.net C#生成缩略图实现思路分解

/// 生成缩略图 
/// </summary> 
/// <param name="originalImagePath">源图路径</param> 
/// <param name="thumbnailPath">缩略图路径</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:HW指定高宽缩放(可能变形);W指定宽，高按比例 H指定高，宽按比例 Cut指定高宽裁减(不变形)</param>　　 
/// <param name="mode">要缩略图保存的格式(gif,jpg,bmp,png) 为空或未知类型都视为jpg</param>　　 
public static void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string mode, string imageType) 
{ 
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 
switch (imageType.ToLower()) 
{ 
case "gif": 
bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Gif); 
break; 
case "jpg": 
bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg); 
break; 
case "bmp": 
bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Bmp); 
break; 
case "png": 
bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Png); 
break; 
default: 
bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg); 
break; 
} 
} 
catch (System.Exception e) 
{ 
throw e; 
} 
finally 
{ 
originalImage.Dispose(); 
bitmap.Dispose(); 
g.Dispose(); 
} 
}