请问一下，在上传图片时，怎么样才能让大图片上传后成为规定大小而且不失真呢？

Question

请问一下，在上传图片时，怎么样才能让大图片上传后成为规定大小而且不失真呢？我想做的是一个图片预览功能，大图片加载的太慢了，所以要把大图片上传的时候就给处理支规定的大小，而且要图片不失真！请求高手们指点，万分感谢！！

Answer 1

生成缩略图
    Public Function GenerateThumbnail(ByVal original As Image, ByVal DescSize As Integer) As Image
        Dim tn As Bitmap
        Dim DescHeight, DescWidth As Integer
        '对图像的高和宽都要判断
        If original.Height >= DescSize Or original.Width >= DescSize Then
            DescHeight = original.Height
            DescWidth = original.Width
            If DescHeight >= DescSize Then
                DescHeight = DescSize
                DescWidth = DescHeight * original.Width \ original.Height
            End If
            If DescWidth >= DescSize Then
                DescWidth = DescSize
                DescHeight = DescWidth * original.Height \ original.Width
            End If        End If        tn = New Bitmap(CInt(DescSize), CInt(DescSize))
        Dim g As Graphics = Graphics.FromImage(tn)
        Dim whiteBrush As New SolidBrush(Color.White)
        Dim Rectangle As Rectangle = New Rectangle((DescSize - DescWidth) / 2, (DescSize - DescHeight) / 2, DescWidth, DescHeight)
        Dim RectangleS As Rectangle = New Rectangle(0, 0, DescSize, (DescSize - DescHeight) / 2)
        Dim RectangleX As Rectangle = New Rectangle(0, (DescSize + DescHeight) / 2, DescSize, (DescSize - DescHeight) / 2)
        Dim RectangleZ As Rectangle = New Rectangle(0, 0, (DescSize - DescWidth) / 2, DescSize)
        Dim RectangleY As Rectangle = New Rectangle((DescSize + DescWidth) / 2, 0, (DescSize - DescWidth) / 2, DescSize)
        g.FillRectangle(whiteBrush, RectangleS)
        g.FillRectangle(whiteBrush, RectangleX)
        g.FillRectangle(whiteBrush, RectangleZ)
        g.FillRectangle(whiteBrush, RectangleY)
        If DescWidth = DescSize Or DescHeight = DescSize Then
            If DescWidth = DescSize Then
                g.DrawImage(original, Rectangle, 0, 0, original.Width, original.Height, GraphicsUnit.Pixel)
            End If
            If DescHeight = DescSize Then
                g.DrawImage(original, Rectangle, 0, 0, original.Width, original.Height, GraphicsUnit.Pixel)
            End If
        Else
            g.DrawImage(original, New Rectangle((DescSize - original.Width) / 2, (DescSize - original.Height) / 2, _
            original.Width, original.Height), 0, 0, original.Width, original.Height, GraphicsUnit.Pixel)        End If
        g.Dispose()        Return CType(tn, Image)
    End Function 'GenerateThumbnail

Answer 2

（转）        /**//// <summary>
        /// 生成缩略图
        /// </summary>
        /// <param name="originalImagePath">源图路径（物理路径）</param>
        /// <param name="thumbnailPath">缩略图路径（物理路径）</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</param>    
        public static void MakeThumbnail(string originalImagePath, string thumbnailPath, int width, int height, string mode)
        {
            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
                bitmap.Save(thumbnailPath, System.Drawing.Imaging.ImageFormat.Jpeg);
            }
            catch(System.Exception e)
            {
                throw e;
            }
            finally
            {
                originalImage.Dispose();
                bitmap.Dispose();                        
                g.Dispose();
            }
        }关键方法Graphics.DrawImage见ms-help://MS.NETFrameworkSDKv1.1.CHS/cpref/html/frlrfsystemdrawinggraphicsclassdrawimagetopic11.htm