Asp.net 使用GDI+实现alpha透明效果

使用alpha的最简单的效果.

 private void button1_Click(object sender, EventArgs e)
        {
            this.Refresh();

            Color startColor = System.Drawing.ColorTranslator.FromHtml(this.textBox1.Text.ToLower());
            int alphaColor = 255;


            //获取起始颜色的三色
              float redColor = startColor.R;
            float greenColor = startColor.G;
            float blueColor = startColor.B;

            //下面创建bitmap开始画图
             Bitmap bm = new Bitmap(100, 100);
            for (int i = 0; i <100; i++)
            {

                if (alphaColor > 255)
                {
                    alphaColor = 255;
                }

                for (int j = 0; j < 100; j++)
                {
                    //通过设置每个像素点的值画图
                    bm.SetPixel(j, i, Color.FromArgb(alphaColor,(int)redColor, (int)greenColor, (int)blueColor));
                }

                alphaColor = alphaColor - 2;
                
            }
            //在窗体上显示,预览
            Graphics g = this.CreateGraphics();
            g.DrawImage(bm, 10, 150);
            bm.Dispose();
            g.Dispose();
        }

修改下代码：

private void button1_Click(object sender, EventArgs e)
        {
            this.Refresh();

            Color startColor = System.Drawing.ColorTranslator.FromHtml(this.textBox1.Text.ToLower());
            int alphaColor = 200;


            //获取起始颜色的三色
            float redColor = startColor.R;
            float greenColor = startColor.G;
            float blueColor = startColor.B;

            //下面创建bitmap开始画图
            Bitmap bm = new Bitmap(100, 100);
            for (int i = 99; i > 0; i--)
            {

                if (alphaColor > 255)
                {
                    alphaColor = 255;
                }

                for (int j = 0; j < 100; j++)
                {
                    //通过设置每个像素点的值画图
                    bm.SetPixel(j, i, Color.FromArgb(alphaColor,(int)redColor, (int)greenColor, (int)blueColor));
                }

                if ( i < 55)
                {
                    alphaColor = alphaColor - 3;
                }
                
            }
            //在窗体上显示,预览
            Graphics g = this.CreateGraphics();
            g.DrawImage(bm, 10, 150);
            bm.Save("c:/"+DateTime.Now.ToString("yyyymmddhhmmssfff")+".gif");
            bm.Dispose();
            g.Dispose();
        }