Perf improvements on stackblur algorithm

carver.go

@@ -75,8 +75,6 @@ func (c *Carver) set(x, y int, px float64) {
 //   - the minimum energy level is calculated by summing up the current pixel value
 //     with the minimum pixel value of the neighboring pixels from the previous row.
 func (c *Carver) ComputeSeams(p *Processor, img *image.NRGBA) (*image.NRGBA, error) {
-	var srcImg *image.NRGBA
-
 	width, height := img.Bounds().Dx(), img.Bounds().Dy()
 	sobel = c.SobelDetector(img, float64(p.SobelThreshold))
 
@@ -215,8 +213,13 @@ func (c *Carver) ComputeSeams(p *Processor, img *image.NRGBA) (*image.NRGBA, err
 		}
 	}
 
+	var srcImg *image.NRGBA
 	if p.BlurRadius > 0 {
-		srcImg = c.StackBlur(sobel, uint32(p.BlurRadius))
+		srcImg = image.NewNRGBA(img.Bounds())
+		err := Stackblur(srcImg, sobel, uint32(p.BlurRadius))
+		if err != nil {
+			return nil, fmt.Errorf("error bluring the image: %w", err)
+		}
 	} else {
 		srcImg = sobel
 	}

carver_test.go

@@ -318,7 +318,9 @@ func TestCarver_ShouldNotRemoveFaceZone(t *testing.T) {
 	pixels := rgbToGrayscale(img)
 
 	sobel := c.SobelDetector(img, float64(p.SobelThreshold))
-	img = c.StackBlur(sobel, uint32(p.BlurRadius))
+
+	err = Stackblur(img, sobel, uint32(p.BlurRadius))
+	assert.NoError(t, err)
 
 	cParams := pigo.CascadeParams{
 		MinSize:     100,

go.mod

@@ -6,9 +6,9 @@ require (
 	gioui.org v0.8.0
 	github.com/disintegration/imaging v1.6.2
 	github.com/esimov/pigo v1.4.5
-	github.com/stretchr/testify v1.8.1
+	github.com/stretchr/testify v1.10.0
 	golang.org/x/exp v0.0.0-20240707233637-46b078467d37
-	golang.org/x/image v0.18.0
+	golang.org/x/image v0.23.0
 	golang.org/x/term v0.0.0-20220722155259-a9ba230a4035
 )
 
@@ -18,7 +18,8 @@ require (
 	github.com/go-text/typesetting v0.2.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	golang.org/x/exp/shiny v0.0.0-20240707233637-46b078467d37 // indirect
-	golang.org/x/sys v0.22.0 // indirect
+	golang.org/x/sys v0.29.0 // indirect
-	golang.org/x/text v0.16.0 // indirect
+	golang.org/x/text v0.21.0 // indirect
+	gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )

