怎样按比例缩小图片的两种方法

第一种：

缩略图代码：
ImageThumbnail.cs
using System;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;

public class ImageThumbnail
{
    public Image ResourceImage;
    private int ImageWidth;
    private int ImageHeight;
    public string ErrorMessage;

    public ImageThumbnail(string ImageFileName)
    {
        ResourceImage = Image.FromFile(ImageFileName);
        ErrorMessage = "";
    }

    public bool ThumbnailCallback()
    {
        return false;
    }


    // 方法1，按大小
    public bool ReducedImage(int Width, int Height, string targetFilePath)
    {
        try
        {
            Image ReducedImage;
            Image.GetThumbnailImageAbort callb = new Image.GetThumbnailImageAbort(ThumbnailCallback);
            ReducedImage = ResourceImage.GetThumbnailImage(Width, Height, callb, IntPtr.Zero);
            ReducedImage.Save(@targetFilePath, ImageFormat.Jpeg);
            ReducedImage.Dispose();
            return true;
        }
        catch (Exception e)
        {
            ErrorMessage = e.Message;
            return false;
        }
    }


    // 方法2，按百分比  缩小60% Percent为0.6 targetFilePath为目标路径
    public bool ReducedImage(double Percent, string targetFilePath)
    {
        try
        {
            Image ReducedImage;
            Image.GetThumbnailImageAbort callb = new Image.GetThumbnailImageAbort(ThumbnailCallback);
            ImageWidth = Convert.ToInt32(ResourceImage.Width * Percent);
            ImageHeight = (ResourceImage.Height)*ImageWidth/ ResourceImage.Width;//等比例缩放
            ReducedImage = ResourceImage.GetThumbnailImage(ImageWidth, ImageHeight, callb, IntPtr.Zero);
            ReducedImage.Save(@targetFilePath, ImageFormat.Jpeg);
            ReducedImage.Dispose();
            return true;
        }
        catch (Exception e)
        {
            ErrorMessage = e.Message;
            return false;
        }
    }


}

后台代码：
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
public partial class _Default : System.Web.UI.Page
{

    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void bt_upload_Click(object sender, EventArgs e)
    {
        try
        {
            if (FileUpload1.PostedFile.FileName == "")
            {
                this.lb_info.Text = "请选择文件！";
            }
            else
            {
                string filepath = FileUpload1.PostedFile.FileName;
                string filename = filepath.Substring(filepath.LastIndexOf("\\") + 1);
                string serverpath1 = Server.MapPath("images/") + filename;
                string serverpath2 = Server.MapPath("images/") + System.DateTime.Now.ToString("yyy-MM-dd-hh-mm-ss") + Session.SessionID + filename;
                FileUpload1.PostedFile.SaveAs(serverpath1);
                ImageThumbnail img = new ImageThumbnail(filepath);
                img.ReducedImage(0.4, serverpath2);//0.4表示缩小40%
                this.lb_info.Text = "上传成功！";
            }
        }
        catch (Exception error)
        {
            this.lb_info.Text = "上传发生错误！原因：" + error.ToString();
        }
    }


}

第二种：

private void btnUploadPicture_Click(object sender, System.EventArgs e) 
{ 
   //检查上传文件的格式是否有效 
   if(this.UploadFile.PostedFile.ContentType.ToLower().IndexOf("image") < 0) 
   { 
    Response.Write("上传图片格式无效！"); 
    return; 
   } 

   //生成原图 
   Byte[] oFileByte = new byte[this.UploadFile.PostedFile.ContentLength]; 
   System.IO.Stream oStream = this.UploadFile.PostedFile.InputStream; 
   System.Drawing.Image oImage = System.Drawing.Image.FromStream(oStream); 

   int oWidth = oImage.Width; //原图宽度 
   int oHeight = oImage.Height; //原图高度 
   int tWidth = 100; //设置缩略图初始宽度 
   int tHeight = 100; //设置缩略图初始高度 

   //按比例计算出缩略图的宽度和高度 
   if(oWidth >= oHeight) 
   { 
    tHeight = (int)Math.Floor(Convert.ToDouble(oHeight) * (Convert.ToDouble(tWidth) / Convert.ToDouble(oWidth))); 
   } 
   else 
   { 
    tWidth = (int)Math.Floor(Convert.ToDouble(oWidth) * (Convert.ToDouble(tHeight) / Convert.ToDouble(oHeight))); 
   } 

   //生成缩略原图 
   Bitmap tImage = new Bitmap(tWidth,tHeight); 
   Graphics g = Graphics.FromImage(tImage); 
   g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High; //设置高质量插值法 
   g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;//设置高质量,低速度呈现平滑程度 
   g.Clear(Color.Transparent); //清空画布并以透明背景色填充 
   g.DrawImage(oImage,new Rectangle(0,0,tWidth,tHeight),new Rectangle(0,0,oWidth,oHeight),GraphicsUnit.Pixel); 

   string oFullName = Server.MapPath(".") + "/" + "o" + DateTime.Now.ToShortDateString().Replace("-","") + DateTime.Now.Hour.ToString() + DateTime.Now.Minute.ToString() + DateTime.Now.Second.ToString() + DateTime.Now.Millisecond.ToString() + ".jpg"; //保存原图的物理路径 
   string tFullName = Server.MapPath(".") + "/" + "t" + DateTime.Now.ToShortDateString().Replace("-","") + DateTime.Now.Hour.ToString() + DateTime.Now.Minute.ToString() + DateTime.Now.Second.ToString() + DateTime.Now.Millisecond.ToString() + ".jpg"; //保存缩略图的物理路径 

   try 
   { 
    //以JPG格式保存图片 
    oImage.Save(oFullName,System.Drawing.Imaging.ImageFormat.Jpeg); 
    tImage.Save(tFullName,System.Drawing.Imaging.ImageFormat.Jpeg); 
   } 
   catch(Exception ex) 
   { 
    throw ex; 
   } 
   finally 
   { 
    //释放资源 
    oImage.Dispose(); 
    g.Dispose(); 
    tImage.Dispose(); 
   } 
}