.net 裁剪图片（不压缩）

using System.Drawing;
using System.Drawing.Imaging;

        /// <summary>  
        /// 生成图片缩略文件  
        /// </summary>  
        /// <param name="originalImage">图片源文件</param>  
        /// <param name="width">缩略图宽度</param>  
        /// <param name="height">缩略图高度</param>  
        /// <param name="mode">生成缩略图的方式</param>  
        /// <returns>缩率处理后图片文件</returns> 
        public static System.Drawing.Image MakeThumbnail(System.Drawing.Image originalImage, int width, int height, ThumbnailModel mode)
        {
            int towidth = width;
            int toheight = height;

            int x = 0;
            int y = 0;
            int ow = originalImage.Width;
            int oh = originalImage.Height;

            switch (mode)
            {
                case ThumbnailModel.HighWidth: //指定高宽缩放（可能变形）   
                    break;
                case ThumbnailModel.Width: //指定宽，高按比例   
                    toheight = originalImage.Height * width / originalImage.Width;
                    break;
                case ThumbnailModel.Hight: //指定高，宽按比例  
                    towidth = originalImage.Width * height / originalImage.Height;
                    break;
                case ThumbnailModel.Default: //指定高，宽按比例  
                    if (ow <= towidth && oh <= toheight)
                    {
                        x = -(towidth - ow) / 2;
                        y = -(toheight - oh) / 2;
                        ow = towidth;
                        oh = toheight;
                    }
                    else
                    {
                        if (ow > oh)//宽大于高
                        {
                            x = 0;
                            y = -(ow - oh) / 2;
                            oh = ow;
                        }
                        else//高大于宽
                        {
                            y = 0;
                            x = -(oh - ow) / 2;
                            ow = oh;
                        }
                    }
                    break;
                case ThumbnailModel.Auto:
                    if (originalImage.Width / originalImage.Height >= width / height)
                    {
                        if (originalImage.Width > width)
                        {
                            towidth = width;
                            toheight = (originalImage.Height * width) / originalImage.Width;
                        }
                        else
                        {
                            towidth = originalImage.Width;
                            toheight = originalImage.Height;
                        }
                    }
                    else
                    {
                        if (originalImage.Height > height)
                        {
                            toheight = height;
                            towidth = (originalImage.Width * height) / originalImage.Height;
                        }
                        else
                        {
                            towidth = originalImage.Width;
                            toheight = originalImage.Height;
                        }
                    }
                    break;
                case ThumbnailModel.Cut: //指定高宽裁减（不变形）   
                    if ((double)originalImage.Width / (double)originalImage.Height > (double)towidth / (double)toheight)
                    {
                        oh = originalImage.Height;
                        ow = originalImage.Height * towidth / toheight;
                        y = 0;
                        x = (originalImage.Width - ow) / 2;
                    }
                    else
                    {
                        ow = originalImage.Width;
                        oh = originalImage.Width * height / towidth;
                        x = 0;
                        y = (originalImage.Height - oh) / 2;
                    }
                    break;
                default:

                    break;
            }

            //新建一个bmp图片  
            System.Drawing.Image bitmap = new System.Drawing.Bitmap(towidth, toheight);

            //新建一个画板  
            System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);

            //设置高质量插值法  
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

            //设置高质量,低速度呈现平滑程度  
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

            //清空画布并以透明背景色填充  
            g.Clear(System.Drawing.Color.White);

            //在指定位置并且按指定大小绘制原图片的指定部分  
            g.DrawImage(originalImage, new System.Drawing.Rectangle(0, 0, towidth, toheight),
                        new System.Drawing.Rectangle(x, y, ow, oh),
                        System.Drawing.GraphicsUnit.Pixel);

            return bitmap;
        }

        /// <summary>
        /// 缩率图处理模式
        /// </summary>
        public enum ThumbnailModel
        {
            /// <summary>
            /// 指定高宽缩放（可能变形）   
            /// </summary>
            HighWidth,

            /// <summary>
            /// 指定宽，高按比例   
            /// </summary>
            Width,

            /// <summary>
            /// 默认  全图不变形   
            /// </summary>
            Default,

            /// <summary>
            /// 指定高，宽按比例
            /// </summary>
            Hight,

            /// <summary>
            /// 指定高宽裁减（不变形）？？指定裁剪区域
            /// </summary>
            Cut,

            /// <summary>
            /// 自动 原始图片按比例缩放
            /// </summary>
            Auto
        }