go.sum

@@ -5,7 +5,6 @@ gioui.org v0.8.0/go.mod h1:vEMmpxMOd/iwJhXvGVIzWEbxMWhnMQ9aByOGQdlQ8rc=
 gioui.org/cpu v0.0.0-20210808092351-bfe733dd3334/go.mod h1:A8M0Cn5o+vY5LTMlnRoK3O5kG+rH0kWfJjeKd9QpBmQ=
 gioui.org/shader v1.0.8 h1:6ks0o/A+b0ne7RzEqRZK5f4Gboz2CfG+mVliciy6+qA=
 gioui.org/shader v1.0.8/go.mod h1:mWdiME581d/kV7/iEhLmUgUK5iZ09XR5XpduXzbePVM=
-github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/disintegration/imaging v1.6.2 h1:w1LecBlG2Lnp8B3jk5zSuNqd7b4DXhcjwek1ei82L+c=
@@ -18,35 +17,35 @@ github.com/go-text/typesetting v0.2.1/go.mod h1:mTOxEwasOFpAMBjEQDhdWRckoLLeI/+q
 github.com/go-text/typesetting-utils v0.0.0-20241103174707-87a29e9e6066 h1:qCuYC+94v2xrb1PoS4NIDe7DGYtLnU2wWiQe9a1B1c0=
 github.com/go-text/typesetting-utils v0.0.0-20241103174707-87a29e9e6066/go.mod h1:DDxDdQEnB70R8owOx3LVpEFvpMK9eeH1o2r0yZhFI9o=
 github.com/golang/freetype v0.0.0-20170609003504-e2365dfdc4a0/go.mod h1:E/TSTwGwJL78qG/PmXZO1EjYhfJinVAhrmmHX6Z8B9k=
+github.com/kr/pretty v0.2.1 h1:Fmg33tUaq4/8ym9TJN1x7sLJnHVwhP33CNkpYV/7rwI=
+github.com/kr/pretty v0.2.1/go.mod h1:ipq/a2n7PKx3OHsz4KJII5eveXtPO4qwEXGdVfWzfnI=
+github.com/kr/pty v1.1.1/go.mod h1:pFQYn66WHrOpPYNljwOMqo10TkYh1fy3cYio2l3bCsQ=
+github.com/kr/text v0.1.0 h1:45sCR5RtlFHMR4UwH9sdQ5TC8v0qDQCHnXt+kaKSTVE=
+github.com/kr/text v0.1.0/go.mod h1:4Jbv+DJW3UT/LiOwJeYQe1efqtUx/iVham/4vfdArNI=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
-github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.10.0 h1:Xv5erBjTwe/5IxqUQTdXv5kgmIvbHo3QQyRwhJsOfJA=
-github.com/stretchr/objx v0.4.0/go.mod h1:YvHI0jy2hoMjB+UWwv71VJQ9isScKT/TqJzVSSt89Yw=
+github.com/stretchr/testify v1.10.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
-github.com/stretchr/objx v0.5.0/go.mod h1:Yh+to48EsGEfYuaHDzXPcE3xhTkx73EhmCGUpEOglKo=
-github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
-github.com/stretchr/testify v1.8.0/go.mod h1:yNjHg4UonilssWZ8iaSj1OCr/vHnekPRkoO+kdMU+MU=
-github.com/stretchr/testify v1.8.1 h1:w7B6lhMri9wdJUVmEZPGGhZzrYTPvgJArz7wNPgYKsk=
-github.com/stretchr/testify v1.8.1/go.mod h1:w2LPCIKwWwSfY2zedu0+kehJoqGctiVI29o6fzry7u4=
 golang.org/x/exp v0.0.0-20240707233637-46b078467d37 h1:uLDX+AfeFCct3a2C7uIWBKMJIR3CJMhcgfrUAqjRK6w=
 golang.org/x/exp v0.0.0-20240707233637-46b078467d37/go.mod h1:M4RDyNAINzryxdtnbRXRL/OHtkFuWGRjvuhBJpk2IlY=
 golang.org/x/exp/shiny v0.0.0-20240707233637-46b078467d37 h1:SOSg7+sueresE4IbmmGM60GmlIys+zNX63d6/J4CMtU=
 golang.org/x/exp/shiny v0.0.0-20240707233637-46b078467d37/go.mod h1:3F+MieQB7dRYLTmnncoFbb1crS5lfQoTfDgQy6K4N0o=
 golang.org/x/image v0.0.0-20191009234506-e7c1f5e7dbb8/go.mod h1:FeLwcggjj3mMvU+oOTbSwawSJRM1uh48EjtB4UJZlP0=
 golang.org/x/image v0.0.0-20200927104501-e162460cd6b5/go.mod h1:FeLwcggjj3mMvU+oOTbSwawSJRM1uh48EjtB4UJZlP0=
-golang.org/x/image v0.18.0 h1:jGzIakQa/ZXI1I0Fxvaa9W7yP25TqT6cHIHn+6CqvSQ=
+golang.org/x/image v0.23.0 h1:HseQ7c2OpPKTPVzNjG5fwJsOTCiiwS4QdsYi5XU6H68=
-golang.org/x/image v0.18.0/go.mod h1:4yyo5vMFQjVjUcVk4jEQcU9MGy/rulF5WvUILseCM2E=
+golang.org/x/image v0.23.0/go.mod h1:wJJBTdLfCCf3tiHa1fNxpZmUI4mmoZvwMCPP0ddoNKY=
 golang.org/x/sys v0.0.0-20191026070338-33540a1f6037/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/sys v0.0.0-20201107080550-4d91cf3a1aaf/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
-golang.org/x/sys v0.22.0 h1:RI27ohtqKCnwULzJLqkv897zojh5/DwS/ENaMzUOaWI=
+golang.org/x/sys v0.29.0 h1:TPYlXGxvx1MGTn2GiZDhnjPA9wZzZeGKHHmKhHYvgaU=
-golang.org/x/sys v0.22.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
+golang.org/x/sys v0.29.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
 golang.org/x/term v0.0.0-20191110171634-ad39bd3f0407/go.mod h1:Nr5EML6q2oocZ2LXRh80K7BxOlk5/8JxuGnuhpl+muw=
 golang.org/x/term v0.0.0-20220722155259-a9ba230a4035 h1:Q5284mrmYTpACcm+eAKjKJH48BBwSyfJqmmGDTtT8Vc=
 golang.org/x/term v0.0.0-20220722155259-a9ba230a4035/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
-golang.org/x/text v0.16.0 h1:a94ExnEXNtEwYLGJSIUxnWoxoRz/ZcCsV63ROupILh4=
+golang.org/x/text v0.21.0 h1:zyQAAkrwaneQ066sspRyJaG9VNi/YJ1NfzcGB3hZ/qo=
-golang.org/x/text v0.16.0/go.mod h1:GhwF1Be+LQoKShO3cGOHzqOgRrGaYc9AvblQOmPVHnI=
+golang.org/x/text v0.21.0/go.mod h1:4IBbMaMmOPCJ8SecivzSH54+73PCFmPWxNTLm+vZkEQ=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c h1:Hei/4ADfdWqJk1ZMxUNpqntNwaWcugrBjAiHlqqRiVk=
+gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c/go.mod h1:JHkPIbrfpd72SG/EVd6muEfDQjcINNoR0C8j2r3qZ4Q=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

