Compositing CGI with Bigger Node Trees - Nuke 101: Professional Compositing and Visual Effects - page 100

3. Delete LayerContactSheet1.
Using the Bounding Box to Speed Up Processing
The bounding box is an element that Nuke uses to define the area of the image for pro-
cessing. It is always there. Chapters 1 and 2 have bounding boxes all over them—you just
may not have noticed them. To understand the bounding box, let's first look at the image
properly.
1. Make sure Read1 (the lemming image) is loaded in Viewer1 by selecting it and
pressing 1 on the keyboard.
2. Look at Read1's alpha channel by pressing A while hovering the mouse pointer
overthe Viewer.Nowgoback toviewing the color channels bypressing Aagain.
Normally, as in most other compositing software, Nuke processes the whole im-
age. If you add a Blur to this image, every pixel is calculated. But that would be
wasteful, because blurring all that blackness in the lemming image won't change
that part of the image at all. There's a way to tell Nuke which part of the image to
process and which to disregard without changing the resolution. You do this with
a bounding box. The bounding box is the rectangular area that defines the area of
the image that should be processed. Pixels outside the bounding box should not
be processed and should remain black.