利用C#实现修改图片透明度功能

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }
    Image new_img;
    private void ChangeAlpha()
    {
        pictureBox1.Refresh();
        Bitmap source = new Bitmap(pictureBox1.Image);
        Bitmap effect = new Bitmap(pictureBox1.Image.Width,pictureBox1.Image.Height);
        Graphics _effect = Graphics.FromImage(effect);
        float[][] matrixItems ={new float[]{1,0,0,0,0},
                                  new float [] {0,1,0,0,0},
                                  new float []{0,0,1,0,0},
                                  new float []{0,0,0,0,0},
                                  new float[]{0,0,0,trackBar1.Value/255f,1}};
        ColorMatrix imgMatrix = new ColorMatrix(matrixItems);
        ImageAttributes imgEffect = new ImageAttributes();
        imgEffect.SetColorMatrix(imgMatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
        if (source.Width <= 368)
        {
            _effect.DrawImage(source, new Rectangle(0, 0, pictureBox1.Image.Width, pictureBox1.Image.Height), 0, 0, pictureBox1.Image.Width, pictureBox1.Image.Height, GraphicsUnit.Pixel, imgEffect);
        }
        else
        {
            _effect.DrawImage(pictureBox1.Image, new Rectangle(0, 0, pictureBox1.Image.Width, pictureBox1.Image.Height), 0, 0, pictureBox1.Image.Width, pictureBox1.Image.Height, GraphicsUnit.Pixel, imgEffect);
        }
        pictureBox1.Image = effect;
    }
    private void Form1_Load(object sender, EventArgs e)
    {

    }

    private void trackBar1_ValueChanged(object sender, EventArgs e)
    {
        ChangeAlpha();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        if (openFileDialog1.ShowDialog() == DialogResult.OK)
        {
            pictureBox1.Image = Image.FromFile(openFileDialog1.FileName);
        }
    }
}

partial class Form1
{
    /// <summary>
    /// 必需的设计器变量。
    /// </summary>
    private System.ComponentModel.IContainer components = null;

    /// <summary>
    /// 清理所有正在使用的资源。
    /// </summary>
    /// <param name="disposing">如果应释放托管资源，为 true；否则为 false。</param>
    protected override void Dispose(bool disposing)
    {
        if (disposing && (components != null))
        {
            components.Dispose();
        }
        base.Dispose(disposing);
    }

    #region Windows 窗体设计器生成的代码

    /// <summary>
    /// 设计器支持所需的方法 - 不要
    /// 使用代码编辑器修改此方法的内容。
    /// </summary>
    private void InitializeComponent()
    {
        this.pictureBox1 = new System.Windows.Forms.PictureBox();
        this.trackBar1 = new System.Windows.Forms.TrackBar();
        this.button1 = new System.Windows.Forms.Button();
        this.openFileDialog1 = new System.Windows.Forms.OpenFileDialog();
        ((System.ComponentModel.ISupportInitialize)(this.pictureBox1)).BeginInit();
        ((System.ComponentModel.ISupportInitialize)(this.trackBar1)).BeginInit();
        this.SuspendLayout();
        // 
        // pictureBox1
        // 
        this.pictureBox1.Location = new System.Drawing.Point(12, 80);
        this.pictureBox1.Name = "pictureBox1";
        this.pictureBox1.Size = new System.Drawing.Size(424, 258);
        this.pictureBox1.SizeMode = System.Windows.Forms.PictureBoxSizeMode.Zoom;
        this.pictureBox1.TabIndex = 0;
        this.pictureBox1.TabStop = false;
        // 
        // trackBar1
        // 
        this.trackBar1.Location = new System.Drawing.Point(12, 41);
        this.trackBar1.Maximum = 100;
        this.trackBar1.Minimum = 1;
        this.trackBar1.Name = "trackBar1";
        this.trackBar1.Size = new System.Drawing.Size(424, 45);
        this.trackBar1.TabIndex = 1;
        this.trackBar1.TickStyle = System.Windows.Forms.TickStyle.None;
        this.trackBar1.Value = 100;
        this.trackBar1.ValueChanged += new System.EventHandler(this.trackBar1_ValueChanged);
        // 
        // button1
        // 
        this.button1.Location = new System.Drawing.Point(12, 12);
        this.button1.Name = "button1";
        this.button1.Size = new System.Drawing.Size(75, 23);
        this.button1.TabIndex = 2;
        this.button1.Text = "打开图片";
        this.button1.UseVisualStyleBackColor = true;
        this.button1.Click += new System.EventHandler(this.button1_Click);
        // 
        // openFileDialog1
        // 
        this.openFileDialog1.Filter = "图片文件|*.jpg;*.jpeg;*.png;*.bmp";
        // 
        // Form1
        // 
        this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 12F);
        this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
        this.ClientSize = new System.Drawing.Size(447, 350);
        this.Controls.Add(this.pictureBox1);
        this.Controls.Add(this.button1);
        this.Controls.Add(this.trackBar1);
        this.Name = "Form1";
        this.Text = "Form1";
        this.Load += new System.EventHandler(this.Form1_Load);
        ((System.ComponentModel.ISupportInitialize)(this.pictureBox1)).EndInit();
        ((System.ComponentModel.ISupportInitialize)(this.trackBar1)).EndInit();
        this.ResumeLayout(false);
        this.PerformLayout();

    }

    #endregion

    private System.Windows.Forms.PictureBox pictureBox1;
    private System.Windows.Forms.TrackBar trackBar1;
    private System.Windows.Forms.Button button1;
    private System.Windows.Forms.OpenFileDialog openFileDialog1;
}