stackblur.go

@@ -1,16 +1,18 @@
-// Go implementation of StackBlur algorithm described here:
+// Go implementation of the StackBlur algorithm
 // http://incubator.quasimondo.com/processing/fast_blur_deluxe.php
 
 package caire
 
 import (
+	"errors"
 	"image"
+	"image/color"
 )
 
-// blurstack is a linked list containing the color value and a pointer to the next struct.
+// blurStack is a linked list containing the color value and a pointer to the next struct.
-type blurstack struct {
+type blurStack struct {
 	r, g, b, a uint32
-	next       *blurstack
+	next       *blurStack
 }
 
 var mulTable = []uint32{
@@ -51,11 +53,86 @@ var shgTable = []uint32{
 	24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
 }
 
-// StackBlur applies a blur filter to the provided image.
+// Stackblur takes the source image and returns it's blurred version by applying the blur radius defined as parameter. The destination image must be a image.NRGBA.
-// The radius defines the bluring average.
+func Stackblur(dst, src image.Image, radius uint32) error {
-func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
+	// Limit the maximum blur radius to 255 to avoid overflowing the multable.
-	var stackEnd, stackIn, stackOut *blurstack
+	if int(radius) >= len(mulTable) {
-	var width, height = uint32(img.Bounds().Dx()), uint32(img.Bounds().Dy())
+		radius = uint32(len(mulTable) - 1)
+	}
+
+	if radius < 1 {
+		return errors.New("blur radius must be greater than 0")
+	}
+
+	img, ok := dst.(*image.NRGBA)
+	if !ok {
+		return errors.New("the destination image must be image.NRGBA")
+	}
+
+	process(img, src, radius)
+	return nil
+}
+
+func process(dst *image.NRGBA, src image.Image, radius uint32) {
+	srcBounds := src.Bounds()
+	srcMinX := srcBounds.Min.X
+	srcMinY := srcBounds.Min.Y
+
+	dstBounds := srcBounds.Sub(srcBounds.Min)
+	dstW := dstBounds.Dx()
+	dstH := dstBounds.Dy()
+
+	switch src0 := src.(type) {
+	case *image.NRGBA:
+		rowSize := srcBounds.Dx() * 4
+		for dstY := 0; dstY < dstH; dstY++ {
+			di := src0.PixOffset(0, dstY)
+			si := src0.PixOffset(srcMinX, srcMinY+dstY)
+			for dstX := 0; dstX < dstW; dstX++ {
+				copy(dst.Pix[di:di+rowSize], src0.Pix[si:si+rowSize])
+			}
+		}
+	case *image.YCbCr:
+		for dstY := 0; dstY < dstH; dstY++ {
+			di := dst.PixOffset(0, dstY)
+			for dstX := 0; dstX < dstW; dstX++ {
+				srcX := srcMinX + dstX
+				srcY := srcMinY + dstY
+				siy := src0.YOffset(srcX, srcY)
+				sic := src0.COffset(srcX, srcY)
+				r, g, b := color.YCbCrToRGB(src0.Y[siy], src0.Cb[sic], src0.Cr[sic])
+				dst.Pix[di+0] = r
+				dst.Pix[di+1] = g
+				dst.Pix[di+2] = b
+				dst.Pix[di+3] = 0xff
+				di += 4
+			}
+		}
+	default:
+		for dstY := 0; dstY < dstH; dstY++ {
+			di := dst.PixOffset(0, dstY)
+			for dstX := 0; dstX < dstW; dstX++ {
+				c := color.NRGBAModel.Convert(src.At(srcMinX+dstX, srcMinY+dstY)).(color.NRGBA)
+				dst.Pix[di+0] = c.R
+				dst.Pix[di+1] = c.G
+				dst.Pix[di+2] = c.B
+				dst.Pix[di+3] = c.A
+				di += 4
+			}
+		}
+	}
+
+	blurImage(dst, radius)
+}
+
+func blurImage(src *image.NRGBA, radius uint32) {
+	var (
+		stackEnd *blurStack
+		stackIn  *blurStack
+		stackOut *blurStack
+	)
+
+	var width, height = uint32(src.Bounds().Dx()), uint32(src.Bounds().Dy())
 	var (
 		div, widthMinus1, heightMinus1, radiusPlus1, sumFactor uint32
 		x, y, i, p, yp, yi, yw,
@@ -65,26 +142,17 @@ func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
 		pr, pg, pb, pa uint32
 	)
 
