C# 水印透明度图片

/// <summary>
        /// 在一张图片的指定位置处加入一张具有水印效果的图片
        /// </summary>
        /// <param name="SourceImage">指定源图片的绝对路径</param>
        /// <param name="WaterMarkImage">指定水印图片的绝对路径</param>
        /// <param name="SaveImage">保存图片的绝对路径</param>
        public static void MakeWaterMark(string SourceImage, string WaterMarkImage, string SaveImage)
        {
            // 创建一个对象用于操作需要加水印的源图片
            Image imgPhoto = Image.FromFile(SourceImage);
            // 获取该源图片的宽度和高度
            int phWidth = imgPhoto.Width;
            int phHeight = imgPhoto.Height;

            // 创建一个BMP格式的空白图片(宽度和高度与源图片一致)
            Bitmap bmPhoto = new Bitmap(phWidth, phHeight, PixelFormat.Format24bppRgb);

            // 设置该新建空白BMP图片的分辨率
            bmPhoto.SetResolution(imgPhoto.HorizontalResolution, imgPhoto.VerticalResolution);

            // 将该BMP图片设置成一个图形对象
            Graphics grPhoto = Graphics.FromImage(bmPhoto);

            // 设置生成图片的质量
            grPhoto.SmoothingMode = SmoothingMode.AntiAlias;

            // 将源图片加载至新建的BMP图片中
            grPhoto.DrawImage(
                imgPhoto,                               // Photo Image object
                new Rectangle(0, 0, phWidth, phHeight), // Rectangle structure
                0,                                      // x-coordinate of the portion of the source image to draw. 
                0,                                      // y-coordinate of the portion of the source image to draw. 
                phWidth,                                // Width of the portion of the source image to draw. 
                phHeight,                               // Height of the portion of the source image to draw. 
                GraphicsUnit.Pixel);                    // Units of measure 

            // 创建水印图片的 Image 对象
            Image imgWatermark = new Bitmap(WaterMarkImage);

            // 获取水印图片的宽度和高度
            int wmWidth = imgWatermark.Width;
            int wmHeight = imgWatermark.Height;

            //------------------------------------------------------------
            // 第一步： 插入水印图片
            //------------------------------------------------------------

            //Create a Bitmap based on the previously modified photograph Bitmap
            Bitmap bmWatermark = new Bitmap(bmPhoto);
            bmWatermark.SetResolution(imgPhoto.HorizontalResolution, imgPhoto.VerticalResolution);
            //Load this Bitmap into a new Graphic Object
            Graphics grWatermark = Graphics.FromImage(bmWatermark);

            //To achieve a transulcent watermark we will apply (2) color 
            //manipulations by defineing a ImageAttributes object and 
            //seting (2) of its properties.
            ImageAttributes imageAttributes = new ImageAttributes();

            //The first step in manipulating the watermark image is to replace 
            //the background color with one that is trasparent (Alpha=0, R=0, G=0, B=0)
            //to do this we will use a Colormap and use this to define a RemapTable
            ColorMap colorMap = new ColorMap();

            //My watermark was defined with a background of 100% Green this will
            //be the color we search for and replace with transparency
            colorMap.OldColor = Color.FromArgb(255, 0, 255, 0);
            colorMap.NewColor = Color.FromArgb(0, 0, 0, 0);

            ColorMap[] remapTable = { colorMap };

            imageAttributes.SetRemapTable(remapTable, ColorAdjustType.Bitmap);

            //The second color manipulation is used to change the opacity of the 
            //watermark.  This is done by applying a 5x5 matrix that contains the 
            //coordinates for the RGBA space.  By setting the 3rd row and 3rd column 
            //to 0.3f we achive a level of opacity
            float[][] colorMatrixElements = { 
                                                new float[] {1.0f,  0.0f,  0.0f,  0.0f, 0.0f},       
                                                new float[] {0.0f,  1.0f,  0.0f,  0.0f, 0.0f},        
                                                new float[] {0.0f,  0.0f,  1.0f,  0.0f, 0.0f},        
                                                new float[] {0.0f,  0.0f,  0.0f,  0.3f, 0.0f},        
                                                new float[] {0.0f,  0.0f,  0.0f,  0.0f, 1.0f}
                                            };

            ColorMatrix wmColorMatrix = new ColorMatrix(colorMatrixElements);

            imageAttributes.SetColorMatrix(wmColorMatrix, ColorMatrixFlag.Default,
                ColorAdjustType.Bitmap);

            //For this example we will place the watermark in the upper right
            //hand corner of the photograph. offset down 10 pixels and to the 
            //left 10 pixles
            int xPosOfWm = ((phWidth - wmWidth) - 10);
            int yPosOfWm = 10;

            grWatermark.DrawImage(imgWatermark,
                new Rectangle(xPosOfWm, yPosOfWm, wmWidth, wmHeight),  //Set the detination Position
                0,                  // x-coordinate of the portion of the source image to draw. 
                0,                  // y-coordinate of the portion of the source image to draw. 
                wmWidth,            // Watermark Width
                wmHeight,            // Watermark Height
                GraphicsUnit.Pixel, // Unit of measurment
                imageAttributes);   //ImageAttributes Object

            //Replace the original photgraphs bitmap with the new Bitmap
            imgPhoto.Dispose();
            imgPhoto = bmWatermark;
            grPhoto.Dispose();
            grWatermark.Dispose();
            bmPhoto.Dispose();

            //------------------------------------------------------------
            // 第三步：保存图片
            //------------------------------------------------------------
            imgPhoto.Save(SaveImage, ImageFormat.Jpeg);

            // 释放使用中的资源
            imgPhoto.Dispose();
            imgWatermark.Dispose();
            bmWatermark.Dispose();
        }