怎样不管图片大小都生成一个等比例（如：4:3）的缩略图？可以裁减！
看到好多新闻系统里面都有这个！

------解决方案--------------------
//****************************************************************************************生成缩略图
/// <summary>
/// 生成缩略图
/// </summary>
/// <param name= "originalImagePath "> 源图路径（物理路径） </param>
/// <param name= "thumbnailPath "> 缩略图路径（物理路径） </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>
public static void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string mode)
{

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 "HW "://指定高宽缩放（可能变形）
break;
case "W "://指定宽，高按比例
toheight = originalImage.Height * width/originalImage.Width;
break;
case "H "://指定高，宽按比例
towidth = originalImage.Width * height/originalImage.Height;
break;
case "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 System.Drawing.Bitmap(towidth,toheight);

//新建一个画板
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(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);

try
{
//以jpg格式保存缩略图
bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
}
catch(System.Exception e)
{
throw e;
}
finally
{
originalImage.Dispose();
bitmap.Dispose();
g.Dispose();
}
}

//*************************************************************************************************

------解决方案--------------------
算法的原理就是
width和height相除
算出比例参数再做运算