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copied!<p>So here is my solution I came with after lot of googling and trial/error coding:</p> <p>The goal here was to find out, how to manipulate images in WinRT, specifically in <a href="http://www.microsoft.com/en-us/download/details.aspx?displaylang=en&id=27411" rel="nofollow noreferrer">Background Tasks</a>. Background Tasks are even more limited than just regular WinRT projects, because they must be of type <a href="http://msdn.microsoft.com/en-us/library/windows/apps/hh441572.aspx" rel="nofollow noreferrer">Windows Runtime Component</a>. 99% of available libraries on NuGet targeting WinRT are targeting only the default WinRT projects, therefore they cannot be used in Windows Runtime Component projects. </p> <p>At first I tried to use the <a href="http://writeablebitmapex.codeplex.com/" rel="nofollow noreferrer">well-known WriteableBitmapEx library</a> - porting the necessary code to my winmd project. There is even <a href="http://writeablebitmapex.codeplex.com/SourceControl/changeset/view/94114" rel="nofollow noreferrer">branch of the WBE project targeting winmd</a>, but it is unfinished. I made it compile after adding [ReadOnlyArray], [WriteOnlyArray] attributes to method parameters of type array and also after changing the project namespace to something not starting with "Windows" - winmd project limitation.</p> <p>Even though I was able to use this library in my Background Task project it wasn't working, because, as I discovered, WriteableBitmap must be instantiated in UI thread and this is not possible as far as I know in Background Task.</p> <p>In the meantime I have also found this <a href="http://msdn.microsoft.com/fr-fr/library/windows/apps/windows.graphics.imaging.aspx" rel="nofollow noreferrer">MSDN article about Image manipulation in WinRT</a>. Most of samples there are only in the JavaScript section, so I had to convert it to C# first. I've also found this helpful <a href="https://stackoverflow.com/questions/10013799/winrt-image-handling">article on StackOverflow about image manipulation in WinRT</a>.</p> <pre class="lang-cs prettyprint-override"><code>internal static async Task LoadTileImageInternalAsync(string imagePath) { string tileName = imagePath.GetHashedTileName(); StorageFile origFile = await ApplicationData.Current.LocalFolder.GetFileAsync(imagePath); // open file for the new tile image file StorageFile tileFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(tileName, CreationCollisionOption.ReplaceExisting); using (IRandomAccessStream tileStream = await tileFile.OpenAsync(FileAccessMode.ReadWrite)) { // get width and height from the original image IRandomAccessStreamWithContentType stream = await origFile.OpenReadAsync(); ImageProperties properties = await origFile.Properties.GetImagePropertiesAsync(); uint width = properties.Width; uint height = properties.Height; // get proper decoder for the input file - jpg/png/gif BitmapDecoder decoder = await GetProperDecoder(stream, imagePath); if (decoder == null) return; // should not happen // get byte array of actual decoded image PixelDataProvider data = await decoder.GetPixelDataAsync(); byte[] bytes = data.DetachPixelData(); // create encoder for saving the tile image BitmapPropertySet propertySet = new BitmapPropertySet(); // create class representing target jpeg quality - a bit obscure, but it works BitmapTypedValue qualityValue = new BitmapTypedValue(TargetJpegQuality, PropertyType.Single); propertySet.Add("ImageQuality", qualityValue); // create the target jpeg decoder BitmapEncoder be = await BitmapEncoder.CreateAsync(BitmapEncoder.JpegEncoderId, tileStream, propertySet); be.SetPixelData(BitmapPixelFormat.Rgba8, BitmapAlphaMode.Straight, width, height, 96.0, 96.0, bytes); // crop the image, if it's too big if (width > MaxImageWidth || height > MaxImageHeight) { BitmapBounds bounds = new BitmapBounds(); if (width > MaxImageWidth) { bounds.Width = MaxImageWidth; bounds.X = (width - MaxImageWidth) / 2; } else bounds.Width = width; if (height > MaxImageHeight) { bounds.Height = MaxImageHeight; bounds.Y = (height - MaxImageHeight) / 2; } else bounds.Height = height; be.BitmapTransform.Bounds = bounds; } // save the target jpg to the file await be.FlushAsync(); } } private static async Task<BitmapDecoder> GetProperDecoder(IRandomAccessStreamWithContentType stream, string imagePath) { string ext = Path.GetExtension(imagePath); switch (ext) { case ".jpg": case ".jpeg": return await BitmapDecoder.CreateAsync(BitmapDecoder.JpegDecoderId, stream); case ".png": return await BitmapDecoder.CreateAsync(BitmapDecoder.PngDecoderId, stream); case ".gif": return await BitmapDecoder.CreateAsync(BitmapDecoder.GifDecoderId, stream); } return null; } </code></pre> <p>In this sample we open one file, decode it into byte array, and encode it back into new file with different size/format/quality.</p> <p>The result is fully working image manipulation even in Windows Runtime Component Class and without WriteableBitmapEx library.</p>

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