Michael Friis' Blog

Some time ago I wrote a post on how to do face detection in C# using OpenCV. I’ve since begun using the Emgu CV wrapper instead of opencvdotnet. Emgu CV is much better, in active development and it even runs on Mono. Two gotchas:

1. You don’t have to install OpenCV, but instead have to copy the relevant dlls (included with the Emgu CV download) to the folder where you code executes.
2. Open CV and X64 are not friends. If you’re running X64 Windows (and unless you are up to recompiling OpenCV) you have to make sure your app is compiled to X86, instead of the usual “Any CPU”.
3. Remember to add PictureBox as per the original tutorial.

Here’s sample code:

using System;
using System.Windows.Forms;
using System.Drawing;
using Emgu.CV;
using Emgu.Util;
using Emgu.CV.Structure;
using Emgu.CV.CvEnum;

namespace opencvtut
{
    public partial class Form1 : Form
    {
		private Capture cap;
		private HaarCascade haar;

        public Form1()
        {
            InitializeComponent();
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
		using (Image<Bgr, byte> nextFrame = cap.QueryFrame())
		{
			if (nextFrame != null)
			{
				// there's only one channel (greyscale), hence the zero index
				//var faces = nextFrame.DetectHaarCascade(haar)[0];
				Image<Gray, byte> grayframe = nextFrame.Convert<Gray, byte>();
				var faces =
					grayframe.DetectHaarCascade(
						haar, 1.4, 4,
						HAAR_DETECTION_TYPE.DO_CANNY_PRUNING,
						new Size(nextFrame.Width/8, nextFrame.Height/8)
						)[0];

				foreach (var face in faces)
				{
					nextFrame.Draw(face.rect, new Bgr(0,double.MaxValue,0), 3);
				}
				pictureBox1.Image = nextFrame.ToBitmap();
			}
		}
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            // passing 0 gets zeroth webcam
			cap = new Capture(0);
            // adjust path to find your xml
			haar = new HaarCascade(
                "..\\..\\..\\..\\lib\\haarcascade_frontalface_alt2.xml");
        }
    }
}

I’ve been mucking around with face detection a bit lately. Here’s a short guide to getting face detection running in C# in a short amount of time.

UPDATE: Recommend EmguCV for C# wrapping OpenCV, read the updated guide.

I’ve identified two free computer vision libraries that do face detection:  Torch3vision and OpenCV(I’m sure there are plenty more, but these seem to be comprehensive, recently updated and freely available). Torch3vision claims to be better than OpenCV but on the other hand more people are building libraries and wrappers around OpenCV and there’s even a wrapper for .Net. While it doesn’t yet wrap the face detection components of OpenCV, it seems to be the most promising solution.

To get face detection in OpenCV to work with C#, do the following:

1. Install OpenCV and OpenCVDotNet as per the instructions
2. Get CVHaar.h from this discussion and place it in C:\Program Files\OpenCVDotNet\src\OpenCVDotNet UPDATE: Dud link, read the updated guide
3. Open OpenCVDotNet.sln, add CVHaar.h to the solution and include it in OpenCVDotNet.cpp
4. Rebuild the solution
5. Create a Windows forms application as described in the tutorial, but do something like the code below

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using OpenCVDotNet;

namespace opencvtut
{
    public partial class Form1 : Form
    {
        private CVCapture cap;
        private CVHaar haar;

        public Form1()
        {
            InitializeComponent();
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            using (CVImage nextFrame = cap.QueryFrame())
            {
                Rectangle[] faces = haar.detectPatterns(1.3, nextFrame, 20, 20, 1.1, 2, 1);
                for (int i = 0; i < faces.Length; i++)
                {
                    nextFrame.DrawRectangle(faces[i], Color.Yellow, 3);
                }
                pictureBox1.Image = nextFrame.ToBitmap();
            }
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            // passing 0 gets zeroth webcam
            cap = new CVCapture(0);
            haar = new CVHaar(
                "C:Program FilesOpenCVdatahaarcascadeshaarcascade_frontalface_alt2.xml");
        }
    }
}

Happy detecting 🙂