asp.net 生成缩略图 .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();
}
}