These are drivers for hardware devices. Although these drivers are at the lowest level for our code, typically they are not device drivers in the common sense. That is, they are not kernel modules which access the hardware directly; rather, they are userland C++ classes which use low-level kernel drivers and APIs to abstract a piece of hardware into a C++ object that is simple and efficient to use. For example, a V4Lgrabber uses the low-level Video4Linux API and whatever underlying kernel driver (e.g., bttv for bttv/connexant chips), to provide an easy way to build a C++ object that can access the framegrabber hardware and grab frames.
for dependency graphs: rankdir: RL
1.4.4