减去形式/s的一半是什么意思?

我在我的新表单中有这个构造函数:

public MagnifierForm(Point MousePosition)
        {
            _doMove = true;
            FormBorderStyle = FormBorderStyle.None;
            ShowInTaskbar = false;
            TopMost = true;
            Width = 150;
            Height = 150;
            GraphicsPath gp = new GraphicsPath();
            gp.AddEllipse(ClientRectangle);
            Region = new Region(gp);
            mTimer = new Timer();
            mTimer.Enabled = true;
            mTimer.Interval = 20;
            mTimer.Tick += new EventHandler(HandleTimer);
            mScreenImage = new Bitmap(Screen.PrimaryScreen.Bounds.Width,
                                     Screen.PrimaryScreen.Bounds.Height);
            //maybe subtract half of the forms width/height from the points (offset them)
            mStartPoint = MousePosition;
            mTargetPoint = MousePosition;
            speed = 0.35F;
        }

在构造函数的底部，我添加了以下代码:

mStartPoint = MousePosition;
mTargetPoint = MousePosition;

在Form1中，我有这个代码:

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
        {
            if (keyData == (Keys.Control | Keys.M))
            {
                if (mf == null)
                {
                    mf = new Magnifier20070401.MagnifierForm(System.Windows.Forms.Cursor.Position);
                    mf.StartPosition = FormStartPosition.Manual;
                    mf.Location = Control.MousePosition;
                    mf.Show();
                    this.Select();
                }
                else if (mf.IsDisposed)
                {
                    mf = new Magnifier20070401.MagnifierForm(System.Windows.Forms.Cursor.Position);
                    mf.StartPosition = FormStartPosition.Manual;
                    mf.Location = Control.MousePosition;
                    mf.Show();
                }
                else
                {
                    mf.Close();
                    mf = null;
                }
            }
            return base.ProcessCmdKey(ref msg, keyData);
        }

一切正常。这个mStartPoint设置的位置/位置的形式也工作得很好。

但是这个句子是什么意思呢?//也许从这些点减去一半的宽度/高度(偏移它们)

这个句子与:mStartPoint和mTargetPoint有关

谁能给我一个例子，如果我想使用这个句子，它应该看起来像什么?

mScreenImage = new Bitmap(Screen.PrimaryScreen.Bounds.Width,
                                     Screen.PrimaryScreen.Bounds.Height);