-	// Limit the maximum blur radius to 255, otherwise it overflows the multable length
-	// and will panic with and index out of range error.
-	if int(radius) >= len(mulTable) {
-		radius = uint32(len(mulTable) - 1)
-	}
-	if radius < 1 {
-		radius = 1
-	}
-
 	div = radius + radius + 1
 	widthMinus1 = width - 1
 	heightMinus1 = height - 1
 	radiusPlus1 = radius + 1
 	sumFactor = radiusPlus1 * (radiusPlus1 + 1) / 2
 
-	stackStart := new(blurstack)
+	stackStart := new(blurStack)
 	stack := stackStart
 
 	for i = 1; i < div; i++ {
-		stack.next = new(blurstack)
+		stack.next = new(blurStack)
 		stack = stack.next
 		if i == radiusPlus1 {
 			stackEnd = stack
@@ -98,10 +166,10 @@ func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
 	for y = 0; y < height; y++ {
 		rInSum, gInSum, bInSum, aInSum, rSum, gSum, bSum, aSum = 0, 0, 0, 0, 0, 0, 0, 0
 
-		pr = uint32(img.Pix[yi])
+		pr = uint32(src.Pix[yi])
-		pg = uint32(img.Pix[yi+1])
+		pg = uint32(src.Pix[yi+1])
-		pb = uint32(img.Pix[yi+2])
+		pb = uint32(src.Pix[yi+2])
-		pa = uint32(img.Pix[yi+3])
+		pa = uint32(src.Pix[yi+3])
 
 		rOutSum = radiusPlus1 * pr
 		gOutSum = radiusPlus1 * pg
@@ -131,10 +199,10 @@ func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
 				diff = i
 			}
 			p = yi + (diff << 2)
-			pr = uint32(img.Pix[p])
+			pr = uint32(src.Pix[p])
-			pg = uint32(img.Pix[p+1])
+			pg = uint32(src.Pix[p+1])
-			pb = uint32(img.Pix[p+2])
+			pb = uint32(src.Pix[p+2])
-			pa = uint32(img.Pix[p+3])
+			pa = uint32(src.Pix[p+3])
 
 			stack.r = pr
 			stack.g = pg
@@ -158,16 +226,16 @@ func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
 
 		for x = 0; x < width; x++ {
 			pa = (aSum * mulSum) >> shgSum
-			img.Pix[yi+3] = uint8(pa)
+			src.Pix[yi+3] = uint8(pa)
 
 			if pa != 0 {
-				img.Pix[yi] = uint8((rSum * mulSum) >> shgSum)
+				src.Pix[yi] = uint8((rSum * mulSum) >> shgSum)
-				img.Pix[yi+1] = uint8((gSum * mulSum) >> shgSum)
+				src.Pix[yi+1] = uint8((gSum * mulSum) >> shgSum)
-				img.Pix[yi+2] = uint8((bSum * mulSum) >> shgSum)
+				src.Pix[yi+2] = uint8((bSum * mulSum) >> shgSum)
 			} else {
-				img.Pix[yi] = 0
+				src.Pix[yi] = 0
-				img.Pix[yi+1] = 0
+				src.Pix[yi+1] = 0
-				img.Pix[yi+2] = 0
+				src.Pix[yi+2] = 0
 			}
 
 			rSum -= rOutSum
@@ -187,10 +255,10 @@ func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
 			}
 			p = (yw + p) << 2
 
