1. Face Recognition in the Video: The first step is to use a face recognition algorithm to detect and locate faces in the video frames. Libraries like OpenCV or dlib can be used for this purpose.
  2. Opening the Image: You need to open the image containing the face that you want to replace the faces in the video with.
  3. Face Replacement: Using the results of face recognition from the video, you will need to replace the detected faces in the video frames with the face from the image. This process may involve overlaying the image face onto the video frame at the detected face position.
  4. Saving the Result: After replacing the faces, you should save the modified video with the replaced faces.

Here’s a C# code example using OpenCV to demonstrate how to perform this task:

using System;
using OpenCvSharp;

class Program
{
    static void Main()
    {
        using (var image = new Mat("path_to_image.jpg"))
        using (var videoCapture = new VideoCapture("path_to_video.mp4"))
        using (var videoWriter = new VideoWriter("output_video.mp4", VideoWriter.FourCC('X', 'V', 'I', 'D'), 30, new Size(640, 480)))
        {
            var faceCascade = new CascadeClassifier("haarcascade_frontalface_default.xml"); // Face recognition model

            while (true)
            {
                var frame = new Mat();
                videoCapture.Read(frame);

                if (frame.Empty())
                    break;

                var faceRectangles = faceCascade.DetectMultiScale(frame);

                foreach (var faceRect in faceRectangles)
                {
                    var resizedFace = new Mat();
                    Cv2.Resize(image, resizedFace, new Size(faceRect.Width, faceRect.Height));

                    var roi = frame[faceRect];
                    resizedFace.CopyTo(roi);
                }

                videoWriter.Write(frame);
            }
        }
    }
}

This code example uses the Haar Cascade classifier from OpenCV for face recognition and replaces the detected faces in the video frames with the face from the image. Adapt the code according to your project’s specific requirements and customize it as needed.