I have created a function that will take as a parameter a StorageFile and return a WriteableBitmap. I use this data type so that I can later encode the image into a base64 string.

My concern is with the intermediary BitmapImage that I create to get the original dimensions. Any other improvements would be welcomed as well.

Also, it should be noted that this is Windows 10 Universal code, so I am unable to use the System.Drawing libraries.

public static async Task<WriteableBitmap> ScaledWImage(StorageFile file, int maxWidth, int maxHeight)
    WriteableBitmap newImage;

    using (var fileStream = await file.OpenReadAsync())
        var bitmap = new BitmapImage();
        var origHeight = bitmap.PixelHeight;
        var origWidth = bitmap.PixelWidth;
        var ratioX = maxWidth / (float)origWidth;
        var ratioY = maxHeight / (float)origHeight;
        var ratio = Math.Min(ratioX, ratioY);
        var newHeight = (int)(origHeight * ratio);
        var newWidth = (int)(origWidth * ratio);

        if (ratio > 1)
            newHeight = origHeight;
            newWidth = origWidth;

        newImage = new WriteableBitmap(newWidth, newHeight);
        var decoder = await BitmapDecoder.CreateAsync(fileStream);

        // Scale image to appropriate size 
        var transform = new BitmapTransform()
            ScaledWidth = Convert.ToUInt32(newImage.PixelWidth),
            ScaledHeight = Convert.ToUInt32(newImage.PixelHeight)
        var pixelData = await decoder.GetPixelDataAsync(
            BitmapPixelFormat.Bgra8, // WriteableBitmap uses BGRA format 
            ExifOrientationMode.IgnoreExifOrientation, // This sample ignores Exif orientation 

        // An array containing the decoded image data, which could be modified before being displayed 
        var sourcePixels = pixelData.DetachPixelData();

        // Open a stream to copy the image contents to the WriteableBitmap's pixel buffer 
        using (var stream = newImage.PixelBuffer.AsStream())
            await stream.WriteAsync(sourcePixels, 0, sourcePixels.Length);

    return newImage;

This code is adapted from a Microsoft sample found here.


1 Answer 1


If you're still looking for a way how to find out the dimensions of a StorageFile image, there is an easy way to do that. I am using it in uwp as well.

I guess it would be less expensive than creation of a new BitmapImage.

        StorageFile image;
        ImageProperties properties = await image.Properties.GetImagePropertiesAsync();
        int height = (int)properties.Height;
        int width = (int)properties.Width;

As well I have this one method which I am using couple of places:

        private async Task<WriteableBitmap> StorageFileToWbmp(StorageFile file)
        ImageProperties props = await file.Properties.GetImagePropertiesAsync();
        WriteableBitmap bitmap = new WriteableBitmap((int) props.Width, (int) props.Height);
        await bitmap.SetSourceAsync(await file.OpenReadAsync());
        return bitmap;

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