2D Tracking - Nuke 101: Professional Compositing and Visual Effects - page 226

3. From Rectangle1's Output drop-down menu, choose Alpha. Make sure you are
viewing Rectangle1 in the Viewer.
To create a nice framing for the statue, use the on-screen controls to move the
rectangle until you are happy with how it looks.
4. Drag the rectangle or enter values in the Properties panel to frame the image. The
Area property will show values similar to: 500, 790, 1580, 2230 ( FIGURE 5.22 ).
Figure 5.22. The edges of Rectangle1's on-screen controls will end up being
the edge of your picture in the frame.
5. YounowneedtomultiplytheRGBwiththealphatocreateapremultipliedimage.
Attach a Premult node from the Merge toolbox after Rectangle1.
Becauseyou'renotgoingtousetheTrackernodetomovethepicturearound,you
will need another transformation tool. CornerPin is specifically designed for this