feat: add mosaic screenshot annotations

internal/application/annotation.go

@@ -81,6 +81,10 @@ func ApplyAnnotationsWithScale(pngBytes []byte, annotations []Annotation, scaleX
 			drawEllipseOutline(dst, ann.Points, scaleX, scaleY, width, c)
 		case "text":
 			drawTextAnnotation(dst, ann, scaleX, scaleY, c)
+		case "mosaic-brush":
+			applyMosaicBrush(dst, ann.Points, scaleX, scaleY, width)
+		case "mosaic-rect":
+			applyMosaicRect(dst, ann.Points, scaleX, scaleY)
 		}
 	}
 
@@ -91,6 +95,93 @@ func ApplyAnnotationsWithScale(pngBytes []byte, annotations []Annotation, scaleX
 	return buf.Bytes(), nil
 }
 
+const mosaicBlockSize = 10
+
+func applyMosaicBrush(img *image.RGBA, points []Point, scaleX, scaleY float64, width int) {
+	if len(points) == 0 {
+		return
+	}
+	mask := image.NewAlpha(img.Bounds())
+	maskColor := color.RGBA{A: 255}
+	if len(points) == 1 {
+		drawDotMask(mask, scalePoint(points[0], scaleX, scaleY), width, maskColor)
+	} else {
+		for i := 1; i < len(points); i++ {
+			drawMaskLine(mask, scalePoint(points[i-1], scaleX, scaleY), scalePoint(points[i], scaleX, scaleY), width, maskColor)
+		}
+	}
+	applyPixelation(img, img.Bounds(), mask)
+}
+
+func applyMosaicRect(img *image.RGBA, points []Point, scaleX, scaleY float64) {
+	if len(points) < 2 {
+		return
+	}
+	a := scalePoint(points[0], scaleX, scaleY)
+	b := scalePoint(points[len(points)-1], scaleX, scaleY)
+	x1, x2 := ordered(a.X, b.X)
+	y1, y2 := ordered(a.Y, b.Y)
+	r := image.Rect(int(math.Floor(x1)), int(math.Floor(y1)), int(math.Ceil(x2)), int(math.Ceil(y2))).Intersect(img.Bounds())
+	applyPixelation(img, r, nil)
+}
+
+func applyPixelation(img *image.RGBA, region image.Rectangle, mask *image.Alpha) {
+	if region.Empty() {
+		return
+	}
+	for y := region.Min.Y; y < region.Max.Y; y += mosaicBlockSize {
+		for x := region.Min.X; x < region.Max.X; x += mosaicBlockSize {
+			block := image.Rect(x, y, min(x+mosaicBlockSize, region.Max.X), min(y+mosaicBlockSize, region.Max.Y))
+			var r, g, b, a, count uint64
+			for py := block.Min.Y; py < block.Max.Y; py++ {
+				for px := block.Min.X; px < block.Max.X; px++ {
+					c := img.RGBAAt(px, py)
+					r += uint64(c.R)
+					g += uint64(c.G)
+					b += uint64(c.B)
+					a += uint64(c.A)
+					count++
+				}
+			}
+			if count == 0 {
+				continue
+			}
+			avg := color.RGBA{uint8(r / count), uint8(g / count), uint8(b / count), uint8(a / count)}
+			for py := block.Min.Y; py < block.Max.Y; py++ {
+				for px := block.Min.X; px < block.Max.X; px++ {
+					if mask == nil || mask.AlphaAt(px, py).A > 0 {
+						img.SetRGBA(px, py, avg)
+					}
+				}
+			}
+		}
+	}
+}
+
+func drawMaskLine(mask *image.Alpha, a, b Point, width int, c color.RGBA) {
+	dx, dy := b.X-a.X, b.Y-a.Y
+	steps := int(math.Max(math.Abs(dx), math.Abs(dy)))
+	if steps == 0 {
+		drawDotMask(mask, a, width, c)
+		return
+	}
+	for i := 0; i <= steps; i++ {
+		t := float64(i) / float64(steps)
+		drawDotMask(mask, Point{X: a.X + dx*t, Y: a.Y + dy*t}, width, c)
+	}
+}
+
+func drawDotMask(mask *image.Alpha, p Point, width int, c color.RGBA) {
+	radius := float64(width) / 2
+	for y := int(math.Floor(p.Y - radius)); y <= int(math.Ceil(p.Y+radius)); y++ {
+		for x := int(math.Floor(p.X - radius)); x <= int(math.Ceil(p.X+radius)); x++ {
+			if image.Pt(x, y).In(mask.Bounds()) && math.Hypot(float64(x)-p.X, float64(y)-p.Y) <= radius {
+				mask.SetAlpha(x, y, color.Alpha{A: c.A})
+			}
+		}
+	}
+}
+
 func parseHexColor(hex string) (color.RGBA, error) {
 	value := strings.TrimPrefix(strings.TrimSpace(hex), "#")
 	if len(value) != 6 {