Image Resize in C# - Algorith to determine resize dimensions (height and width)

Question

I need to scale down an image that has a height or width greater than a predefined pixel value.

I wrote some code that takes a look at the original image, checks to see if either the width, the height, or the height and width are greater than the Max Width/Max Height settings.

I now need to figure out what dimensions to resize to based on the max of the latter value.

For example: If the image is 900h x 300w and the MAX Height is 700h I will need to resize the height to 700 and the width to ???? <-- this is what I need to calculate..

Creating and saving the image file is simple, and outside the scope of this post:

// First I get the max height and width allowed:

int resizeMaxHeight =  int.Parse(Utility.GetConfigValue("ResizeMaxHeight")); // in config: 700px
int resizeMaxWidth =  int.Parse(Utility.GetConfigValue("ResizeMaxWidth"));  //  in config: 500px

// Save original: 
try
{
    filebase.SaveAs(savedFileName);
}
catch (System.IO.DirectoryNotFoundException ex)
{
    Logger.Instance.LogException(ex, 0, "FileTransfer");
}

// Determin original dimensions:
Image image = System.Drawing.Image.FromFile(Server.MapPath(savedFileName));

int resizeHeight, resizeWidth;
bool doResize = true;

// both height and width are greater than the allowed height and width:
if (image.Width > resizeMaxWidth && image.Height > resizeMaxHeight)
{
    if (image.Height > image.Width) 
        resizeHeight = resizeMaxHeight;
    else
        resizeWidth = resizeMaxWidth;
}
else if (image.Width > resizeMaxWidth)
{
    // width is too great, but height is ok
    resizeWidth = resizeMaxWidth;
}
else if (image.Height > resizeMaxHeight)
{
    // height is too great, but width is ok
    resizeHeight = resizeMaxHeight;
}
else
{
    // image is ok size, don't resize:
    doResize = false;
}

Create thumbnail: This is what I'm working now... not complete:

if (doResize)
{
    ImageUtilities.ResizeImage(image, resizeWidth, resizeHeight);
}

Answer 1

The solution posted by Nathaniel actually fails if the image height is larger than the image width. The following example yields the correct result :

private Size ResizeFit(Size originalSize, Size maxSize)
{
    var widthRatio = (double)maxSize.Width / (double)originalSize.Width;
    var heightRatio = (double) maxSize.Height/(double) originalSize.Height;
    var minAspectRatio = Math.Min(widthRatio, heightRatio);
    if (minAspectRatio > 1)
        return originalSize;
    return new Size((int)(originalSize.Width*minAspectRatio), (int)(originalSize.Height*minAspectRatio));
}