ASP.NET 生成缩略图(防失真)。

BuildSourImage()，避免了使用传统的 SaveAs()方法保存文件。

BuildThumbImage()方法，是生成缩略图的，这个生成缩略图的步骤是，首先读取原图。根据原图的大小，按照比例缩放原图。像下面的代码：

//问题1
//这个判断就是判断，哪一个除数大（即高度大），那长度不变，高度乘以比例。反之，类推。
if (((decimal)width) / height <= ((decimal)intThumbWidth) / intThumbHeight)
{
smallWidth = intThumbWidth;
smallHeight = intThumbWidth * height / width;
//问题2 <br>
//其实可以写成下面这样子，不知道对不对？我的理解就是，拿缩略图的和原图的宽的比例，然后乘以高度。就是两者比例下的高度了。
//smallHeight = intThumbWidth / width * height;
}
else {
smallWidth = intThumbHeight / height * width;
smallHeight = intThumbHeight;
}

全部代码如下

protected void Button1_Click(object sender, EventArgs e)
{
UpLoadImage(File1, "~/UPLOAD/", "X.JPG", 20, 15);
}

/// <summary>
/// asp.net上传图片并生成缩略图
/// </summary>
/// <param name="upImage">HtmlInputFile控件</param>
/// <param name="sSavePath">保存的路径,些为相对服务器路径的下的文件夹</param>
/// <param name="sThumbExtension">缩略图的thumb</param>
/// <param name="intThumbWidth">生成缩略图的宽度</param>
/// <param name="intThumbHeight">生成缩略图的高度</param>
/// <returns>缩略图名称</returns>
public string UpLoadImage(HtmlInputFile upImage, string sSavePath, string sThumbExtension, int intThumbWidth, int intThumbHeight)
{
string sThumbFile = "";
string sFilename = "";
if (upImage.PostedFile != null)
{
sFilename = BuildSourImage(upImage, sSavePath);//上传原图
sThumbFile = BuildThumbImage(sSavePath + sFilename, sFilename, sSavePath + "small/", 300, 200); //上传缩略图
return sThumbFile;
}
return "没有选择图片";
}

/// <summary>
/// 上传大图
/// </summary>
/// <param name="upImage"></param>
/// <param name="sSavePath"></param>
/// <returns></returns>
public string BuildSourImage(HtmlInputFile upImage, string sSavePath)
{
string sFilename = "";
HttpPostedFile myFile = upImage.PostedFile;
int nFileLen = myFile.ContentLength;
if (nFileLen == 0)
return "没有选择上传图片";
//获取upImage选择文件的扩展名
string extendName = System.IO.Path.GetExtension(myFile.FileName).ToLower();
//判断是否为图片格式
if (extendName != ".jpg" && extendName != ".jpge" && extendName != ".gif" && extendName != ".bmp" && extendName != ".png")
return "图片格式不正确";

byte[] myData = new Byte[nFileLen];
myFile.InputStream.Read(myData, 0, nFileLen);
sFilename = DateTime.Now.ToString("yyyyMMddHHmmssfff") + extendName;

//检查当前文件夹下是否有同名图片,有则在文件名+1

System.IO.FileStream newFile
= new System.IO.FileStream(System.Web.HttpContext.Current.Server.MapPath(sSavePath) + sFilename,
System.IO.FileMode.Create, System.IO.FileAccess.Write);
newFile.Write(myData, 0, myData.Length);
newFile.Close();

return sFilename;
}

/// <summary>
/// 生成缩略图
/// </summary>
/// <param name="sSourceFile">原图地址</param>
/// <param name="thumbName">缩略图名称</param>
/// <param name="sSavePath">保存地址</param>
/// <param name="intThumbWidth">缩略图宽度</param>
/// <param name="intThumbHeight">缩略图高度</param>
/// <returns></returns>
public string BuildThumbImage(string sSourceFile, string thumbName, string sSavePath, int intThumbWidth, int intThumbHeight)
{

try
{
//原图加载
using (System.Drawing.Image sourceImage = System.Drawing.Image.FromFile(System.Web.HttpContext.Current.Server.MapPath(sSourceFile)))
{
//原图宽度和高度
int width = sourceImage.Width;
int height = sourceImage.Height;

int smallWidth;
int smallHeight;
//获取第一张绘制图的大小,(比较 原图的宽/缩略图的宽 和 原图的高/缩略图的高)
if (((decimal)width) / height <= ((decimal)intThumbWidth) / intThumbHeight)
{
smallWidth = intThumbWidth;
smallHeight = intThumbWidth * height / width;
}
else
{
smallWidth = intThumbHeight * width / height;
smallHeight = intThumbHeight;
}

//缩略图保存的绝对路径
string smallImagePath = System.Web.HttpContext.Current.Server.MapPath(sSavePath) + thumbName;
//新建一个图板,以最小等比例压缩大小绘制原图
using (System.Drawing.Image bitmap = new System.Drawing.Bitmap(smallWidth, smallHeight))
{
//绘制中间图
using (System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap))
{
//高清,平滑
g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
g.Clear(Color.Black);
g.DrawImage(
sourceImage,
new System.Drawing.Rectangle(0, 0, smallWidth, smallHeight),
new System.Drawing.Rectangle(0, 0, width, height),
System.Drawing.GraphicsUnit.Pixel
);

}
//新建一个图板,以缩略图大小绘制中间图
using (System.Drawing.Image bitmap1 = new System.Drawing.Bitmap(intThumbWidth, intThumbHeight))
{
//绘制缩略图
using (System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap1))
{
//高清,平滑
g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;
g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
g.Clear(Color.Black);
int lwidth = (smallWidth - intThumbWidth) / 2;
int bheight = (smallHeight - intThumbHeight) / 2;
g.DrawImage(bitmap,
new Rectangle(0, 0, intThumbWidth, intThumbHeight),
lwidth, bheight, intThumbWidth, intThumbHeight,
GraphicsUnit.Pixel);
g.Dispose();
bitmap1.Save(smallImagePath, System.Drawing.Imaging.ImageFormat.Jpeg);
}
}
}
}
}
catch
{
//出错则删除
System.IO.File.Delete(System.Web.HttpContext.Current.Server.MapPath(sSavePath + thumbName));
return "图片格式不正确";
}

return thumbName;
}