生成高质量缩略图

调用：

int width = 190;
int height = 190;
Bitmap source = new Bitmap("c:\\someimage.jpg");
System.Drawing.Image thumb = source.GetThumbnailImage(width,height,null,IntPtr.Zero);
thumb.Save("C:\\someimageshot.jpg");
thumb.Dispose();

函数：

public static Bitmap CreateThumbnail(Bitmap source, int thumbWi, int thumbHi, bool maintainAspect)
        {
            if (source.Width < thumbWi && source.Height < thumbHi) return source;
            System.Drawing.Bitmap ret = null;
            try
            {
                int wi, hi;

                wi = thumbWi;
                hi = thumbHi;

                if (maintainAspect)
                {
                    if (source.Width > source.Height)
                    {
                        wi = thumbWi;
                        hi = (int)(source.Height * ((decimal)thumbWi / source.Width));
                    }
                    else
                    {
                        hi = thumbHi;
                        wi = (int)(source.Width * ((decimal)thumbHi / source.Height));
                    }
                }
                ret = new Bitmap(wi, hi);
                using (Graphics g = Graphics.FromImage(ret))
                {
                    g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
                    g.FillRectangle(Brushes.White, 0, 0, wi, hi);
                    g.DrawImage(source, 0, 0, wi, hi);
                }
            }
            catch
            {
                ret = null;
            }

            return ret;
        }

另类用途：

比如某张云图特别大。要在首页上缩小显示。第一想法就是强制设定img的width和height。这个方法对ie8完美运行。但是ie7和6会出现图片压缩严重失真现象。这时就可以用大图转成缩略图来实现。读取大图的时候生成缩略图，并读取缩略图。下次再thumb.Save同一个缩略图文件。这样也不占用空间。