利用最大高度和最大宽度约束按比例调整图像大小

像这样吗？

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;
}

工作方法:

用于调整大小小于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);

更多参考资料请浏览 http://net4attack.blogspot.com/

长得多的解决方案，但考虑到以下情况:

1. 图像是否小于边框？
2. 图像和包围盒是正方形的吗？
3. 是图像正方形，而边框不是
4. 图像比边框更宽更高
5. 图像比边框宽

6. 图像高于边框

   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;
   }
   }
   }