using System;
using System.Collections.Generic;
using System.Text;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
namespace Project
{class ImageOperation{/// <summary>/// Resize图片/// </summary>/// <param name="bmp">原始Bitmap </param>/// <param name="newW">新的宽度</param>/// <param name="newH">新的高度</param>/// <param name="Mode">保留着，暂时未用</param>/// <returns>处理以后的图片</returns>public static Bitmap ResizeImage(Bitmap bmp, int newW, int newH, int Mode){try{Bitmap b = new Bitmap(newW, newH);Graphics g = Graphics.FromImage(b);// 插值算法的质量g.InterpolationMode = InterpolationMode.HighQualityBicubic;g.DrawImage(bmp, new Rectangle(0, 0, newW, newH), new Rectangle(0, 0, bmp.Width, bmp.Height), GraphicsUnit.Pixel);g.Dispose();return b;}catch{return null;}}/// <summary>/// 剪裁 -- 用GDI+/// </summary>/// <param name="b">原始Bitmap</param>/// <param name="StartX">开始坐标X</param>/// <param name="StartY">开始坐标Y</param>/// <param name="iWidth">宽度</param>/// <param name="iHeight">高度</param>/// <returns>剪裁后的Bitmap</returns>public static Bitmap Cut(Bitmap b, int StartX, int StartY, int iWidth, int iHeight){if (b == null){return null;}int w = b.Width;int h = b.Height;if (StartX >= w || StartY >= h){return null;}if (StartX + iWidth > w){iWidth = w - StartX;}if (StartY + iHeight > h){iHeight = h - StartY;}try{Bitmap bmpOut = new Bitmap(iWidth, iHeight, PixelFormat.Format24bppRgb);Graphics g = Graphics.FromImage(bmpOut);g.DrawImage(b, new Rectangle(0, 0, iWidth, iHeight), new Rectangle(StartX, StartY, iWidth, iHeight), GraphicsUnit.Pixel);g.Dispose();return bmpOut;}catch{return null;}}}
}
















实例：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace CompressImageTest
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        Image SourceImage = null;
        
        private void button_SelectImage_Click(object sender, EventArgs e)
        {
            OpenFileDialog openFileDialog = new OpenFileDialog();
            string filePath = "";
            if(openFileDialog.ShowDialog()==DialogResult.OK)
            {

filePath = openFileDialog.FileName;

}
             SourceImage=Image.FromFile(filePath);
            this.textBox_CurrentWidth.Text = Convert.ToString(SourceImage.Width);
            this.textBox_CurrentHeight.Text = Convert.ToString(SourceImage.Height);
            this.textBox_SetWidth.Text = "";
            this.textBox_SetHeight.Text = "";
            this.pictureBox1.Image = SourceImage;
            SaveImageToStream(SourceImage);//保存图像
           
        }
        public  Bitmap GetThumbnail(Bitmap b,  int destWidth, int destHeight)
        {
            System.Drawing.Image imgSource = b;
            System.Drawing.Imaging.ImageFormat thisFormat = imgSource.RawFormat;
            int sW = 0, sH = 0;
            // 按比例缩放           
            int sWidth = imgSource.Width;
            int sHeight = imgSource.Height;
            if (sHeight > destHeight || sWidth > destWidth)
            {
                if ((sWidth * destHeight) > (sHeight * destWidth))
                {
                    sW = destWidth;
                    sH = (destWidth * sHeight) / sWidth;
                }
                else
                {
                    sH = destHeight;
                    sW = (sWidth * destHeight) / sHeight;
                }
            }
            else
            {
                sW = sWidth;
                sH = sHeight;
            }
            Bitmap outBmp = new Bitmap(destWidth, destHeight);
            Graphics g = Graphics.FromImage(outBmp);
            g.Clear(Color.Transparent);
            // 设置画布的描绘质量         
            g.CompositingQuality = CompositingQuality.HighQuality;
            g.SmoothingMode = SmoothingMode.HighQuality;
            g.InterpolationMode = InterpolationMode.HighQualityBicubic;
            g.DrawImage(imgSource, new Rectangle((destWidth - sW) / 2, (destHeight - sH) / 2, sW, sH), 0, 0, imgSource.Width, imgSource.Height, GraphicsUnit.Pixel);
            g.Dispose();
            // 以下代码为保存图片时，设置压缩质量     
            EncoderParameters encoderParams = new EncoderParameters();
            long[] quality = new long[1];
            quality[0] = 100;
            EncoderParameter encoderParam = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, quality);
            encoderParams.Param[0] = encoderParam;
            imgSource.Dispose();
            return outBmp;
        }

private void compress_Click(object sender, EventArgs e)
        {
            SourceImage=SaveImage = GetImageFromStream(Ms);//获取图像
            Bitmap bitmap = new Bitmap(SourceImage);
            SourceImage=  GetThumbnail(bitmap, Convert.ToInt32(textBox_SetWidth.Text), Convert.ToInt32(textBox_SetHeight.Text));
            //SourceImage=ResizeImage(bitmap, Convert.ToInt32(textBox_SetWidth.Text), Convert.ToInt32(textBox_SetHeight.Text), 0);
            pictureBox1.Image = SourceImage;
            pictureBox1.i
          this.textBox_CurrentWidth.Text = Convert.ToString(SourceImage.Width);
          this.textBox_CurrentHeight.Text = Convert.ToString(SourceImage.Height);
        }

public  Bitmap ResizeImage(Bitmap bmp, int newW, int newH, int Mode)
        {
            try
            {
                Bitmap b = new Bitmap(newW, newH);
                Graphics g = Graphics.FromImage(b);
                // 插值算法的质量
                g.InterpolationMode = InterpolationMode.HighQualityBicubic;
                g.DrawImage(bmp, new Rectangle(0, 0, newW, newH), new Rectangle(0, 0, bmp.Width, bmp.Height), GraphicsUnit.Pixel);
                g.Dispose();
                return b;
            }
            catch
            {
                return null;
            }
        }

public static Image SaveImage;//保存图像
        public static MemoryStream Ms;
        public static void SaveImageToStream(Image image)//保存图像
        {

Ms = new MemoryStream();
            image.Save(Ms, System.Drawing.Imaging.ImageFormat.Png);

}
        public static Image GetImageFromStream(Stream stream)//获取图像
        {

MemoryStream ms = stream as MemoryStream;
            ms.Position = 0;
            Image image = Image.FromStream(ms);
            return image;
            //ms.Close();

}
    }
}