Print a moustache on everyone's face using openCV

moustache.go

package main

import (
	"fmt"
	"image"
	"image/color"
	"math"

	"gocv.io/x/gocv"
)

const filled = -1
const n = 50

var black = color.RGBA{0, 0, 0, 0}

// adapted from gocv tutorial
func main() {
	webcam, err := gocv.VideoCaptureDevice(0)
	if err != nil {
		panic(err)
	}
	defer webcam.Close()

	window := gocv.NewWindow("Snorbot")
	defer window.Close()
	img := gocv.NewMat()
	defer img.Close()

	classifier := gocv.NewCascadeClassifier()
	defer classifier.Close()
	if !classifier.Load("../../gocv/data/haarcascade_frontalface_default.xml") {
		fmt.Println("Error reading cascade file: data/haarcascade_frontalface_default.xml")
		return
	}

	for {
		if ok := webcam.Read(&img); !ok {
			fmt.Printf("cannot read device\n")
			return
		}
		if img.Empty() {
			continue
		}

		rects := classifier.DetectMultiScale(img)
		for _, r := range rects {
			mid := r.Min.Add(r.Size().Div(2)).Add(image.Point{0, r.Dy() / 4})
			w := float64(r.Dx()) / 4.0
			h := float64(r.Dy()) / 16.0

			moustacheHalf(mid, &img, w, h, 1)
			moustacheHalf(mid, &img, w, h, -1)
		}

		window.IMShow(img)
		window.WaitKey(1)
	}
}

func moustacheHalf(mid image.Point, img *gocv.Mat, w, h float64, mod int) {
	pts := []image.Point{image.Pt(mid.X, mid.Y)}
	for i := 0; i < n; i++ {
		step := float64(i+1) / float64(n)
		p := image.Pt(mid.X+mod*int(step*w), mid.Y-int(math.Sin(0.7*step*2*math.Pi)*h))
		pts = append(pts, p)
	}
	points := [][]image.Point{pts}
	pv := gocv.NewPointsVectorFromPoints(points)
	defer pv.Close()
	gocv.FillPoly(img, pv, black)
}