拍摄WP7中使用矩形的快照

Im working on a WP7-app where I would like to record video and before the video is saved take a snapshot of the video so that it could be used as a thumbnail picture. The thumbnail picture is temporary save in the isolated storage before it is used. For the camera I use a rectangle to record video and later I want to display the picture on the phone. The problem is that the picture is displays only a black screen. Even when i tried to save the picture in the media library the picture is also there displayed as black. What can be the cause of this problem and how do i solve it?Im working on a WP7-app where I would like to r