C#中在PictureBox上使用橡皮筋画线

Question

想法是在原有图层上加上一个透明图层，在那上面画线段。来获取两点位置和线段长度。

在winform项目中添加一个PictureBox控件，然后添加鼠标在PictureBox上的事件。目前遇到了两个问题，1.透明图层的添加，BufferedGraphics bg对象使用bg.Graphics.Clear(Color.Transparent);时背景为黑色，如何设置透明色？

在网上看了许多，是用图像缓存技术来实现的。具体代码如下：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace 橡皮技术3
{
    public partial class Form2 : Form
    {
        //存放直线对象的集合
        List<Line> lines = new List<Line>();
        //起始点
        Point mStartPoint = Point.Empty;
        public Form2()
        {
            InitializeComponent();
        }

        //在图片对象上点击时
        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                //点击左键，存入起始点
                mStartPoint = e.Location;
            }
        }

        //在图片上移动时，绘制直线
        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button==MouseButtons.Left&& mStartPoint!=null)
            {
                drawlines(pictureBox1.CreateGraphics(), mStartPoint, e.Location);//按住左键不放绘制
            }
        }

        //在松开左键时，存储点击,需要判定有无起始点，每次绘制结束后都要清空起始点
        private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
        {
            if (!mStartPoint.IsEmpty)
            {
                Line line = new Line(mStartPoint, e.Location);
                lines.Add(line);
                mStartPoint = Point.Empty;
            }
        }

        private void drawlines(Graphics g, Point mPoint1, Point mPoint2)
        {
            BufferedGraphicsContext context = BufferedGraphicsManager.Current;//开辟空间
            BufferedGraphics bg = context.Allocate(g,new Rectangle(0,0,PictureBox1.Width,PictureBox1.Height));//开辟空间的大小
            bg.Graphics.Clear(Color.White);
            //在缓冲区上画线
            foreach (Line line in lines)
            {
                line.Draw(bg.Graphics);
            }
            //在缓存图层中画直线的样式，并没有实际显示，尽量与绘图的样式一致
            bg.Graphics.DrawLine(SystemPens.ControlText, mPoint1, mPoint2);
            bg.Render();
            bg.Dispose();
            bg = null;
        }

        //通过paint绘制
        private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            foreach (Line line in lines)
            {
                line.Draw(pictureBox1.CreateGraphics());
            }
        }


    }
}

Answer 1

保存原始图片作为背景

        //原始图片
        Image mImage;

        private void Form2_Load(object sender, EventArgs e)
        {
            mImage = pictureBox1.Image;
        }

        private void drawlines(Graphics g, Point mPoint1, Point mPoint2)
        {
            BufferedGraphicsContext context = BufferedGraphicsManager.Current;//开辟空间
            BufferedGraphics bg = context.Allocate(g, new Rectangle(0, 0, pictureBox1.Width, pictureBox1.Height));//开辟空间的大小
            bg.Graphics.Clear(Color.White);
            bg.Graphics.DrawImage(mImage, new Rectangle(0, 0,mImage.Width,mImage.Height));
            //在缓冲区上画线
            ...
        }