Bitmap保存图片全是黑的

[cunnor]

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

namespace frmSxqm
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private Graphics g;
        private int sX, sY; 
        private Bitmap bitmap; 

        private void Form1_Load(object sender, EventArgs e)
        {
            bitmap = new Bitmap(300, 300);
            g = Graphics.FromImage(bitmap); 
            Cursor.Current = Cursors.Default;
        }

        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            sX = e.X;
            sY = e.Y; 
        }

        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            g.DrawLine(new Pen(Color.Black, 3), sX, sY, e.X, e.Y); 
            sX = e.X; sY = e.Y; 
            this.pictureBox1.Image = bitmap; 
        }

        private void button1_Click(object sender, EventArgs e)
        {
            bitmap = new Bitmap(300, 300);
            g = Graphics.FromImage(bitmap); 
            this.pictureBox1.Image = bitmap;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            DialogResult result = MessageBox.Show("确定签名完成了吗？", "提示", MessageBoxButtons.OKCancel, MessageBoxIcon.Question, MessageBoxDefaultButton.Button2); 
            if (result == DialogResult.Cancel) 
                return; 
            else 
            { 
               string  signaturePath = DateTime.Now.ToString("yyMMddHHmmssms") + ".jpg"; 
                compressionPicture(bitmap, 300, 300, signaturePath, ImageFormat.Jpeg); 
                //this.Close();
            } 
        }
        private void compressionPicture(Bitmap bmap, int newWidth, int newHeight, string saveFileName, ImageFormat format) 
        { 
            Bitmap newImg = new Bitmap(newWidth, newHeight); 
            Rectangle srcRec = new Rectangle(0, 0, bmap.Width, bmap.Height); 
            Rectangle destRec = new Rectangle(0, 0, newWidth, newHeight); 
            Graphics g = Graphics.FromImage(newImg);
            g.DrawImage(bmap, destRec, srcRec, GraphicsUnit.Pixel); 
            newImg.Save(saveFileName, format); 
        } 
    }
}

为什么保存图片全是黑的啊
求高手指点

[娃都会打酱油了]

Graphics g = Graphics.FromImage(newImg); g.Clear(Color.White);

[智商余额不足]

为什么要写多这个啊

 private void button1_Click(object sender, EventArgs e)
        {
            bitmap = new Bitmap(300, 300);
            g = Graphics.FromImage(bitmap); 
            this.pictureBox1.Image = bitmap;
        }

[智商余额不足]

两个解决方法： 1.把绘图表明用指定颜色清除下（new bitmap 缺省像素值是(0,0,0)） 参考一楼 2.把位图保存为 png 格式

string  signaturePath = DateTime.Now.ToString("yyMMddHHmmssms") + ".png"; 
compressionPicture(bitmap, 300, 300, signaturePath, ImageFormat.Png); 

[智商余额不足]

绘图表明--->>绘图表面

[卧_槽]

存png就好了。