How to resize Image in C# WinRT/winmd?

Question

I've got simple question, but so far I've found no answer: how to resize jpeg image in C# WinRT/WinMD project and save it as new jpeg?

I'm developing Windows 8 Metro application for downloading daily image form certain site and displaying it on a Live Tile. The problem is the image must be smaller than 1024x1024 and smaller than 200kB, otherwise it won't show on the tile: http://msdn.microsoft.com/en-us/library/windows/apps/hh465403.aspx

If I got larger image, how to resize it to be fit for the Live Tile? I'm thinking just about simple resize like width/2 and height/2 with keeping the aspect ration.

The specific requirement here is that the code must run as Windows Runtime Component, so WriteableBitmapEx library won't work here - it's only available for regular WinRT projects. There is even a branch for WriteableBitmapEx as winmd project, but it's far from ready.

Answer 1

Example of how to scale and crop taken from here:

async private void BitmapTransformTest()
{
    // hard coded image location
    string filePath = "C:\\Users\\Public\\Pictures\\Sample Pictures\\fantasy-dragons-wallpaper.jpg";

    StorageFile file = await StorageFile.GetFileFromPathAsync(filePath);
    if (file == null)
        return;

    // create a stream from the file and decode the image
    var fileStream = await file.OpenAsync(Windows.Storage.FileAccessMode.Read);
    BitmapDecoder decoder = await BitmapDecoder.CreateAsync(fileStream);


    // create a new stream and encoder for the new image
    InMemoryRandomAccessStream ras = new InMemoryRandomAccessStream();
    BitmapEncoder enc = await BitmapEncoder.CreateForTranscodingAsync(ras, decoder);

    // convert the entire bitmap to a 100px by 100px bitmap
    enc.BitmapTransform.ScaledHeight = 100;
    enc.BitmapTransform.ScaledWidth = 100;


    BitmapBounds bounds = new BitmapBounds();
    bounds.Height = 50;
    bounds.Width = 50;
    bounds.X = 50;
    bounds.Y = 50;
    enc.BitmapTransform.Bounds = bounds;

    // write out to the stream
    try
    {
        await enc.FlushAsync();
    }
    catch (Exception ex)
    {
        string s = ex.ToString();
    }

    // render the stream to the screen
    BitmapImage bImg = new BitmapImage();
    bImg.SetSource(ras);
    img.Source = bImg; // image element in xaml

}

Answer 2

More simpler code to re-size the image, not crop. The below code re-size the image as 80x80

using (var sourceStream = await sourceFile.OpenAsync(FileAccessMode.Read))
{
    BitmapDecoder decoder = await BitmapDecoder.CreateAsync(sourceStream);
    BitmapTransform transform = new BitmapTransform() { ScaledHeight = 80, ScaledWidth = 80 };
    PixelDataProvider pixelData = await decoder.GetPixelDataAsync(
        BitmapPixelFormat.Rgba8,
        BitmapAlphaMode.Straight,
        transform,
        ExifOrientationMode.RespectExifOrientation,
        ColorManagementMode.DoNotColorManage);

    using (var destinationStream = await destinationFile.OpenAsync(FileAccessMode.ReadWrite))
    {
        BitmapEncoder encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, destinationStream);
        encoder.SetPixelData(BitmapPixelFormat.Rgba8, BitmapAlphaMode.Premultiplied, 80, 80, 96, 96, pixelData.DetachPixelData());                        
        await encoder.FlushAsync();
    }
}

Source