Taking snapshots of WPF animation

I've been playing around with WPF animation, and ended up wanting to take a snapshot of an animation at a specific place.

Specifically, I have a canvas that has some pictures on it that are moving, and I want to be able to save a bitmap of what it looks like at a specific time.

Getting the bitmap is pretty easy, using a RenderTargetBitmap and a VisualBrush.

private BitmapSource CaptureScreen(Visual target)
{

    Rect bounds = VisualTreeHelper.GetDescendantBounds(target);

    RenderTargetBitmap renderBitmap = new RenderTargetBitmap(800, 600, 96, 96, PixelFormats.Pbgra32);

    DrawingVisual dv = new DrawingVisual();
    using (DrawingContext ctx = dv.RenderOpen())
{
        VisualBrush vb = new VisualBrush(target);

ctx.DrawRectangle(vb,

null, new Rect(new Point(), bounds.Size));
}
renderBitmap.Render(dv);

    return renderBitmap;
}

That gives you the state of the objects pre-animation, but what I really wanted was the state of the objects during the animation. I had some code to move the image around:

DoubleAnimation

anim = new DoubleAnimation(50, 400, TimeSpan.FromSeconds(10), FillBehavior.HoldEnd);
    AnimationClock clock = anim.CreateClock();
m_imageLeft.ApplyAnimationClock(Canvas.LeftProperty, clock);

After a bit of research, I settled on the following bit of code to set the animation to the point that I wanted:

clock.Controller.SeekAlignedToLastTick(

TimeSpan.FromSeconds(5.0), TimeSeekOrigin.BeginTime);

Which should have worked fine, but didn't. When I looked at the properties, I noticed that the Left property was changing, but it wasn't showing up. It was almost as if the Canvas didn't know that the object property had changed and a re-layout was necessary...

m_canvas.UpdateLayout();

was the magic incantation to make that happen.

(Note that I'm a novice at WPF, and there may be a better way to do this...)

Comments

• Anonymous
  February 27, 2007
  The comment has been removed
• Anonymous
  March 12, 2007
  Hi, I am trying to produce a screenshot of my WPF canvas in VB.NET. However when I use the above code, as well as another similar approach I have found, it produces a totally black image. The dimensions are correct etc but nothing is actually contained in the image but black pixels. Does anyone have any ideas what might be causing this? MW