如何设置图片水印的透明度？

[xunyz]

System.Drawing.Image copyImage = Image.FromFile(HttpContext.Current.Server.MapPath(watermarkImgPath));

g.DrawImage(copyImage, new Rectangle(bitmap.Width - copyImage.Width, bitmap.Height - copyImage.Height, copyImage.Width, copyImage.Height), 0, 0, copyImage.Width, copyImage.Height, GraphicsUnit.Pixel);

图片水印的透明度程序怎么写？
各位帮帮忙啊!

[huacha]

缩略图水印组件wsImage3.5
组件支持文字水印和图片水印，文字水印可设定透明度、阴影及阴影模糊,自定义字体、颜色、旋转角度等。图片水印也支持透明度

[qxs]

//设置颜色矩阵
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, (float)lucencyPercent/100f, 0},
new float[] {0, 0, 0, 0, 1}};

System.Drawing.Imaging.ColorMatrix colorMatrix = new System.Drawing.Imaging.ColorMatrix(matrixItems);
System.Drawing.Imaging.ImageAttributes imgAttr = new System.Drawing.Imaging.ImageAttributes();
imgAttr.SetColorMatrix(colorMatrix, System.Drawing.Imaging.ColorMatrixFlag.Default, System.Drawing.Imaging.ColorAdjustType.Bitmap);