How to create WriteableBitmap from BitmapImage? - c#-4.0

Question

I could create WriteableBitmap from pictures in Assets.
Uri imageUri1 = new Uri("ms-appx:///Assets/sample1.jpg");
WriteableBitmap writeableBmp = await new WriteableBitmap(1, 1).FromContent(imageUri1);
but, I can't create WriteableBitmap from Pictures Directory,(I'm using WinRT XAML Toolkit)
//open image
StorageFolder picturesFolder = KnownFolders.PicturesLibrary;
StorageFile file = await picturesFolder.GetFileAsync("sample2.jpg");
var stream = await file.OpenReadAsync();
//create bitmap
BitmapImage bitmap2 = new BitmapImage();
bitmap2.SetSource();
bitmap2.SetSource(stream);
//create WriteableBitmap, but cannot
WriteableBitmap writeableBmp3 =
await WriteableBitmapFromBitmapImageExtension.FromBitmapImage(bitmap2);
Is this correct ?

Answer 1

This is a total contrivance, but it does seem to work ...
// load a jpeg, be sure to have the Pictures Library capability in your manifest
var folder = KnownFolders.PicturesLibrary;
var file = await folder.GetFileAsync("test.jpg");
var data = await FileIO.ReadBufferAsync(file);
// create a stream from the file
var ms = new InMemoryRandomAccessStream();
var dw = new Windows.Storage.Streams.DataWriter(ms);
dw.WriteBuffer(data);
await dw.StoreAsync();
ms.Seek(0);
// find out how big the image is, don't need this if you already know
var bm = new BitmapImage();
await bm.SetSourceAsync(ms);
// create a writable bitmap of the right size
var wb = new WriteableBitmap(bm.PixelWidth, bm.PixelHeight);
ms.Seek(0);
// load the writable bitpamp from the stream
await wb.SetSourceAsync(ms);

Answer 2

Here's the way reading an image to a WriteableBitmap works as Filip noted:
StorageFile imageFile = ...
WriteableBitmap writeableBitmap = null;
using (IRandomAccessStream imageStream = await imageFile.OpenReadAsync())
{
BitmapDecoder bitmapDecoder = await BitmapDecoder.CreateAsync(
imageStream);
BitmapTransform dummyTransform = new BitmapTransform();
PixelDataProvider pixelDataProvider =
await bitmapDecoder.GetPixelDataAsync(BitmapPixelFormat.Bgra8,
BitmapAlphaMode.Premultiplied, dummyTransform,
ExifOrientationMode.RespectExifOrientation,
ColorManagementMode.ColorManageToSRgb);
byte[] pixelData = pixelDataProvider.DetachPixelData();
writeableBitmap = new WriteableBitmap(
(int)bitmapDecoder.OrientedPixelWidth,
(int)bitmapDecoder.OrientedPixelHeight);
using (Stream pixelStream = writeableBitmap.PixelBuffer.AsStream())
{
await pixelStream.WriteAsync(pixelData, 0, pixelData.Length);
}
}
Note that I am using the pixel format and alpha mode Writeable Bitmap uses and that I pass .

Answer 3

WriteableBitmapFromBitmapImageExtension.FromBitmapImage() works by using the original Uri used to load BitmapImage and IIRC it only works with BitmapImages from the appx. In your case there isn't even a Uri since loading from Pictures folder can only be done by loading from stream, so your options from the fastest to slowest (I think) are:
Open the image as WriteableBitmap from the get go, so that you don't need to reopen it or copy bits around.
If you need to have two copies - open it as WriteableBitmap and then create a new WriteableBitmap of same size and copy the pixel buffer.
If you need to have two copies - track the path used to open the first bitmap and then create a new WriteableBitmap by loading it from the same file as the original one.
I think option 2 might be faster than option 3 since you avoid decoding a compressed image twice.