Achieving wipe effect in video with (motion tracked) mask revealing 3D object An object in a video clip should be tracked to move a mask across the screen, revealing a 3D object (extruded text). Imagine a shot of a sheet of paper on a desktop. The paper is pulled out of the view sideways, revealing the rendered object which appears to be on top of the desk.

So I imagine I need to composite the video in the background, and an animated mask on top of the text, so that the visibility of the text (= opacity of the mask) is zero where the paper is in the video, hiding it.

What I have: a video clip in the Timeline, an extruded text in the Scene, and two nodes "Render Layers -> Composite" in the Compositing Editor.
When Rendering, the result shows only the video, but not the object.

I'd be happy with a short outline of the easiest steps I need to take to achieve what I want. But a few pointers to the tools I need to use or a rephrasing of what I am trying to do in the proper terminology would help in searching the information on my own.
Motion tracking is not really necessary since it is a short clip and I'd gladly learn how to animate the mask manually.

I am using Blender 2.78a on Windows.
If I'm guessing right, you're moving a sheet of paper in a previously taken video to uncover some 3D (extruded) text that's sitting in Blender's 3D window.  

I've done it here with a single animated masking plane.

 

The first example is the (pre-recorded) video I manufactured for the purpose.

The second is the end result when using the first as the bkd video and overlaying it's paper sheet with a matching sized plane. (in Blender's 3D work area)  The plane is textured with a screenshot of the video's paper sheet.

The text is there all the time.  It's the copycat paper (plane) that's hiding it from view.

The plane moves in unison with the video's paper using keyframes and shape keys to match it's changing dimensions as the frames progress.

It is in fact manual tracking and is not difficult in many circumstances.

Once the text is in view, the masking plane is dispensed with and the video can proceed on it's own.

This is but one solution.  Another might be to use Blender's tracking and inbuilt masking.