Rendering XAML to a JPEG using Silverlight 3

Something that had been relatively easily do in WPF that was near impossible in silverlight was to easily take the XAML that had been rendered on the screen and allow the user to save it as an image file.

With the release of version 3 of silverlight this is now possible through the use of the WriteableBitmap class. Once the XAML has been rendered to the to the writablebitmap it can then be written to a Bitmap file or using a third party code encoded as a JPEG or PNG as shown below. This can have a range of applications form allowing users create avitars through to saving an image of a silverlight game. Once you have the image it can then be saved to isolated storage, a web server via a web service or to the users own machine.

Install Microsoft Silverlight

In the sample I have used the FJcore library to encode it as a JPEG to keep the file small, however it is just as easy to use the Joe Stegman’s PNG encoder.

The first step is to write your XAML to the writeable bitmap which can easily be done in one line through the constructor. To render the image identical to the screen we pass null into the transform argument.

WriteableBitmap bitmap = new WriteableBitmap(sourceElement, null);

Using a type of panel such as a canvas or grid will allow you can render multiple elements together in one image. One thing to note is that it ignores the Background property of the pannel placing all elements on default bitmap black background. To get around this you can first drawn a white rectangle as a background to your image.

The easiest way to save the image and get the best mix of file size and quality is to use the FjCore Library  which is design to be a simple lightweight JPEG encoder/decoder for use with silverlight and distributed under the MIT License licence.

The fjcore encode method expect a byte array of the image so the only additional method in the project converts the WriteableBitmap (using a variation on code provided by RHLopez in a stack overflow question) and adds it to a file stream which in the case of this sample is created from the save dialog class so is inturn saved to the users machine.

private static void SaveToFile(WriteableBitmap bitmap,Stream fs)
            //Convert the Image to pass into FJCore
            int width = bitmap.PixelWidth;
            int height = bitmap.PixelHeight;
            int bands = 3;

            byte[][,] raster = new byte[bands][,]; 

            for (int i = 0; i < bands; i++)
                raster[i] = new byte[width, height];

            for (int row = 0; row < height; row++)
                for (int column = 0; column < width; column++)
                    int pixel = bitmap.Pixels[width * row + column];
                    raster[0][column, row] = (byte)(pixel >> 16);
                    raster[1][column, row] = (byte)(pixel >> 8);
                    raster[2][column, row] = (byte)pixel;

            ColorModel model = new ColorModel {colorspace = ColorSpace.RGB };

            FluxJpeg.Core.Image img = new FluxJpeg.Core.Image(model, raster);

            //Encode the Image as a JPEG
            MemoryStream stream = new MemoryStream();
            FluxJpeg.Core.Encoder.JpegEncoder encoder = new FluxJpeg.Core.Encoder.JpegEncoder(img, 100, stream);


            //Move back to the start of the stream
            stream.Seek(0, SeekOrigin.Begin);
            //Get the Bytes and write them to the stream
            byte[] binaryData = new Byte[stream.Length];
            long bytesRead = stream.Read(binaryData, 0, (int)stream.Length);

            fs.Write(binaryData, 0, binaryData.Length ); 


The full code sample can be downloaded here

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Tuesday, July 21st, 2009 General 17 Comments

The use of PART_ in Control Templates

One of the most useful features of WPF is the ability to completely redesign the look of a control such a button, progress bar or slider bar relatively easily through the use of control templates. Meaning a progress bar can become round, a slider bar 3D or some other shape.

With the more complex controls (such as the progress bar) you can also keep the functionality of the original control and save having to re-write the base logic. This is done by specifying that elements in your design are the same as part of the original control. The link is created through simply naming your elements by set names usually prefixed with PART_.

In the example below we have a ControlTemplate for a ProgressBar. In order for it to keep it’s function as a progress bar the I have named the elements “PART_Track” and “PART_Indicator” to link to the two parts of original functionality. In the case of a progress bar without this when the value property is set on the progresses bar it would not be reflected in the redesigned control.

        <ControlTemplate x:Key="DarkBar" TargetType="{x:Type ProgressBar}">
                <Border Name="PART_Track" CornerRadius="10" BorderThickness="2" BorderBrush="DarkGray"/>
                <Border Name="PART_Indicator" CornerRadius="10" Background="Gray" HorizontalAlignment="Left" />

For each control type the names that you use to specify the link are different, although they are all prefixed with the word PART_ . These is no full list online however as these are specified as TemplatePartAttributeattributes on the controls it means that they are relatively easy to discover.

Using reflection (code sample below) it is possible to get a full list.

Dim extAssembly As System.Reflection.Assembly = System.Reflection.Assembly.LoadFile("C:Program FilesReference AssembliesMicrosoftFrameworkv3.0PresentationFramework.dll")
For Each t As System.Type In extAssembly.GetTypes
For Each att As System.Windows.TemplatePartAttribute In t.GetCustomAttributes(GetType(System.Windows.TemplatePartAttribute), True)
Console.WriteLine(t.ToString & " : " & att.Name.ToString & " Type of " & att.Type.ToString)


Alternatively and often more practical is to use Red Gates Reflector tool. With this free application you can search through and look at the source code of the controls. The part will be specified at the top along with the other attributes as shown below.

Reflector Progress Bar Attribute Example

This has been a quick overview and further reading is often need to discover just how to use the parts as they differ per control however hopefully you can start to see how useful they are when redesigning controls.

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Sunday, February 8th, 2009 General 4 Comments