C# - Detect face and crop image

Question

I'm writing a HttpHandler in C# which serves resized images and blah blah blah... No troubles, we have millions of handlers to use as reference.

The problem is that I have pictures of my users taken with "traditional" sizes, as 4:3 and 16:9. But this handler will need to serve the picture in a Photo ID size (4cm by 3cm) and obviously has need of cropping around the user face. The faces positions vary a lot (aren't always at the picture center).

So, what kind of algorithm I could use to detect the face center and then crop the image around this point?

Answer 1

You can use HaarCascade class in EmguCV (DotNet port of OpenCV) http://www.emgu.com/wiki/index.php/Face_detection

Notes in order to run this example:

• Create a Windows Form Application
• Add a PictureBox and a Timer (and Enable it) - Run it on a x86 system
• Be sure you have the OpenCV relevant dlls (included with the Emgu CV download) in the folder where you code executes.
• Adjust the path to find the Haarcascade xml (last line of the 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");
        }
    }
}