Bounding Box and Black Outside
The bounding box
The bounding box is a region that defines the limits of what is being processed by Nuke. If this is set too large for what needs to be processed, then Nuke wastes a lot of processing power. This is particularly so in filtering operations such as blur and defocus. Certainly, it is rare that anything needs to be processed that lays outside the limits of the Format.
They most important node in the management of the bounding box is the Merge node. If two images are laid over each other, and one is larger than the other, then by default Nuke will assume that the bounding box should be large enough to accommodate them both. In the image below, a green rectangle has been made very large with a Transform node. In the viewer, the bounding box is the dotted line that extends beyond the format.
If the 'set bbox to' parameter is changed to 'B' then the bounding box shrinks to the size of the format.
Other bounding box related nodes are:
AdjBBox: this will increase or decrease the size of the bounding box.
CopyBBox: this will copy the bounding box from one input to another.
Crop: Similar to AdjBBox, this adjusts the size of the bounding box. Howvere, it offers more controls than AdjBBox, allowing arbitrary repositioning of the edges. using it, the format can also be changed.
A clear video explaining the bounding box can be found here.
Occasionally in your Nuke travels you will find yourself looking at weird streaky pixels in the Viewer (the Nuke manual calls them stripes). These typically appear after using a Reformat or Transform node. Example below:
The black outside parameter also effects the alpha, so it can impact upon your merging operations. There is, by the way, a dedicated BlackOutside node that forces everything outside the bounding box to be black.