From 3695a6ef5935c1e73666144bd568ab40e9265921 Mon Sep 17 00:00:00 2001
From: Thomas Friedel <tomee6@gmail.com>
Date: Mon, 13 May 2019 11:56:36 +0200
Subject: [PATCH] improved box blur accuracy

---
 backend/softwarebackend/blur.go | 142 ++++++++++++++++++++++++++++++--
 1 file changed, 135 insertions(+), 7 deletions(-)

diff --git a/backend/softwarebackend/blur.go b/backend/softwarebackend/blur.go
index 2038472..dde33e5 100644
--- a/backend/softwarebackend/blur.go
+++ b/backend/softwarebackend/blur.go
@@ -18,14 +18,142 @@ func (b *SoftwareBackend) drawBlurred(size float64) {
 	draw.Draw(b.Image, b.Image.Rect, blurred, image.ZP, draw.Over)
 }
 
+/*
+func gauss(img *image.RGBA, size float64) *image.RGBA {
+	const maxDist = 128
+
+	var buf [2*maxDist + 1]float64
+	kernel := buf[:]
+	gaussianKernel(size, kernel)
+
+	dist := maxDist
+	for x := 0; x < maxDist; x++ {
+		if kernel[x] < 0.002 {
+			continue
+		}
+		dist = maxDist - x
+		kernel = kernel[x : len(kernel)-x]
+		break
+	}
+
+	fmt.Println(dist)
+
+	img = gaussx(img, kernel, dist)
+	return gaussy(img, kernel, dist)
+}
+
+func gaussx(img *image.RGBA, kernel []float64, maxDist int) *image.RGBA {
+	bounds := img.Bounds()
+	result := image.NewRGBA(bounds)
+	w, h := bounds.Dx(), bounds.Dy()
+
+	for y := 0; y < h; y++ {
+		for x := 0; x < w; x++ {
+			imgFrom, imgTo := x-maxDist, x+maxDist
+			kernelFrom := 0
+			if imgFrom < 0 {
+				kernelFrom -= imgFrom
+				imgFrom = 0
+			}
+			if imgTo >= w {
+				imgTo = w - 1
+			}
+			kernelSum := 0.0
+			var r, g, b, a float64
+			for xx, j := imgFrom, kernelFrom; xx <= imgTo; xx++ {
+				k := kernel[j]
+				kernelSum += k
+				col := img.RGBAAt(xx, y)
+				r += float64(col.R) * k
+				g += float64(col.G) * k
+				b += float64(col.B) * k
+				a += float64(col.A) * k
+				j++
+			}
+			col := color.RGBA{
+				R: uint8(math.Round(r / kernelSum)),
+				G: uint8(math.Round(g / kernelSum)),
+				B: uint8(math.Round(b / kernelSum)),
+				A: uint8(math.Round(a / kernelSum)),
+			}
+			result.SetRGBA(x, y, col)
+		}
+	}
+
+	return result
+}
+
+func gaussy(img *image.RGBA, kernel []float64, maxDist int) *image.RGBA {
+	bounds := img.Bounds()
+	result := image.NewRGBA(bounds)
+	w, h := bounds.Dx(), bounds.Dy()
+
+	for y := 0; y < h; y++ {
+		imgFrom, imgTo := y-maxDist, y+maxDist
+		kernelFrom := 0
+		if imgFrom < 0 {
+			kernelFrom -= imgFrom
+			imgFrom = 0
+		}
+		if imgTo >= h {
+			imgTo = h - 1
+		}
+		for x := 0; x < w; x++ {
+			kernelSum := 0.0
+			var r, g, b, a float64
+			for yy, j := imgFrom, kernelFrom; yy <= imgTo; yy++ {
+				k := kernel[j]
+				kernelSum += k
+				col := img.RGBAAt(x, yy)
+				r += float64(col.R) * k
+				g += float64(col.G) * k
+				b += float64(col.B) * k
+				a += float64(col.A) * k
+				j++
+			}
+			col := color.RGBA{
+				R: uint8(math.Round(r / kernelSum)),
+				G: uint8(math.Round(g / kernelSum)),
+				B: uint8(math.Round(b / kernelSum)),
+				A: uint8(math.Round(a / kernelSum)),
+			}
+			result.SetRGBA(x, y, col)
+		}
+	}
+
+	return result
+}
+
+func gaussianKernel(stddev float64, target []float64) {
+	stddevSqr := stddev * stddev
+	center := float64(len(target) / 2)
+	factor := 1.0 / math.Sqrt(2*math.Pi*stddevSqr)
+	for i := range target {
+		x := float64(i) - center
+		target[i] = factor * math.Pow(math.E, -x*x/(2*stddevSqr))
+	}
+}
+*/
+
 func box3(img *image.RGBA, size float64) *image.RGBA {
-	rsize := int(math.Round(size * 0.75)) // todo properly size
-	img = box3x(img, rsize)
-	img = box3x(img, rsize)
-	img = box3x(img, rsize)
-	img = box3y(img, rsize)
-	img = box3y(img, rsize)
-	img = box3y(img, rsize)
+	size *= 1 - 1/(size+1) // this just seems to improve the accuracy
+
+	fsize := math.Floor(size)
+	sizea := int(fsize)
+	sizeb := sizea
+	sizec := sizea
+	if size-fsize > 0.333333333 {
+		sizeb++
+	}
+	if size-fsize > 0.666666666 {
+		sizec++
+	}
+	img = box3x(img, sizea)
+	img = box3x(img, sizeb)
+	img = box3x(img, sizec)
+	img = box3y(img, sizea)
+	img = box3y(img, sizeb)
+	img = box3y(img, sizec)
 	return img
 }