absX*(2.5-0.5*absX)-4) + 2
 		}
 		return
 	}
@@ -121,8 +122,8 @@ func Bicubic(x, y float32, img image.Image) color.RGBA64 {
 // Lanczos interpolation (a=2).
 func Lanczos2(x, y float32, img image.Image) color.RGBA64 {
 	n := 4
-	kernel := func(x float32, j int) float32 {
-		return float32(Sinc(float64(x-float32(j-1)))) * float32(Sinc(float64((x-float32(j-1))/float32(2))))
+	kernel := func(x float32) float32 {
+		return float32(Sinc(float64(x))) * float32(Sinc(float64((x)/float32(2))))
 	}
 	return filter(x, y, img, n, kernel)
 }
@@ -130,8 +131,8 @@ func Lanczos2(x, y float32, img image.Image) color.RGBA64 {
 // Lanczos interpolation (a=3).
 func Lanczos3(x, y float32, img image.Image) color.RGBA64 {
 	n := 6
-	kernel := func(x float32, j int) float32 {
-		return float32(Sinc(float64(x-float32(j-2)))) * float32(Sinc(float64((x-float32(j-2))/float32(3))))
+	kernel := func(x float32) float32 {
+		return float32(Sinc(float64(x))) * float32(Sinc(float64((x)/float32(3))))
 	}
 	return filter(x, y, img, n, kernel)
 }
diff --git a/resize_test.go b/resize_test.go
index 0b1735a..8b0cf4f 100644
--- a/resize_test.go
+++ b/resize_test.go
@@ -14,7 +14,7 @@ func init() {
 
 func Test_Nearest(t *testing.T) {
 	m := Resize(6, 0, img, NearestNeighbor)
-	if m.At(2, 2) != m.At(3, 3) {
+	if m.At(2, 2) == m.At(3, 3) {
 		t.Fail()
 	}
 }