C#图片按比例缩放的实现代码

using system.drawing;
using system.drawing.drawing2d;
using system.drawing.imaging;

namespace publics
{
    public class imghelper
    {
        public static void adjustphoto(int towidth, int toheight, string filepath, string fromfilename, string tofilename, int maxwidth, int maxheight)
        {
            image originalimage = image.fromfile(filepath + "/" + fromfilename);
            //如果尺寸不够返回保存原图
            if (originalimage.width < towidth && originalimage.height < toheight)
            {
                originalimage.save(filepath + "/" + tofilename);
                originalimage.dispose();
                return;
            }

            //根据图片大小获取新图片从原图片截取的区域
            int x, y, w, h;
            if (toheight > 0)
            {
                if (towidth > 0)
                {
                    if (originalimage.width > towidth && originalimage.height > toheight)
                    {
                        w = towidth;
                        h = towidth * originalimage.height / originalimage.width;

                        if (h > toheight)
                        {
                            h = toheight;
                            w = toheight * originalimage.width / originalimage.height;
                            x = (towidth - w) / 2;
                            y = 0;
                        }
                        else
                        {
                            x = 0;
                            y = (toheight - h) / 2;
                        }
                    }
                    else if (originalimage.width > towidth)
                    {
                        w = towidth;
                        h = towidth * originalimage.height / originalimage.width;
                        x = 0;
                        y = (toheight - h) / 2;
                    }
                    else if (originalimage.height > toheight)
                    {
                        h = toheight;
                        w = toheight * originalimage.width / originalimage.height;
                        x = (towidth - w) / 2;
                        y = 0;
                    }
                    else
                    {
                        w = originalimage.width;
                        h = originalimage.height;
                        x = (towidth - w) / 2;
                        y = (toheight - h) / 2;
                    }
                }
                else
                {
                    if (originalimage.height > maxheight)
                    {
                        towidth = toheight * originalimage.width / originalimage.height;
                        x = 0;
                        y = 0;
                        w = towidth;
                        h = toheight;

                    }
                    else
                    {
                        x = 0;
                        y = 0;
                        w = originalimage.width;
                        h = originalimage.height;
                        towidth = originalimage.width;
                        toheight = originalimage.height;
                    }
                }
            }
            else
            {
                if (originalimage.width > maxwidth)
                {
                    toheight = towidth * originalimage.height / originalimage.width;
                    x = 0;
                    y = 0;
                    w = towidth;
                    h = toheight;

                }
                else
                {
                    x = 0;
                    y = 0;
                    w = originalimage.width;
                    h = originalimage.height;
                    towidth = originalimage.width;
                    toheight = originalimage.height;
                }
            }
            bitmap bm = new bitmap(towidth, toheight);
            graphics g = graphics.fromimage(bm);

            g.smoothingmode = smoothingmode.highquality;
            g.interpolationmode = interpolationmode.highqualitybicubic;

            g.clear(color.white);
            g.drawimage(originalimage, new rectangle(x, y, w, h), 0, 0, originalimage.width, originalimage.height, graphicsunit.pixel);

            long[] quality = new long[1];
            quality[0] = 80;

            encoderparameters encoderparams = new encoderparameters();
            encoderparameter encoderparam = new encoderparameter(encoder.quality, quality);
            encoderparams.param[0] = encoderparam;
            imagecodecinfo[] arrayici = imagecodecinfo.getimageencoders();//获得包含有关内置图像编码解码器的信息的imagecodecinfo 对象。
            imagecodecinfo jpegici = null;
            for (int i = 0; i < arrayici.length; i++)
            {
                if (arrayici[i].formatdescription.equals("jpeg"))
                {
                    jpegici = arrayici[i];//设置jpeg编码
                    break;
                }
            }
            if (jpegici != null)
            {

                //bm.save(server.mappath(path + "/thumb_" + filename), jpegici, encoderparams);
                bm.save(filepath + "/" + tofilename, jpegici, encoderparams);
            }

            bm.dispose();
            originalimage.dispose();
            g.dispose();
        }

        /// <summary>
        /// 保持比例图像缩放简易算法
        /// </summary>
        /// <param name="spcwidth"></param>
        /// <param name="spcheight"></param>
        /// <param name="orgwidth"></param>
        /// <param name="orgheight"></param>
        /// <returns></returns>
        public static dictionary<string, int> adjustsize(int spcwidth, int spcheight, int orgwidth, int orgheight)
        {
            dictionary<string, int> size = new dictionary<string, int>();
            // 原始宽高在指定宽高范围内，不作任何处理 
            if (orgwidth <= spcwidth && orgheight <= spcheight)
            {
                size["width"] = orgwidth;
                size["height"] = orgheight;
            }
            else
            {
                // 取得比例系数 
                float w = orgwidth / (float)spcwidth;
                float h = orgheight / (float)spcheight;
                // 宽度比大于高度比 
                if (w > h)
                {
                    size["width"] = spcwidth;
                    size["height"] = (int)(w >= 1 ? math.round(orgheight / w) : math.round(orgheight * w));
                }
                // 宽度比小于高度比 
                else if (w < h)
                {
                    size["height"] = spcheight;
                    size["width"] = (int)(h >= 1 ? math.round(orgwidth / h) : math.round(orgwidth * h));
                }
                // 宽度比等于高度比 
                else
                {
                    size["width"] = spcwidth;
                    size["height"] = spcheight;
                }
            }
            return size;
        }
    }
}