feat: add line and arrow annotations

internal/application/annotation.go

@@ -75,6 +75,10 @@ func ApplyAnnotationsWithScale(pngBytes []byte, annotations []Annotation, scaleX
 		switch ann.Tool {
 		case "pen":
 			drawPolyline(dst, ann.Points, scaleX, scaleY, width, c)
+		case "line":
+			drawStraightLine(dst, ann.Points, scaleX, scaleY, width, c)
+		case "arrow":
+			drawArrow(dst, ann.Points, scaleX, scaleY, width, c)
 		case "rect":
 			drawRectOutline(dst, ann.Points, scaleX, scaleY, width, c)
 		case "ellipse":
@@ -95,6 +99,34 @@ func ApplyAnnotationsWithScale(pngBytes []byte, annotations []Annotation, scaleX
 	return buf.Bytes(), nil
 }
 
+func drawStraightLine(img *image.RGBA, points []Point, scaleX, scaleY float64, width int, c color.RGBA) {
+	if len(points) < 2 {
+		return
+	}
+	drawLine(img, scalePoint(points[0], scaleX, scaleY), scalePoint(points[len(points)-1], scaleX, scaleY), width, c)
+}
+
+func drawArrow(img *image.RGBA, points []Point, scaleX, scaleY float64, width int, c color.RGBA) {
+	if len(points) < 2 {
+		return
+	}
+	start := scalePoint(points[0], scaleX, scaleY)
+	end := scalePoint(points[len(points)-1], scaleX, scaleY)
+	dx, dy := end.X-start.X, end.Y-start.Y
+	length := math.Hypot(dx, dy)
+	if length < 1 {
+		return
+	}
+	drawLine(img, start, end, width, c)
+	headLength := math.Min(length*0.45, math.Max(10*math.Max(scaleX, scaleY), float64(width)*4))
+	angle := math.Atan2(dy, dx)
+	spread := math.Pi / 6
+	left := Point{X: end.X - headLength*math.Cos(angle-spread), Y: end.Y - headLength*math.Sin(angle-spread)}
+	right := Point{X: end.X - headLength*math.Cos(angle+spread), Y: end.Y - headLength*math.Sin(angle+spread)}
+	drawLine(img, end, left, width, c)
+	drawLine(img, end, right, width, c)
+}
+
 const mosaicBlockSize = 10
 
 func applyMosaicBrush(img *image.RGBA, points []Point, scaleX, scaleY float64, width int) {