added shadow blur to software implementation

.gitignore

@@ -2,6 +2,7 @@ test*.html
 plan.txt
 glowgen
 testapp
+testblur
 videocap
 .DS_Store
 .vscode

backend/softwarebackend/blur.go

@@ -0,0 +1,188 @@
+package softwarebackend
+
+import (
+	"image"
+	"image/color"
+	"image/draw"
+	"math"
+)
+
+func (b *SoftwareBackend) activateBlurTarget() {
+	b.blurSwap = b.Image
+	b.Image = image.NewRGBA(b.Image.Rect)
+}
+
+func (b *SoftwareBackend) drawBlurred(size float64) {
+	blurred := box3(b.Image, size)
+	b.Image = b.blurSwap
+	draw.Draw(b.Image, b.Image.Rect, blurred, image.ZP, draw.Over)
+}
+
+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)
+	return img
+}
+
+func box3x(img *image.RGBA, size int) *image.RGBA {
+	bounds := img.Bounds()
+	result := image.NewRGBA(bounds)
+	w, h := bounds.Dx(), bounds.Dy()
+
+	for y := 0; y < h; y++ {
+		if size >= w {
+			var r, g, b, a float64
+			for x := 0; x < w; x++ {
+				col := img.RGBAAt(x, y)
+				r += float64(col.R)
+				g += float64(col.G)
+				b += float64(col.B)
+				a += float64(col.A)
+			}
+
+			factor := 1.0 / float64(w)
+			col := color.RGBA{
+				R: uint8(math.Round(r * factor)),
+				G: uint8(math.Round(g * factor)),
+				B: uint8(math.Round(b * factor)),
+				A: uint8(math.Round(a * factor)),
+			}
+			for x := 0; x < w; x++ {
+				result.SetRGBA(x, y, col)
+			}
+			continue
+		}
+
+		var r, g, b, a float64
+		for x := 0; x <= size; x++ {
+			col := img.RGBAAt(x, y)
+			r += float64(col.R)
+			g += float64(col.G)
+			b += float64(col.B)
+			a += float64(col.A)
+		}
+
+		samples := size + 1
+		x := 0
+		for {
+			factor := 1.0 / float64(samples)
+			col := color.RGBA{
+				R: uint8(math.Round(r * factor)),
+				G: uint8(math.Round(g * factor)),
+				B: uint8(math.Round(b * factor)),
+				A: uint8(math.Round(a * factor)),
+			}
+			result.SetRGBA(x, y, col)
+
+			if x >= w-1 {
+				break
+			}
+
+			if left := x - size; left >= 0 {
+				col = img.RGBAAt(left, y)
+				r -= float64(col.R)
+				g -= float64(col.G)
+				b -= float64(col.B)
+				a -= float64(col.A)
+				samples--
+			}
+
+			x++
+
+			if right := x + size; right < w {
+				col = img.RGBAAt(right, y)
+				r += float64(col.R)
+				g += float64(col.G)
+				b += float64(col.B)
+				a += float64(col.A)
+				samples++
+			}
+		}
+	}
+
+	return result
+}
+
+func box3y(img *image.RGBA, size int) *image.RGBA {
+	bounds := img.Bounds()
+	result := image.NewRGBA(bounds)
+	w, h := bounds.Dx(), bounds.Dy()
+
+	for x := 0; x < w; x++ {
+		if size >= h {
+			var r, g, b, a float64
+			for y := 0; y < h; y++ {
+				col := img.RGBAAt(x, y)
+				r += float64(col.R)
+				g += float64(col.G)
+				b += float64(col.B)
+				a += float64(col.A)
+			}
+
+			factor := 1.0 / float64(h)
+			col := color.RGBA{
+				R: uint8(math.Round(r * factor)),
+				G: uint8(math.Round(g * factor)),
+				B: uint8(math.Round(b * factor)),
+				A: uint8(math.Round(a * factor)),
+			}
+			for y := 0; y < h; y++ {
+				result.SetRGBA(x, y, col)
+			}
+			continue
+		}
+
+		var r, g, b, a float64
+		for y := 0; y <= size; y++ {
+			col := img.RGBAAt(x, y)
+			r += float64(col.R)
+			g += float64(col.G)
+			b += float64(col.B)
+			a += float64(col.A)
+		}
+
+		samples := size + 1
+		y := 0
+		for {
+			factor := 1.0 / float64(samples)
+			col := color.RGBA{
+				R: uint8(math.Round(r * factor)),
+				G: uint8(math.Round(g * factor)),
+				B: uint8(math.Round(b * factor)),
+				A: uint8(math.Round(a * factor)),
+			}
+			result.SetRGBA(x, y, col)
+
+			if y >= h-1 {
+				break
+			}
+
+			if top := y - size; top >= 0 {
+				col = img.RGBAAt(x, top)
+				r -= float64(col.R)
+				g -= float64(col.G)
+				b -= float64(col.B)
+				a -= float64(col.A)
+				samples--
+			}
+
+			y++
+
+			if bottom := y + size; bottom < h {
+				col = img.RGBAAt(x, bottom)
+				r += float64(col.R)
+				g += float64(col.G)
+				b += float64(col.B)
+				a += float64(col.A)
+				samples++
+			}
+		}
+	}
+
+	return result
+}

backend/softwarebackend/fill.go

@@ -20,6 +20,11 @@ func (b *SoftwareBackend) Clear(pts [4][2]float64) {
 }
 
 func (b *SoftwareBackend) Fill(style *backendbase.FillStyle, pts [][2]float64) {
+	if style.Blur > 0 {
+		b.activateBlurTarget()
+		defer b.drawBlurred(style.Blur)
+	}
+
 	if lg := style.LinearGradient; lg != nil {
 		lg := lg.(*LinearGradient)
 		from := [2]float64{style.Gradient.X0, style.Gradient.Y0}

backend/softwarebackend/software.go

@@ -12,6 +12,8 @@ type SoftwareBackend struct {
 
 	MSAA int
 
+	blurSwap *image.RGBA
+
 	clip *image.Alpha
 	mask *image.Alpha
 	w, h int