使用MaxHeight和MaxWidth约束按比例调整图像大小

使用System.Drawing.Image。

如果图像的宽度或高度超过最大值，则需要按比例调整大小。调整大小后，需要确保宽度或高度均不超过限制。

宽度和高度将自动调整大小，直到不超过最大值和最小值（可能的最大尺寸），并保持该比例。

像这样？

public static void Test()
{
    using (var image = Image.FromFile(@"c:\logo.png"))
    using (var newImage = ScaleImage(image, 300, 400))
    {
        newImage.Save(@"c:\test.png", ImageFormat.Png);
    }
}

public static Image ScaleImage(Image image, int maxWidth, int maxHeight)
{
    var ratioX = (double)maxWidth / image.Width;
    var ratioY = (double)maxHeight / image.Height;
    var ratio = Math.Min(ratioX, ratioY);

    var newWidth = (int)(image.Width * ratio);
    var newHeight = (int)(image.Height * ratio);

    var newImage = new Bitmap(newWidth, newHeight);

    using (var graphics = Graphics.FromImage(newImage))
        graphics.DrawImage(image, 0, 0, newWidth, newHeight);

    return newImage;
}

工作方案：

调整大小小于100Kb的图像

WriteableBitmap bitmap = new WriteableBitmap(140,140);
bitmap.SetSource(dlg.File.OpenRead());
image1.Source = bitmap;

Image img = new Image();
img.Source = bitmap;
WriteableBitmap i;

do
{
    ScaleTransform st = new ScaleTransform();
    st.ScaleX = 0.3;
    st.ScaleY = 0.3;
    i = new WriteableBitmap(img, st);
    img.Source = i;
} while (i.Pixels.Length / 1024 > 100);

有关更多参考，请访问http://net4attack.blogspot.com/

解决方案长得多，但是考虑了以下情况：

1. 图像是否小于边框？
2. 图像和边界框是否正方形？
3. 图片是正方形，边框不是吗
4. 图像是否比边界框宽且高
5. 图像是否比边界框宽

6. 图片比边框高吗

   private Image ResizePhoto(FileInfo sourceImage, int desiredWidth, int desiredHeight)
   {
       //throw error if bouning box is to small
       if (desiredWidth < 4 || desiredHeight < 4)
           throw new InvalidOperationException("Bounding Box of Resize Photo must be larger than 4X4 pixels.");            
       var original = Bitmap.FromFile(sourceImage.FullName);
   
       //store image widths in variable for easier use
       var oW = (decimal)original.Width;
       var oH = (decimal)original.Height;
       var dW = (decimal)desiredWidth;
       var dH = (decimal)desiredHeight;
   
       //check if image already fits
       if (oW < dW && oH < dH)
           return original; //image fits in bounding box, keep size (center with css) If we made it bigger it would stretch the image resulting in loss of quality.
   
       //check for double squares
       if (oW == oH && dW == dH)
       {
           //image and bounding box are square, no need to calculate aspects, just downsize it with the bounding box
           Bitmap square = new Bitmap(original, (int)dW, (int)dH);
           original.Dispose();
           return square;
       }
   
       //check original image is square
       if (oW == oH)
       {
           //image is square, bounding box isn't.  Get smallest side of bounding box and resize to a square of that center the image vertically and horizontally with Css there will be space on one side.
           int smallSide = (int)Math.Min(dW, dH);
           Bitmap square = new Bitmap(original, smallSide, smallSide);
           original.Dispose();
           return square;
       }
   
       //not dealing with squares, figure out resizing within aspect ratios            
       if (oW > dW && oH > dH) //image is wider and taller than bounding box
       {
           var r = Math.Min(dW, dH) / Math.Min(oW, oH); //two dimensions so figure out which bounding box dimension is the smallest and which original image dimension is the smallest, already know original image is larger than bounding box
           var nH = oH * r; //will downscale the original image by an aspect ratio to fit in the bounding box at the maximum size within aspect ratio.
           var nW = oW * r;
           var resized = new Bitmap(original, (int)nW, (int)nH);
           original.Dispose();
           return resized;
       }
       else
       {
           if (oW > dW) //image is wider than bounding box
           {
               var r = dW / oW; //one dimension (width) so calculate the aspect ratio between the bounding box width and original image width
               var nW = oW * r; //downscale image by r to fit in the bounding box...
               var nH = oH * r;
               var resized = new Bitmap(original, (int)nW, (int)nH);
               original.Dispose();
               return resized;
           }
           else
           {
               //original image is taller than bounding box
               var r = dH / oH;
               var nH = oH * r;
               var nW = oW * r;
               var resized = new Bitmap(original, (int)nW, (int)nH);
               original.Dispose();
               return resized;
           }
       }
   }