When evaluating libraries for facial recognition and privacy protection in .NET, developers often compare DlibDotNet and ViewFaceCore. While both libraries are capable, ViewFaceCore offers a more straightforward coordinate system that maps intuitively to image pixels. This tutorial outlines the process of creating a Windows Forms application to mask faces in images using ViewFaceCore, focusing on three distinct obfuscation techniques.

First, create a WinForms project and install the ViewFaceCore package via the NuGet Package Manager. The implementation consists of two primary stages: identifying the location of faces using the FaceDetector class, and then applying a visual mask to the detected regions.

1. Solid Color Masking

The simplest approach involves drawing a solid rectangle over the detected face. This method utilizes GDI+ to render a shape on the canvas. The following code iterates through the collection of detected faces and fills the corresponding rectangular areas with a user-defined color.

private void ApplySolidMask(Graphics canvas, List<FaceInfo> detectedFaces, Color maskColor)
{
    using (var brush = new SolidBrush(maskColor))
    {
        foreach (var faceItem in detectedFaces)
        {
            var location = faceItem.Face.Location;
            
            // Draw the rectangle directly at the detected coordinates
            canvas.FillRectangle(brush, 
                location.X, 
                location.Y, 
                location.Width, 
                location.Height);
        }
    }
}

2. Image Overlay Masking

Instead of a flat color, an application may require covering the face with another image, such as a emoji or icon. This is achieved by using the DrawImage method to stretch the overlay image to fit the dimensions of the detected face bounding box.

private void ApplyImageMask(Graphics canvas, List<FaceInfo> detectedFaces, Image overlayGraphic)
{
    if (overlayGraphic == null) return;

    foreach (var faceItem in detectedFaces)
    {
        var location = faceItem.Face.Location;
        
        var targetRect = new Rectangle(
            location.X, 
            location.Y, 
            location.Width, 
            location.Height);

        canvas.DrawImage(overlayGraphic, targetRect);
    }
}

3. Pixelation (Mosaic) Effect

Unlike solid colors or overlays, which can be drawn directly onto a control's surface, a mosaic efffect requires direct manipulation of the image's pixel data. The following implementation demonstrates a mosaic effect by downscaling the image region and upscaling it back using NearestNeighbor interpolation to create the blocky pixelated look.

private void ApplyMosaicEffect(Bitmap sourceBitmap, List<FaceInfo> detectedFaces, int blockSize)
{
    using (var graphics = Graphics.FromImage(sourceBitmap))
    {
        foreach (var faceItem in detectedFaces)
        {
            var region = faceItem.Face.Location;

            // Calculate reduced dimensions to create the mosaic blocks
            int reducedWidth = Math.Max(1, region.Width / blockSize);
            int reducedHeight = Math.Max(1, region.Height / blockSize);

            // Create a temporary bitmap for the downscaled region
            using (var tempBmp = new Bitmap(reducedWidth, reducedHeight))
            using (var g = Graphics.FromImage(tempBmp))
            {
                // Draw the original face area into the smaller bitmap
                g.DrawImage(sourceBitmap, 0, 0, reducedWidth, reducedHeight);

                // Draw the small bitmap back onto the original image using NearestNeighbor
                // This preserves the hard edges of the pixels
                graphics.InterpolationMode = InterpolationMode.NearestNeighbor;
                graphics.DrawImage(tempBmp, region);
            }
        }
    }
}