-			stackIn.r = uint32(img.Pix[p])
+			stackIn.r = uint32(src.Pix[p])
-			stackIn.g = uint32(img.Pix[p+1])
+			stackIn.g = uint32(src.Pix[p+1])
-			stackIn.b = uint32(img.Pix[p+2])
+			stackIn.b = uint32(src.Pix[p+2])
-			stackIn.a = uint32(img.Pix[p+3])
+			stackIn.a = uint32(src.Pix[p+3])
 
 			rInSum += stackIn.r
 			gInSum += stackIn.g
@@ -230,10 +298,10 @@ func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
 		rInSum, gInSum, bInSum, aInSum, rSum, gSum, bSum, aSum = 0, 0, 0, 0, 0, 0, 0, 0
 
 		yi = x << 2
-		pr = uint32(img.Pix[yi])
+		pr = uint32(src.Pix[yi])
-		pg = uint32(img.Pix[yi+1])
+		pg = uint32(src.Pix[yi+1])
-		pb = uint32(img.Pix[yi+2])
+		pb = uint32(src.Pix[yi+2])
-		pa = uint32(img.Pix[yi+3])
+		pa = uint32(src.Pix[yi+3])
 
 		rOutSum = radiusPlus1 * pr
 		gOutSum = radiusPlus1 * pg
@@ -259,10 +327,10 @@ func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
 
 		for i = 1; i <= radius; i++ {
 			yi = (yp + x) << 2
-			pr = uint32(img.Pix[yi])
+			pr = uint32(src.Pix[yi])
-			pg = uint32(img.Pix[yi+1])
+			pg = uint32(src.Pix[yi+1])
-			pb = uint32(img.Pix[yi+2])
+			pb = uint32(src.Pix[yi+2])
-			pa = uint32(img.Pix[yi+3])
+			pa = uint32(src.Pix[yi+3])
 
 			stack.r = pr
 			stack.g = pg
@@ -293,16 +361,16 @@ func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
 		for y = 0; y < height; y++ {
 			p = yi << 2
 			pa = (aSum * mulSum) >> shgSum
-			img.Pix[p+3] = uint8(pa)
+			src.Pix[p+3] = uint8(pa)
 
 			if pa > 0 {
-				img.Pix[p] = uint8((rSum * mulSum) >> shgSum)
+				src.Pix[p] = uint8((rSum * mulSum) >> shgSum)
-				img.Pix[p+1] = uint8((gSum * mulSum) >> shgSum)
+				src.Pix[p+1] = uint8((gSum * mulSum) >> shgSum)
-				img.Pix[p+2] = uint8((bSum * mulSum) >> shgSum)
+				src.Pix[p+2] = uint8((bSum * mulSum) >> shgSum)
 			} else {
-				img.Pix[p] = 0
+				src.Pix[p] = 0
-				img.Pix[p+1] = 0
+				src.Pix[p+1] = 0
-				img.Pix[p+2] = 0
+				src.Pix[p+2] = 0
 			}
 
 			rSum -= rOutSum
@@ -322,10 +390,10 @@ func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
 			}
 			p = (x + (p * width)) << 2
 
-			stackIn.r = uint32(img.Pix[p])
+			stackIn.r = uint32(src.Pix[p])
-			stackIn.g = uint32(img.Pix[p+1])
+			stackIn.g = uint32(src.Pix[p+1])
-			stackIn.b = uint32(img.Pix[p+2])
+			stackIn.b = uint32(src.Pix[p+2])
-			stackIn.a = uint32(img.Pix[p+3])
+			stackIn.a = uint32(src.Pix[p+3])
 
 			rInSum += stackIn.r
 			gInSum += stackIn.g
@@ -359,5 +427,4 @@ func (c *Carver) StackBlur(img *image.NRGBA, radius uint32) *image.NRGBA {
 			yi += width
 		}
 	}
-	return img
 }