ProPictureBox.cs

using System;
using System.Drawing;
using System.Windows.Forms;

namespace FindCircles
{
	public struct ProTransformation
	{
		public Point Translation { get { return _translation; } }
		public double Scale { get { return _scale; } }
		private readonly Point _translation;
		private readonly double _scale;

		public ProTransformation(Point translation, double scale)
		{
			_translation = translation;
			_scale = scale;
		}

		public Point ConvertToIm(Point p)
		{
			return new Point((int)(p.X * _scale + _translation.X), (int)(p.Y * _scale + _translation.Y));
		}

		public Size ConvertToIm(Size p)
		{
			return new Size((int)(p.Width * _scale), (int)(p.Height * _scale));
		}

		public Rectangle ConvertToIm(Rectangle r)
		{
			return new Rectangle(ConvertToIm(r.Location), ConvertToIm(r.Size));
		}

		public Point ConvertToPb(Point p)
		{
			return new Point((int)((p.X - _translation.X) / _scale), (int)((p.Y - _translation.Y) / _scale));
		}

		public Size ConvertToPb(Size p)
		{
			return new Size((int)(p.Width / _scale), (int)(p.Height / _scale));
		}

		public Rectangle ConvertToPb(Rectangle r)
		{
			return new Rectangle(ConvertToPb(r.Location), ConvertToPb(r.Size));
		}

		public ProTransformation SetTranslate(Point p)
		{
			return new ProTransformation(p, _scale);
		}

		public ProTransformation AddTranslate(Point p)
		{
			return SetTranslate(new Point(p.X + _translation.X, p.Y + _translation.Y));
		}

		public ProTransformation SetScale(double scale)
		{
			return new ProTransformation(_translation, scale);
		}
	}

	public class ProPictureBox : PictureBox
	{
		private Point? _clickedPoint;
		private ProTransformation _transformation;
		public ProTransformation Transformation
		{
			set
			{
				_transformation = FixTranslation(value);
				Invalidate();
			}
			get
			{
				return _transformation;
			}
		}

		public ProPictureBox()
		{
			_transformation = new ProTransformation(new Point(100, 0), .5f);
			MouseDown += OnMouseDown;
			MouseMove += OnMouseMove;
			MouseUp += OnMouseUp;
			MouseWheel += OnMouseWheel;
			Resize += OnResize;
		}

		private ProTransformation FixTranslation(ProTransformation value)
		{
			var maxScale = Math.Max((double)Image.Width / ClientRectangle.Width, (double)Image.Height / ClientRectangle.Height);
			if (value.Scale > maxScale)
				value = value.SetScale(maxScale);
			if (value.Scale < 0.3)
				value = value.SetScale(0.3);
			var rectSize = value.ConvertToIm(ClientRectangle.Size);
			var max = new Size(Image.Width - rectSize.Width, Image.Height - rectSize.Height);

			value = value.SetTranslate((new Point(Math.Min(value.Translation.X, max.Width), Math.Min(value.Translation.Y, max.Height))));
			if (value.Translation.X < 0 || value.Translation.Y < 0)
			{
				value = value.SetTranslate(new Point(Math.Max(value.Translation.X, 0), Math.Max(value.Translation.Y, 0)));
			}
			return value;
		}

		private void OnResize(object sender, EventArgs eventArgs)
		{
			if (Image == null)
				return;
			Transformation = Transformation;
		}

		private void OnMouseWheel(object sender, MouseEventArgs e)
		{
			var transformation = _transformation;
			var pos1 = transformation.ConvertToIm(e.Location);
			if (e.Delta > 0)
				transformation = (transformation.SetScale(Transformation.Scale / 1.25));
			else
				transformation = (transformation.SetScale(Transformation.Scale * 1.25));
			var pos2 = transformation.ConvertToIm(e.Location);
			transformation = transformation.AddTranslate(pos1 - (Size)pos2);
			Transformation = transformation;
		}

		private void OnMouseUp(object sender, MouseEventArgs mouseEventArgs)
		{
			_clickedPoint = null;
		}

		private void OnMouseMove(object sender, MouseEventArgs e)
		{
			if (_clickedPoint == null)
				return;
			var p = _transformation.ConvertToIm((Size)e.Location);
			Transformation = _transformation.SetTranslate(_clickedPoint.Value - p);
		}

		private void OnMouseDown(object sender, MouseEventArgs e)
		{
			Focus();
			_clickedPoint = _transformation.ConvertToIm(e.Location);
		}

		protected override void OnPaint(PaintEventArgs e)
		{
			var imRect = Transformation.ConvertToIm(ClientRectangle);
			if (Image == null)
				return;
			e.Graphics.DrawImage(Image, ClientRectangle, imRect, GraphicsUnit.Pixel);
		}

		public void DecideInitialTransformation()
		{
			Transformation = new ProTransformation(Point.Empty, int.MaxValue);
		}
	}
}