/*
 * Class:WaterImage
 * Use for add a water Image to the picture both words and image
 * By Gary Gong From Demetersoft.com | 和信诚软件(北京)有限公司
 * 2007.07.23   create the file 
 *
 *
 * 使用说明：
 * 　建议先定义一个WaterImage实例
 * 　然后利用实例的属性，去匹配需要进行操作的参数
 * 　然后定义一个WaterImageManage实例
 * 　利用WaterImageManage实例进行DrawImage（），印图片水印
 * 　DrawWords（）印文字水印
 *
-*/

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;
using System.IO;
/// <summary>
/// 图片位置
/// </summary>
public enum ImagePosition
{
    LeftTop,        //左上
    LeftBottom,    //左下
    RightTop,       //右上
    RigthBottom,  //右下
    TopMiddle,     //顶部居中
    BottomMiddle, //底部居中
    Center           //中心
}

/// <summary>
/// 水印图片的操作管理 Design by Gary Gong From Demetersoft.com
/// </summary>
public class WaterImageManage
{
    /// <summary>
    /// 生成一个新的水印图片制作实例
    /// </summary>
 public WaterImageManage ()
 {
  //
  // TODO: Add constructor logic here
  //
 }

/// <summary>
    /// 添加图片水印
    /// </summary>
    /// <param name="sourcePicture">源图片文件名</param>
    /// <param name="waterImage">水印图片文件名</param>
    /// <param name="alpha">透明度(0.1-1.0数值越小透明度越高)</param>
    /// <param name="position">位置</param>
    /// <param name="PicturePath" >图片的路径</param>
    /// <returns>返回生成于指定文件夹下的水印文件名</returns>
    public string  DrawImage(string sourcePicture,
                                      string waterImage,
                                      float alpha,
                                      ImagePosition position,
                                      string PicturePath )
    {
        //
        // 判断参数是否有效
        //
        if (sourcePicture == string.Empty || waterImage == string.Empty || alpha == 0.0 || PicturePath == string.Empty)
        {
            return sourcePicture;
        }

//
        // 源图片，水印图片全路径
        //
        string sourcePictureName = PicturePath + sourcePicture;
        string waterPictureName = PicturePath + waterImage;
        string fileSourceExtension = System.IO.Path.GetExtension(sourcePictureName).ToLower();
        string fileWaterExtension = System.IO.Path.GetExtension(waterPictureName).ToLower();
        //
        // 判断文件是否存在,以及类型是否正确
        //
        if (System.IO.File.Exists(sourcePictureName) == false ||
            System.IO.File.Exists(waterPictureName) == false ||(
            fileSourceExtension != ".gif" &&
            fileSourceExtension != ".jpg" &&
            fileSourceExtension != ".png") || (
            fileWaterExtension != ".gif" &&
            fileWaterExtension != ".jpg" &&
            fileWaterExtension != ".png")
            )
        {
            return sourcePicture;
        }

//
        // 目标图片名称及全路径
        //
        string targetImage = sourcePictureName.Replace ( System.IO.Path.GetExtension(sourcePictureName),"") + "_1101.jpg";

//
        // 将需要加上水印的图片装载到Image对象中
        //
        Image imgPhoto = Image.FromFile(sourcePictureName);
        //
        // 确定其长宽
        //
        int phWidth = imgPhoto.Width;
        int phHeight = imgPhoto.Height;

//
        // 封装 GDI+ 位图，此位图由图形图像及其属性的像素数据组成。
        //
        Bitmap bmPhoto = new Bitmap(phWidth, phHeight, PixelFormat.Format24bppRgb);

//
        // 设定分辨率
        //
        bmPhoto.SetResolution(imgPhoto.HorizontalResolution, imgPhoto.VerticalResolution);

//
        // 定义一个绘图画面用来装载位图
        //
        Graphics grPhoto = Graphics.FromImage(bmPhoto);

//
        //同样，由于水印是图片，我们也需要定义一个Image来装载它
        //
        Image imgWatermark = new Bitmap(waterPictureName);
       
        //
        // 获取水印图片的高度和宽度
        //
        int wmWidth = imgWatermark.Width;
        int wmHeight = imgWatermark.Height;

//SmoothingMode：指定是否将平滑处理（消除锯齿）应用于直线、曲线和已填充区域的边缘。
        // 成员名称   说明
        // AntiAlias      指定消除锯齿的呈现。 
        // Default        指定不消除锯齿。 
        // HighQuality  指定高质量、低速度呈现。 
        // HighSpeed   指定高速度、低质量呈现。 
        // Invalid        指定一个无效模式。 
        // None          指定不消除锯齿。
        grPhoto.SmoothingMode = SmoothingMode.AntiAlias;

