在其上绘画时保留 Picturebox 图像

Question

在我的表单中，我有一个图片框。我希望能够在图像上绘制箭头。我设法走到了一半。在表单加载事件中，我将图像分配给图片框。我可以使用下面的代码创建箭头。问题是每次我创建箭头时，我在表单加载事件中分配的图片都会被删除。为什么我的图像会被删除？如何维护我在加载表单时分配的图像，同时在其顶部绘制箭头？

    private bool isMoving = false;
    private Point mouseDownPosition = Point.Empty;
    private Point mouseMovePosition = Point.Empty;
    private List<Tuple<Point, Point>> lines = new List<Tuple<Point, Point>>();
    Pen _Pen;

    private void Test_Load(object sender, EventArgs e)
    {
        pictureBox1.Image = Properties.Resources.background;          
    }

    private void pictureBox1_Paint(object sender, PaintEventArgs e)
    {

        if (isMoving)
        {
            if (pictureBox1.Image == null)
            {
                Bitmap bmp = new Bitmap(pictureBox1.Width, pictureBox1.Height);
                using (Graphics g = Graphics.FromImage(bmp))
                {
                    g.Clear(Color.White);
                }
                pictureBox1.Image = bmp;
            }

            using (Graphics g = Graphics.FromImage(pictureBox1.Image))
            {
                g.Clear(pictureBox1.BackColor);

                AdjustableArrowCap bigArrow = new AdjustableArrowCap(5, 5);
                _Pen = new Pen(Color.IndianRed, 3);
                _Pen.CustomEndCap = bigArrow;
                g.DrawLine(_Pen, mouseDownPosition, mouseMovePosition);
                _Pen.Dispose();
            }
        }
    }



    private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
    {
        isMoving = true;
        mouseDownPosition = e.Location;
    }

    private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
    {
        if (isMoving)
        {
            mouseMovePosition = e.Location;
            pictureBox1.Invalidate();
        }
    }

    private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
    {
        if (isMoving)
        {
            lines.Add(Tuple.Create(mouseDownPosition, mouseMovePosition));
        }
        isMoving = false;
    }

我认为问题在于这里的这条线g.Clear(pictureBox1.BackColor);

Answer 1

是的，这里的这条线有问题g.Clear(pictureBox1.BackColor);你在画线之前擦除了整个控制区域。

你应该直接画到e.Graphics：

private void pictureBox1_Paint(object sender, PaintEventArgs e)
{
    if (isMoving)
    {
        if (pictureBox1.Image == null) e.Graphics.Clear(Color.White);

        // Add this line for high quality drawing:
        e.Graphics.SmoothingMode = SmoothingMode.HighQuality;

        AdjustableArrowCap bigArrow = new AdjustableArrowCap(5, 5);
        _Pen = new Pen(Color.IndianRed, 3);
        _Pen.CustomEndCap = bigArrow;
        e.Graphics.DrawLine(_Pen, mouseDownPosition, mouseMovePosition);
        _Pen.Dispose();
    }
}