Resize an image to fill a picture box without stretching

Question

I was wanting to get an image to fill a picture box, but not leaving any whitespace. Thus cutting off parts of the image to fit when its not resized to the aspect ratio of the pictureBox. And to adjust as the user resizes the window/pictureBox. The existing options, Sizemode = Zoom leaves whitespace, as its afraid to cut off any of the image and Sizemode = StretchImage stretches the image, distorting it.

The only way I can think of doing this is creating an algorithm to resize the image, keeping the contrast ratio, and setting the width or length of the image to the pictureBox width or length and creating some runtime loop which runs the algorithm once a frame. It seems kind of performance heavy for what it does and kind of hackish. Is there a better option?

Edit: For anyone coming by, I implemented Ivan Stoev's solution slightly differently:

class ImageHandling
{
    public static Rectangle GetScaledRectangle(Image img, Rectangle thumbRect)
    {
        Size sourceSize = img.Size;
        Size targetSize = thumbRect.Size;
        float scale = Math.Max((float) targetSize.Width / sourceSize.Width, (float) targetSize.Height / sourceSize.Height);
        var rect = new RectangleF();
        rect.Width = scale * sourceSize.Width;
        rect.Height = scale * sourceSize.Height;
        rect.X = (targetSize.Width - rect.Width) / 2;
        rect.Y = (targetSize.Height - rect.Height) / 2;
        return Rectangle.Round(rect);
    }

    public static Image GetResizedImage(Image img, Rectangle rect)
    {
        Bitmap b = new Bitmap(rect.Width, rect.Height);
        Graphics g = Graphics.FromImage((Image) b);
        g.InterpolationMode = InterpolationMode.HighQualityBicubic;
        g.DrawImage(img, 0, 0, rect.Width, rect.Height);
        g.Dispose();
        try
        {
            return (Image)b.Clone();
        }
        finally
        {
            b.Dispose();
            b = null;
            g = null;
        }
    }

    public Form1()
    {
        InitializeComponent();
        updateMainBackground();
    }

    void updateMainBackground()
    {
        Image img = Properties.Resources.BackgroundMain;
        Rectangle newRect = ImageHandling.GetScaledRectangle(img, mainBackground.ClientRectangle);
        mainBackground.Image = ImageHandling.GetResizedImage(img, newRect);
    }

    private void Form1_Resize(object sender, EventArgs e)
    {
        updateMainBackground();
    }
}

Answer 1

If I understand correctly, you are seeking for a "Fill" mode (similar to Windows background picture). There is no standard way of doing that, but it's not so hard to make your own with the help of a small calculation and GDI+:

using System;
using System.Drawing;
using System.IO;
using System.Net;
using System.Windows.Forms;

namespace Samples
{
    public class ImageFillBox : Control
    {
        public ImageFillBox()
        {
            SetStyle(ControlStyles.Selectable | ControlStyles.SupportsTransparentBackColor, false);
            SetStyle(ControlStyles.AllPaintingInWmPaint | ControlStyles.OptimizedDoubleBuffer | ControlStyles.Opaque | ControlStyles.UserPaint | ControlStyles.ResizeRedraw, true);
        }
        private Image image;
        public Image Image
        {
            get { return image; }
            set
            {
                if (image == value) return;
                image = value;
                Invalidate();
            }
        }
        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);
            if (image == null)
                e.Graphics.Clear(BackColor);
            else
            {
                Size sourceSize = image.Size, targetSize = ClientSize;
                float scale = Math.Max((float)targetSize.Width / sourceSize.Width, (float)targetSize.Height / sourceSize.Height);
                var rect = new RectangleF();
                rect.Width = scale * sourceSize.Width;
                rect.Height = scale * sourceSize.Height;
                rect.X = (targetSize.Width - rect.Width) / 2;
                rect.Y = (targetSize.Height - rect.Height) / 2;
                e.Graphics.DrawImage(image, rect);
            }
        }
    }
    static class Test
    {
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            var testForm = new Form();
            testForm.Controls.Add(new ImageFillBox
            {
                Dock = DockStyle.Fill,
                Image = GetImage(@"http://www.celebrityrockstarguitars.com/rock/images/Metall_1.jpg")
            });
            Application.Run(testForm);
        }
        static Image GetImage(string path)
        {
            var uri = new Uri(path);
            if (uri.IsFile) return Image.FromFile(path);
            using (var client = new WebClient())
                return Image.FromStream(new MemoryStream(client.DownloadData(uri)));
        }
    }
}