//
        // 第一次描绘，将我们的底图描绘在绘图画面上
        //
        grPhoto.DrawImage(imgPhoto,                                         
                                    new Rectangle(0, 0, phWidth, phHeight),
                                    0,    
                                    0,      
                                    phWidth,  
                                    phHeight,     
                                    GraphicsUnit.Pixel);

//
        // 与底图一样，我们需要一个位图来装载水印图片。并设定其分辨率
        //
        Bitmap bmWatermark = new Bitmap(bmPhoto);
        bmWatermark.SetResolution(imgPhoto.HorizontalResolution, imgPhoto.VerticalResolution);
       
        //
        // 继续，将水印图片装载到一个绘图画面grWatermark
        //
        Graphics grWatermark = Graphics.FromImage(bmWatermark);

//
        //ImageAttributes 对象包含有关在呈现时如何操作位图和图元文件颜色的信息。
        //      
        ImageAttributes imageAttributes = new ImageAttributes();

//
        //Colormap: 定义转换颜色的映射
        //
        ColorMap colorMap = new ColorMap();

//
        //我的水印图被定义成拥有绿色背景色的图片被替换成透明
        //
        colorMap.OldColor = Color.FromArgb(255, 0, 255, 0);
        colorMap.NewColor = Color.FromArgb(0, 0, 0, 0);

ColorMap[] remapTable = { colorMap };

imageAttributes.SetRemapTable(remapTable, ColorAdjustType.Bitmap);

float[][] colorMatrixElements = {
           new float[] {1.0f,  0.0f,  0.0f,  0.0f, 0.0f}, // red红色
           new float[] {0.0f,  1.0f,  0.0f,  0.0f, 0.0f}, //green绿色
           new float[] {0.0f,  0.0f,  1.0f,  0.0f, 0.0f}, //blue蓝色      
           new float[] {0.0f,  0.0f,  0.0f,  alpha, 0.0f}, //透明度    
           new float[] {0.0f,  0.0f,  0.0f,  0.0f, 1.0f}};//

//  ColorMatrix:定义包含 RGBA 空间坐标的 5 x 5 矩阵。
        //  ImageAttributes 类的若干方法通过使用颜色矩阵调整图像颜色。
        ColorMatrix wmColorMatrix = new ColorMatrix(colorMatrixElements);

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

//
        //上面设置完颜色，下面开始设置位置
        //
        int xPosOfWm;
        int yPosOfWm;

switch (position)
        {
            case ImagePosition .BottomMiddle :
                xPosOfWm = (phWidth-wmWidth ) / 2 ;
                yPosOfWm = phHeight- wmHeight -10;
                break ;
            case ImagePosition .Center :
                xPosOfWm = (phWidth - wmWidth) / 2;
                yPosOfWm = (phHeight-wmHeight ) / 2;
                break ;
            case ImagePosition .LeftBottom :
                xPosOfWm = 10;
                yPosOfWm = phHeight - wmHeight - 10;
                break ;
            case ImagePosition .LeftTop :
                xPosOfWm = 10;
                yPosOfWm = 10;
                break;
            case ImagePosition .RightTop :
                xPosOfWm = phWidth - wmWidth - 10;
                yPosOfWm = 10;
                break ;
            case ImagePosition .RigthBottom :
                xPosOfWm = phWidth - wmWidth - 10;
                yPosOfWm = phHeight - wmHeight - 10;
                break ;
            case ImagePosition.TopMiddle :
                xPosOfWm = (phWidth - wmWidth) / 2;
                yPosOfWm = 10;
                break ;
            default:
                xPosOfWm = 10;
                yPosOfWm = phHeight - wmHeight - 10;
                break;
        }

//
        // 第二次绘图，把水印印上去
        //
        grWatermark.DrawImage(imgWatermark,
         new Rectangle(xPosOfWm,
                             yPosOfWm,
                             wmWidth,
                             wmHeight),
                             0,             
                             0,         
                             wmWidth, 
                             wmHeight,  
                             GraphicsUnit.Pixel, 
                             imageAttributes);

imgPhoto = bmWatermark;
        grPhoto.Dispose();
        grWatermark.Dispose();

//
        // 保存文件到服务器的文件夹里面
        //
        imgPhoto.Save(targetImage, ImageFormat.Jpeg);
        imgPhoto.Dispose();
        imgWatermark.Dispose();
        return targetImage.Replace (PicturePath,"");
    }

