重绘半透明线

Question

我的可移动面板包含图像控件和图像前面的透明用户控件（PathDraw）。

我在PathDraw类中使用以下代码使其透明。

public partial class PathDraw : UserControl
{
    public PathDraw()
    {
        ...
        InitializeComponent();
        SetStyle(ControlStyles.SupportsTransparentBackColor, true);
        BackColor = Color.Transparent;
    }
    protected override CreateParams CreateParams
    {
        get
        {
            CreateParams cp = base.CreateParams;
            cp.ExStyle |= 0x20;
            return cp;
        }
    }

    protected override void OnPaintBackground(System.Windows.Forms.PaintEventArgs pevent)
    {
        // Do nothing.
    }

    ...

在MouseDown事件中，绘制一条包含alpha 100的行并添加到pathList以供将来使用

    private void PathDraw_MouseDown(object sender, MouseEventArgs e)
    {
        ...
        var gr = this.CreateGraphics();
        gr.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;

        var path = GetPath(pointList[pointList.Count - 1], e.Location);
        gr.DrawPath(pathPen, path);
        pointList.Add(e.Location);
        pathList.Add(path);
    }

    private GraphicsPath GetPath(Point p1, Point p2)
    {
        GraphicsPath path = new GraphicsPath();
        path.AddLine(p1, p2);
        return path;
    }
    public PathDraw()
    {
        // create semi transparent pen
        Color semiTransparentColor = Color.FromArgb(100, Color.Blue);
        pathPen = new Pen(semiTransparentColor, 4);
        pathPen.LineJoin = System.Drawing.Drawing2D.LineJoin.Round;
        pathPen.StartCap = System.Drawing.Drawing2D.LineCap.Round;
        pathPen.EndCap = System.Drawing.Drawing2D.LineCap.Round;
        ...
    }

问题：当移动面板和图像刷新时，重绘路径基于paint事件中的ClipRectangle。

检查e.ClipRectangle以及路径是否与ClipRectangle相交，重绘

但是一条线会失去透明度并变得不透明。

    private void PathDraw_Paint(object sender, PaintEventArgs e)
    {
        e.Graphics.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.AntiAlias;
        for (int i = 0; i < pathList.Count; i++)
        {
            if (e.ClipRectangle.IntersectsWith(Rectangle.Round(pathList[i].GetBounds())))
            {
                e.Graphics.DrawPath(pathPen, pathList[i]);
            }
        }
    }

我认为我的错误是在不清除的情况下绘制完整的行。

如何在重绘之前清除线条？或任何解决方案。

修改 我出于某种原因使用此控件来显示图像

public partial class CompoundControl : UserControl
{
    private System.Windows.Forms.Label backLabel;
    public CompoundControl()
    {
        InitializeComponent();
    }

    //Image _img;
    public Image Image
    {
        get
        {
            return backLabel.Image;
        }
        set
        {
            backLabel.Image = value;
            //_img = value;
        }
    }

public event PaintEventHandler Painted;

    private void backLabel_Paint(object sender, PaintEventArgs e)
    {
        if (this.Painted != null)
        {
            this.Painted(this, e);
        }
    }   
    ...

并在表单类中检查Painted事件

    private void image_Painted(object sender, PaintEventArgs e)
    {
        var clip = e.ClipRectangle;
        clip.Offset((sender as CompoundControl).Location);
        pathDrawObj.Invalidate(clip);
    }

我更改此控件并正常工作。 CompoundControl中有什么问题？