在图片上画线或矩形

public partial class Form4 : Form
{
	#region 成员变量
	Point m_p1, m_p2;
	bool m_flgKeuDowm = false;
	Bitmap m_mapStart;
	Bitmap m_mapEnd;
	Bitmap m_mapInit;
	Graphics m_graphics;
	#endregion

	public Form4()
	{
		InitializeComponent();
		m_graphics = pictureBox1.CreateGraphics();

		//Image image = Image.FromFile(@"d:\mjy\Led.jpg");
		//Bitmap bitmap = new Bitmap(image);
		//pictureBox1.Image = bitmap;

		// 最初的背景图存起来，清除绘制图形时用
		m_mapInit = (Bitmap)pictureBox1.BackgroundImage;
	}

	private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
	{
		m_flgKeuDowm = true;
		_initPoint(e);

	}

	private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
	{
		m_flgKeuDowm = false;

		// 最终的图片设为背景图
		pictureBox1.BackgroundImage = m_mapEnd;

		// 起止点初始化
		_initPoint(e);
	}

	private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
	{
		if (!m_flgKeuDowm)
		{
			return;
		}
		else
		{
			m_p2 = new Point(e.X, e.Y);
		}

		int width = Math.Abs(e.X - m_p1.X);
		int height = Math.Abs(e.Y - m_p1.Y);


		_draw();
	}

	private void _draw()
	{

		// 每次的【终止图】都是取自【起始图】
		m_mapEnd = (Bitmap)m_mapStart.Clone();
		Graphics g = Graphics.FromImage(m_mapEnd);
		Pen pen = new Pen(Color.Red, 3);
		if (rdoRect.Checked)
		{
			Point p1, p2;
			_swapPoint(out p1, out p2);
			int width = Math.Abs(p2.X - p1.X);
			int height = Math.Abs(p2.Y - p1.Y);
			g.DrawRectangle(pen, p1.X, p1.Y, width, height);
		}
		else if (rdoLine.Checked)
		{
			// 画直线不用转换点坐标，直接用成员变量的Point
			g.DrawLine(pen, m_p1, m_p2);
		}

		m_graphics.DrawImage(m_mapEnd, new Point(0, 0));
	}

	private void _initPoint(MouseEventArgs e)
	{
		m_p1 = new Point(e.X, e.Y);
		m_p2 = m_p1;

		if (pictureBox1.BackgroundImage != null)
		{
			m_mapStart = (Bitmap)pictureBox1.BackgroundImage;
		}
	}

	private void _printPoint(Point p)
	{
		Console.WriteLine(p.X + "," + p.Y);
	}

	private void _swapPoint(out Point _p1, out Point _p2)
	{
		//实现画框随意翻转
		_p1 = m_p1;
		_p2 = m_p2;
		if (m_p1.X > m_p2.X)
		{
			int tmp = _p1.X;
			_p1.X = _p2.X;
			_p2.X = tmp;
		}
		if (m_p1.Y > m_p2.Y)
		{
			int tmp = _p1.Y;
			_p1.Y = _p2.Y;
			_p2.Y = tmp;
		}
	}

	private void btnSave_Click(object sender, EventArgs e)
	{
		// 保存
		SaveFileDialog dlg = new SaveFileDialog();
		dlg.DefaultExt = "png";
		dlg.Filter = "Png Files|*.png";
		dlg.FileName = "截图";

		DialogResult dlgRet = dlg.ShowDialog();
		if (dlgRet == DialogResult.OK)
		{
			pictureBox1.BackgroundImage.Save(dlg.FileName, System.Drawing.Imaging.ImageFormat.Png);
		}
	}

	private void btnClear_Click(object sender, EventArgs e)
	{
		pictureBox1.BackgroundImage = m_mapInit;
	}


}