/// <summary>
    /// 在图片上添加水印文字
    /// </summary>
    /// <param name="sourcePicture">源图片文件</param>
    /// <param name="waterWords">需要添加到图片上的文字</param>
    /// <param name="alpha">透明度</param>
    /// <param name="position">位置</param>
    /// <param name="PicturePath">文件路径</param>
    /// <returns></returns>
    public string DrawWords(string sourcePicture,
                                      string waterWords,
                                      float alpha,
                                      ImagePosition position,
                                      string PicturePath)
    {
        //
        // 判断参数是否有效
        //
        if (sourcePicture == string.Empty || waterWords == string.Empty || alpha == 0.0 || PicturePath == string.Empty)
        {
            return sourcePicture;
        }

//
        // 源图片全路径
        //
        string sourcePictureName = PicturePath + sourcePicture;
        string fileExtension = System.IO.Path.GetExtension(sourcePictureName).ToLower();

//
        // 判断文件是否存在,以及文件名是否正确
        //
        if (System.IO.File.Exists(sourcePictureName) == false || (
            fileExtension != ".gif"  &&
            fileExtension != ".jpg" &&
            fileExtension != ".png" ))
        {
            return sourcePicture;
        }

//
        // 目标图片名称及全路径
        //
        string targetImage = sourcePictureName.Replace(System.IO.Path.GetExtension(sourcePictureName), "") + "_0703.jpg";

//创建一个图片对象用来装载要被添加水印的图片
        Image imgPhoto = Image.FromFile(sourcePictureName);

//获取图片的宽和高
        int phWidth = imgPhoto.Width;
        int phHeight = imgPhoto.Height;

//
        //建立一个bitmap，和我们需要加水印的图片一样大小
        Bitmap bmPhoto = new Bitmap(phWidth, phHeight, PixelFormat.Format24bppRgb);

//SetResolution：设置此 Bitmap 的分辨率
        //这里直接将我们需要添加水印的图片的分辨率赋给了bitmap
        bmPhoto.SetResolution(imgPhoto.HorizontalResolution, imgPhoto.VerticalResolution);

//Graphics：封装一个 GDI+ 绘图图面。
        Graphics grPhoto = Graphics.FromImage(bmPhoto);

//设置图形的品质
        grPhoto.SmoothingMode = SmoothingMode.AntiAlias;

//将我们要添加水印的图片按照原始大小描绘（复制）到图形中
        grPhoto.DrawImage(
         imgPhoto,                                           //   要添加水印的图片
         new Rectangle(0, 0, phWidth, phHeight), //  根据要添加的水印图片的宽和高
         0,                                                     //  X方向从0点开始描绘
         0,                                                     // Y方向
         phWidth,                                            //  X方向描绘长度
         phHeight,                                           //  Y方向描绘长度
         GraphicsUnit.Pixel);                              // 描绘的单位，这里用的是像素

//根据图片的大小我们来确定添加上去的文字的大小
        //在这里我们定义一个数组来确定
        int[] sizes = new int[] { 16, 14, 12, 10, 8, 6, 4 };

//字体
        Font crFont = null;
        //矩形的宽度和高度，SizeF有三个属性，分别为Height高，width宽，IsEmpty是否为空
        SizeF crSize = new SizeF();

//利用一个循环语句来选择我们要添加文字的型号
        //直到它的长度比图片的宽度小
        for (int i = 0; i < 7; i++)
        {
            crFont = new Font("arial", sizes[i], FontStyle.Bold);

//测量用指定的 Font 对象绘制并用指定的 StringFormat 对象格式化的指定字符串。
            crSize = grPhoto.MeasureString(waterWords, crFont);

// ushort 关键字表示一种整数数据类型
            if ((ushort)crSize.Width < (ushort)phWidth)
                break;
        }

//截边5%的距离，定义文字显示(由于不同的图片显示的高和宽不同，所以按百分比截取)
        int yPixlesFromBottom = (int)(phHeight * .05);

