Cover.UpdateGeometryState C# (CSharp) Method

    public override void UpdateGeometryState(double atTime)
    {
        fRevealed = (int)Math.Floor((fDistance * atTime) + 0.5);

        switch (fDirection)
        {
            case DOWN:
                fSourceFrame = Rectangle.FromLTRB(Frame.Left, Frame.Top, Frame.Right, Revealed);
				fSourceBoundary = new Rectangle(0, SourcePixelBuffer.Height - Revealed, SourcePixelBuffer.Width, Revealed);
                break;
            case UP:
                fSourceFrame = Rectangle.FromLTRB(Frame.Left, Frame.Bottom - Revealed, Frame.Right, Frame.Bottom - Revealed);
                fSourceBoundary = new Rectangle(0, 0, SourcePixelBuffer.Width, Revealed);
                break;
            case LEFT:
                fSourceFrame = Rectangle.FromLTRB(Frame.Right - Revealed, Frame.Top, Frame.Right, Frame.Bottom);
                fSourceBoundary = new Rectangle(0, 0, Revealed, SourcePixelBuffer.Height);
               break;
            case RIGHT:
                fSourceFrame = Rectangle.FromLTRB(Frame.Left, Frame.Top, Frame.Left + Revealed, Frame.Bottom);
                fSourceBoundary = new Rectangle(SourcePixelBuffer.Width - Revealed, 0, Revealed, SourcePixelBuffer.Height);
                break;
        }


    }