Compositing CGI with Bigger Node Trees - Nuke 101: Professional Compositing and Visual Effects - page 106

You can see in FIGURE 3.13 that the bounding box has changed dramatically. If
you move back and forth a couple of frames, you will see that the bounding box
changes to engulf only the areas where something is happening. Having a bound-
ingboxthatonlyengulfsthelemming,therebyreducingthesizeoftheimagethat
is being processed, dramatically speeds up processing.
Slapping Things Together: Foreground Over Background
To achieve the desired results, the lemming needs to be composited over a background.
Let's bring it in.
1. Using a Read node, read another sequence from disk: chapter03/bg/bg.####.png.
2. LoadRead2intotheViewerandclickthePlaybuttontoviewit.Whendone,stop
and go to frame 143 in the Timebar.
Thisisyourbackground( FIGURE 3.14 ) .Thecheekylemmingisgoingtoconnect
itself to the back of the car for a little surfing action.
Figure 3.14. The background image.