//定义在图片上文字的位置
        float wmHeight =  crSize.Height;
        float wmWidth = crSize .Width ;

float  xPosOfWm;
        float  yPosOfWm;

switch (position)
        {
            case ImagePosition .BottomMiddle :
                xPosOfWm = phWidth / 2 ;
                yPosOfWm = phHeight- wmHeight -10;
                break ;
            case ImagePosition .Center :
                xPosOfWm = phWidth / 2;
                yPosOfWm = phHeight / 2;
                break ;
            case ImagePosition .LeftBottom :
                xPosOfWm = wmWidth;
                yPosOfWm = phHeight - wmHeight - 10;
                break ;
            case ImagePosition .LeftTop :
                xPosOfWm = wmWidth/2 ;
                yPosOfWm = wmHeight / 2;
                break;
            case ImagePosition .RightTop :
                xPosOfWm = phWidth - wmWidth - 10;
                yPosOfWm = wmHeight;
                break ;
            case ImagePosition .RigthBottom :
                xPosOfWm = phWidth - wmWidth - 10;
                yPosOfWm = phHeight - wmHeight - 10;
                break ;
            case ImagePosition.TopMiddle :
                xPosOfWm = phWidth / 2;
                yPosOfWm = wmWidth;
                break ;
            default:
                xPosOfWm = wmWidth;
                yPosOfWm = phHeight - wmHeight - 10;
                break;
        }

//封装文本布局信息（如对齐、文字方向和 Tab 停靠位），显示操作（如省略号插入和国家标准 (National) 数字替换）和 OpenType 功能。
        StringFormat StrFormat = new StringFormat();

//定义需要印的文字居中对齐
        StrFormat.Alignment = StringAlignment.Center;

//SolidBrush:定义单色画笔。画笔用于填充图形形状，如矩形、椭圆、扇形、多边形和封闭路径。
        //这个画笔为描绘阴影的画笔，呈灰色
        int m_alpha = Convert .ToInt32 ( 256 * alpha);
        SolidBrush semiTransBrush2 = new SolidBrush(Color.FromArgb(m_alpha, 0, 0, 0));

//描绘文字信息，这个图层向右和向下偏移一个像素，表示阴影效果
        //DrawString 在指定矩形并且用指定的 Brush 和 Font 对象绘制指定的文本字符串。
        grPhoto.DrawString(waterWords,                                    //string of text
                                   crFont,                                         //font
                                   semiTransBrush2,                            //Brush
                                   new PointF(xPosOfWm + 1, yPosOfWm + 1),  //Position
                                   StrFormat);

//从四个 ARGB 分量（alpha、红色、绿色和蓝色）值创建 Color 结构，这里设置透明度为153
        //这个画笔为描绘正式文字的笔刷，呈白色
        SolidBrush semiTransBrush = new SolidBrush(Color.FromArgb(153, 255, 255, 255));

//第二次绘制这个图形，建立在第一次描绘的基础上
        grPhoto.DrawString(waterWords,                 //string of text
                                   crFont,                                   //font
                                   semiTransBrush,                           //Brush
                                   new PointF(xPosOfWm, yPosOfWm),  //Position
                                   StrFormat);

//imgPhoto是我们建立的用来装载最终图形的Image对象
        //bmPhoto是我们用来制作图形的容器，为Bitmap对象
        imgPhoto = bmPhoto;
        //释放资源，将定义的Graphics实例grPhoto释放，grPhoto功德圆满
        grPhoto.Dispose();

//将grPhoto保存
        imgPhoto.Save(targetImage, ImageFormat.Jpeg);
        imgPhoto.Dispose();

return targetImage.Replace(PicturePath, "");
    }
}

/// <summary>
/// 装载水印图片的相关信息
/// </summary>
public class WaterImage
{
    public WaterImage ()
    {

}

private string m_sourcePicture;
    /// <summary>
    /// 源图片地址名字(带后缀)
    /// </summary>
    public string SourcePicture
    {
        get { return m_sourcePicture; }
        set { m_sourcePicture = value; }
    }

private string  m_waterImager;
    /// <summary>
    /// 水印图片名字(带后缀)
    /// </summary>
    public string  WaterPicture
    {
        get { return m_waterImager; }
        set { m_waterImager = value; }
    }

private float  m_alpha;
    /// <summary>
    /// 水印图片文字的透明度
    /// </summary>
    public float  Alpha
    {
        get { return m_alpha; }
        set { m_alpha = value; }
    }

private ImagePosition  m_postition;
    /// <summary>
    /// 水印图片或文字在图片中的位置
    /// </summary>
    public ImagePosition  Position
    {
        get { return m_postition; }
        set { m_postition = value; }
    }

private string  m_words;
    /// <summary>
    /// 水印文字的内容
    /// </summary>
    public string  Words
    {
        get { return m_words; }
        set { m_words = value; }
    }
}

ps: 感谢清清月儿的文章。