The Bounding Box (BBox) of a layer, also referred to as the Region of Definiton (ROD) in some software, defines an area containing pixels that will be used to perform an operation. One of the first things a modifier does is it inspects the BBox of the layer on which it's applied in order to know which source it should work on.
The notion of a BBox can appear both in modifier and generator parameters, among others. It shouldn’t be confused with the notion of a Format. A BBox can evolve and be modified throughout processing.
One of the most obvious modifiers that will change the BBox is Erode/Dilate. In the following example, the layer silhouette is defined by an Alpha channel and we want to expand these contours to make this shape fatter. New pixels will be added around the silhouette to make it grow, which will have the effect of increasing the BBox of the layer as well.
We have the possibility to:
- Expand (default): increase the size of the BBox according to the newly created pixels
- Crop to format: define the BBox of the layer based on the format; that could expand or crop it
- Same as input: keep the BBox similar to what it was before the modifier
Other modifiers have the same parameter, such as Blur for example:
As you can see, the BBox that used to touch the edges of this image gradually moves away as the blur evolves. Autograph chooses to expand the BBox by default so the content won't be cropped. If we keep the same BBox as the input, this is the result:
This concept is important to understand because you'll find it in many places in the software, not only in modifiers. For example, when you check the Crop Transparent Borders option in the Reader, you potentially reduce the size of the BBox to only keep relevant pixels.