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Frame-based Bitmap Animations in Silverlight

Despite the fact that I have woefully neglected my incipient series on developing a WPF application with TDD (aka ChumChase), like many others this week I've been taken by the Silverlight Lust.

I decided to make a simple game, and settled on an Asteroids clone. I drew some inspiration and insight from the blogs of Bill Reiss, Andy Beaulieu, and Robby Ingebretsen.

I started off with a basic triangular ship, drawn using this:

<UserControl x:Class="BlueSpire.Roids.Symbols.StarShip"
    <Canvas x:Name="LayoutRoot" 
            Width="50" Height="105">
        <Path x:Name="Ship"
              Data="M50,0 L25,105 L50,80 L75,105 z"/>

a very simple vector ship I was more interested in gameplay, than graphics. :-)

However, as my imagination began to flame, I had a idea that required animated bitmap graphics.

Animations that use bitmaps are essentially little flip-books, where you display a sequence of bitmaps in rapid succession providing the illusion of animation. There are lots of ways to achieve this technically. I found a StackOverflow question asking about the best approach. The aforementioned Bill Reiss provided an answer and I decided to implement it.

Sprite Sheet

First you create a single image that contains all of frames. For my proof of concept, I decided to make a little engine glow over the course of 8 frames. The results was the 512x64 image.

an example sprite sheet

The idea is that we display just a portion of the image, in this case 64x64 square portion. The actual UIElement that will represent the ship on screen is a Rectangle. We'll make it 64x64 in size, and set it's Fill property with an ImageBrush pointing to my sprite sheet. Then I animated a TranslateTransform on the ImageBrush to display the each frame in sequence. Well, code speaks louder than words, so:

private void SpriteSheetTest()
    const int sprite_size = 64;    //the frame is 64x64
    const int num_of_frames = 8;
    var ship_element = new Rectangle();
    ship_element.Width = ship_element.Height = sprite_size;

    var ship_sprite_sheet = new ImageBrush
                                    Stretch = Stretch.None,
                                    AlignmentX = AlignmentX.Left,
                                    AlignmentY = AlignmentY.Top

    // If the brush alignment is not set, the image will be
    // centered and translation transform won't be right. 

    var sprite_sheet_position = new TranslateTransform();
    ship_sprite_sheet.Transform = sprite_sheet_position;
    ship_sprite_sheet.ImageSource = ResourceHelper.GetBitmap("Resources/Roid.png");

    ship_element.Fill = ship_sprite_sheet;

    // I'm dynamically creating the frames, displaying a 
    // new frame every 80 ms.
    var sprite_anim = new DoubleAnimationUsingKeyFrames();
    for (int i = 0; i < num_of_frames; i++)
        var frame_span = new TimeSpan(0, 0, 0, 0, i * 80);
        sprite_anim.KeyFrames.Add(new DiscreteDoubleKeyFrame
                                 Value = (-sprite_size*i),
                                 KeyTime = KeyTime.FromTimeSpan(frame_span)

    // finally, we need a storyboard to tie the animation
    // to the transform
    var sb = new Storyboard {RepeatBehavior = RepeatBehavior.Forever};
    Storyboard.SetTarget(sprite_anim, sprite_sheet_position);
                     new PropertyPath(TranslateTransform.XProperty));

    //Stage is a Canvas that represent the main area of the game

The ResourceHelper class is a handy bit for working with embedded resources, that I picked up here. Hint, embedded resources have a Build Action of Resource and not Embedded Resource. I wasted 15 or 20 minutes on that one...

Posted 10-17-2008 11:39 AM by Christopher Bennage


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