How to create a writeablebitmap image from an image file in windows8

Question

I want to load an image file from local disk to a writeablebitmap image, so I can let users to edit it. when I create a WriteableBitmap object, the constructor need pixelwidth and pixelheight parameters, I don't know where to get these two, anyone can help?

Answer 1

If you want to have the correct pixel width and height, you need to load it into a BitmapImage first to populate it correctly:

StorageFile storageFile =
    await StorageFile.GetFileFromApplicationUriAsync("ms-appx:///myimage.png");
using (IRandomAccessStream fileStream = await storageFile.OpenAsync(FileAccessMode.Read))
{
    BitmapImage bitmapImage = new BitmapImage();
    await bitmapImage.SetSourceAsync(fileStream);

    WriteableBitmap writeableBitmap =
        new WriteableBitmap(bitmapImage.PixelWidth, bitmapImage.PixelHeight);
    fileStream.Seek(0);
    await writeableBitmap.SetSourceAsync(fileStream);
}

(For WinRT apps)

Answer 2

Don't care the pixelWidth and pixelHeight when define the WriteableBitmap image, please try this:

using (IRandomAccessStream stream = await file.OpenAsync(FileAccessMode.ReadWrite))
{
    WriteableBitmap image = new WriteableBitmap(1, 1);
    image.SetSource(stream);
    WriteableBitmapImage.Source = image;
}

Answer 3

Try the following code. There is an straightforward way to do this (load the image using BitmapImage and then pass this object directly to WriteableBitmap constructor, but I'm not sure if this works as expected or it has performance problem, don't remember):

BitmapSource bmp = BitmapFrame.Create(
    new Uri(@"C:\Users\Public\Pictures\Sample Pictures\Koala.jpg", UriKind.Relative),
    BitmapCreateOptions.None, BitmapCacheOption.OnLoad);

if (bmp.Format != PixelFormats.Bgra32)
    bmp = new FormatConvertedBitmap(bmp, PixelFormats.Bgra32, null, 1);
    // Just ignore the last parameter

WriteableBitmap wbmp = new WriteableBitmap(bmp.PixelWidth, bmp.PixelHeight,
    kbmp.DpiX, bmp.DpiY, bmp.Format, bmp.Palette);

Int32Rect r = new Int32Rect(0, 0, bmp.PixelWidth, bmp.PixelHeight);
wbmp.Lock();
bmp.CopyPixels(r, wbmp.BackBuffer, wbmp.BackBufferStride * wbmp.PixelHeight,
    wbmp.BackBufferStride);

wbmp.AddDirtyRect(r);
wbmp.Unlock();