【转载】C#生成图片的缩略图

图片处理是c#程序开发中时常会涉及到的一个业务，除了图像的上传、保存以及下载等功能外，根据上传的图片生成一个缩略图也是常见业务，在c#语言中，可以通过image类提供的相关方法对图片进行操作，如指定宽高对图片进行缩放， 指定高宽裁减裁剪图片、生成图片水印等。

定义一个图片处理工具类picdeal，该类实现了生成缩略图、在图片上生成图片水印等相应功能。具体的业务代码如下：

    /// <summary>
    /// 枚举,生成缩略图模式
    /// </summary>
    public enum thumbnailmod : byte
    {
        /// <summary>
        /// hw
        /// </summary>
        hw,
        /// <summary>
        /// w
        /// </summary>
        w,
        /// <summary>
        /// h
        /// </summary>
        h,
        /// <summary>
        /// cut
        /// </summary>
        cut
    };
    /// <summary>
    /// 操作图片类, 生成缩略图,添加水印
    /// </summary>
    public static class picdeal
    {
        private static hashtable htmimes = new hashtable();
        internal static readonly string allowext = ".jpe|.jpeg|.jpg|.png|.tif|.tiff|.bmp";
        #region 生成缩略图
        /// <summary>
        /// 生成缩略图
        /// </summary>
        /// <param name="originalimagepath"></param>
        /// <param name="width"></param>
        /// <param name="height"></param>
        /// <param name="mode"></param>
        /// <returns></returns>
        public static bool makethumbnail(string originalimagepath, int width, int height, thumbnailmod mode)
        {
            string thumbnailpath = originalimagepath.substring(0, originalimagepath.lastindexof('.')) + "s.jpg";
            image originalimage = image.fromfile(originalimagepath);
            int towidth = width;
            int toheight = height;
            int x = 0;
            int y = 0;
            int ow = originalimage.width;
            int oh = originalimage.height;
            switch (mode)
            {
                case thumbnailmod.hw://指定高宽缩放（可能变形）                
                    break;
                case thumbnailmod.w://指定宽，高按比例                    
                    toheight = originalimage.height * width / originalimage.width;
                    break;
                case thumbnailmod.h://指定高，宽按比例
                    towidth = originalimage.width * height / originalimage.height;
                    break;
                case thumbnailmod.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
            image bitmap = new bitmap(towidth, toheight);
            //新建一个画板
            graphics g = 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.transparent);
            //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 rectangle(0, 0, towidth, toheight),
                new rectangle(x, y, ow, oh),
                graphicsunit.pixel);
            bool isok = false;
            try
            {
                //以jpg格式保存缩略图
                bitmap.save(thumbnailpath, imageformat.jpeg);
                isok = true;
            }
            catch (exception)
            {
                thumbnailpath = originalimagepath;
            }
            finally
            {
                originalimage.dispose();
                bitmap.dispose();
                g.dispose();
            }
            return isok;
        }
        #endregion
        #region 在图片上生成图片水印
        ///// <summary>
        ///// 在图片上生成图片水印
        ///// </summary>
        ///// <param name="path">原服务器图片路径</param>
        ///// <param name="path_syp">生成的带图片水印的图片路径</param>
        ///// <param name="path_sypf">水印图片路径</param>
        /// <summary>
        /// 在图片上生成图片水印
        /// </summary>
        /// <param name="path">原服务器图片路径</param>
        /// <param name="path_sypf">水印图片路径</param>
        public static void addwaterpic(string path, string path_sypf)
        {
            try
            {
                image image = image.fromfile(path);
                image copyimage = image.fromfile(path_sypf);
                graphics g = graphics.fromimage(image);
                g.drawimage(copyimage, new system.drawing.rectangle(image.width - copyimage.width, image.height - copyimage.height, copyimage.width, copyimage.height), 0, 0, copyimage.width, copyimage.height, system.drawing.graphicsunit.pixel);
                g.dispose();
                image.save(path + ".temp");
                image.dispose();
                system.io.file.delete(path);
                file.move(path + ".temp", path);
            }
            catch
            { }
        }
        #endregion
        /// <summary>
        /// 公共方法
        /// </summary>
        private static void getimgtype()
        {
            htmimes[".jpe"] = "image/jpeg";
            htmimes[".jpeg"] = "image/jpeg";
            htmimes[".jpg"] = "image/jpeg";
            htmimes[".png"] = "image/png";
            htmimes[".tif"] = "image/tiff";
            htmimes[".tiff"] = "image/tiff";
            htmimes[".bmp"] = "image/bmp";
        }

        #region 返回新图片尺寸
        /// <summary>
        /// 返回新图片尺寸
        /// </summary>
        /// <param name="width">原始宽</param>
        /// <param name="height">原始高</param>
        /// <param name="maxwidth">新图片最大宽</param>
        /// <param name="maxheight">新图片最大高</param>
        /// <returns></returns>
        public static size resizeimage(int width, int height, int maxwidth, int maxheight)
        {
            decimal max_width = (decimal)maxwidth;
            decimal max_height = (decimal)maxheight;
            decimal aspect_ratio = max_width / max_height;
            int newwidth, newheight;
            decimal originalwidth = (decimal)width;
            decimal originalheight = (decimal)height;
            if (originalwidth > max_width || originalheight > max_height)
            {
                decimal factor;
                // determine the largest factor 
                if (originalwidth / originalheight > aspect_ratio)
                {
                    factor = originalwidth / max_width;
                    newwidth = convert.toint32(originalwidth / factor);
                    newheight = convert.toint32(originalheight / factor);
                }
                else
                {
                    factor = originalheight / max_height;
                    newwidth = convert.toint32(originalwidth / factor);
                    newheight = convert.toint32(originalheight / factor);
                }
            }
            else
            {
                newwidth = width;
                newheight = height;
            }
            return new size(newwidth, newheight);
        }
        #endregion
    }

 

备注：原文转载自c#生成图片的缩略图_it技术小趣屋。