FaceDetection.cs

using Emgu.CV;
using Emgu.CV.CvEnum;
using Emgu.CV.Dnn;
using Emgu.CV.Structure;
using System;
using System.Collections.Generic;
using System.Drawing;

namespace FaceDetection
{
    /// <summary>
    /// Wraps the Emgu CV YuNet face-detector and returns the bounding rectangles
    /// of all faces found in a bitmap.
    /// </summary>
    public sealed class FaceRectangleDetector : IDisposable
    {
        private readonly FaceDetectorYN _model;

        /// <param name="inputSize">
        /// The width/height that YuNet expects.  
        /// Pass the frame size of your camera / video source.
        /// </param>
        public FaceRectangleDetector(Size inputSize)
        {
            _model = new FaceDetectorYN(
                model: "face_detection_yunet_2022mar.onnx",
                config: string.Empty,
                inputSize: inputSize,
                scoreThreshold: 0.9f,
                nmsThreshold: 0.3f,
                topK: 5,
                backendId: Emgu.CV.Dnn.Backend.Default,
                targetId: Target.Cpu);
        }

        /// <summary>
        /// Detects faces in <paramref name="image"/> and returns their rectangles.
        /// </summary>
        /// <param name="image">Source frame (BGR or RGB bitmap).</param>
        /// <returns>Zero or more face rectangles.</returns>
        public IReadOnlyList<Rectangle> GetFaceRectangles(Bitmap image)
        {
            using var frame = image.ToMat();      // convert to Mat
            using var faces = new Mat();
            _model.Detect(frame, faces);

            var results = new List<Rectangle>();

            if (faces.Rows <= 0)
                return results;

            var facesData = (float[,])faces.GetData(jagged: true);

            for (int i = 0; i < facesData.GetLength(0); i++)
            {
                var rect = new Rectangle(
                    x: (int)facesData[i, 0],
                    y: (int)facesData[i, 1],
                    width: (int)facesData[i, 2],
                    height: (int)facesData[i, 3]);

                results.Add(rect);
            }

            return results;
        }

        public void Dispose() => _model?.Dispose();
    }
}