Tiny solver is targeted towards small dense least square
solves, where the overhead of calling normal Ceres is too
high. For example, when solving for inverse camera
distortion for every pixel location in a many-megapixel
image. Anecdotally, at one point in the past, tiny solver
was ~20x faster than Ceres for the problems it's intended
for. This is due to two key aspects:
1. Memory is allocated up front: repeated solves incur no
allocation overhead beyond a few scalars on the stack.
2. The cost function is fully inlined into the solver
loop, removing even the cost function call overhead.
Tiny solver originated many years ago as part of
libmv/Blender, where it is still used for distortion solving
today, but the time has come for it to migrate into Ceres.
This commit is just the initial import into Ceres. Follow
up patches will add further cleanups, and add CostFunction
and Jet adapters to make it easier to call tiny solver
(though by using adapters, some performance advantages will
be lost).
Change-Id: I8079535cd41382b1e0ac0ca2fca141711c72b7f8
Keir Mierle