Document boundary case(s)

[hlapp]

Document how boundary case(s) is/are handled. One boundary case specifically is if the expansion of the bounding box in some direction is not possible by the requested amount (2.5% in each direction by default, right?) because doing so would exceed the boundary of the image. What happens in such a case?

Also, when calculating the pixels to add in each direction, are fractions subject to floor or ceiling?