标记要裁剪的区域后绘图矩形消失

Question

目标：

在松开鼠标左键时绘制一个红色矩形。保留矩形。

代码：

private void button1_Click(object sender, EventArgs e)
{
    if (Clipboard.ContainsImage())
    {
        pictureBox1.Image?.Dispose();
        pictureBox1.Image = Clipboard.GetImage();
    }
    else
    {
        MessageBox.Show("No Image detected in clipboard." + Environment.NewLine + "Have you print screened the label?", "Print Screen", MessageBoxButtons.OK, MessageBoxIcon.Warning);
    }
}

private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
        LocationXY = e.Location;
}

private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        LocationX1Y1 = e.Location;
        pictureBox1.Invalidate();
    }
}

private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        LocationX1Y1 = e.Location;

        pictureBox1.Invalidate();
        pictureBox2.Invalidate();
    }
}

private void pictureBox1_Paint(object sender, PaintEventArgs e)
{
    if (MouseButtons == MouseButtons.Left)
    {
        e.Graphics.DrawRectangle(Pens.Red, GetRect());
    }
}

private void pictureBox2_Paint(object sender, PaintEventArgs e)
{
    var src = GetRect();

    if (src == Rectangle.Empty) return;

    var des = new Rectangle(0, 0, src.Width, src.Height);

    e.Graphics.DrawImage(pictureBox1.Image,
        des, src, GraphicsUnit.Pixel);
}


private Rectangle GetRect()
{
    return new Rectangle(
        Math.Min(LocationXY.X, LocationX1Y1.X),
        Math.Min(LocationXY.Y, LocationX1Y1.Y),
        Math.Abs(LocationXY.X - LocationX1Y1.X),
        Math.Abs(LocationXY.Y - LocationX1Y1.Y)
        );
}

private Bitmap GetCroppedImage()
{
    var des = GetRect();

    if (des == Rectangle.Empty) return null;

    var b = new Bitmap(des.Width, des.Height);

    using (var g = Graphics.FromImage(b))
    {
        g.InterpolationMode = InterpolationMode.HighQualityBicubic;
        g.SmoothingMode = SmoothingMode.HighQuality;
        g.DrawImage(pictureBox1.Image, new Rectangle(0, 0, des.Width, des.Height), des, GraphicsUnit.Pixel);
    }
    return b;
}

结果：

点击一个按钮，这个打印的屏幕图像就会显示出来：

[![在此处输入图片描述][1]][1]

如果我使用鼠标左键单击并拖动，会出现这个红色矩形：

[![在此处输入图片描述][2]][2]

我一发布：

[![在此处输入图片描述][1]][1]

问题：

如何标记要裁剪的区域并将红色矩形保留在那里？没有消失

Answer 1

因为你只在按下鼠标左键时才绘制红色矩形，

它不是由释放鼠标左键后随后的pictureBox1_Paint 调用绘制的。

通过以下小改动，我们可以绘制红色矩形并使其持久化：

// Draw the red rectangle not only when
// the left mouse button is pressed
// but only when GetRect() is not empty
private void pictureBox1_Paint(object sender, PaintEventArgs e)
{
    Rectangle currentSelection = GetRect();
    if (currentSelection != Rectangle.Empty)
    {
        e.Graphics.DrawRectangle(Pens.Red, currentSelection);
    }
}

注意：如果 pictureBox1.Image 为空，pictureBox2_Paint 会失败。