use faster unit quaternion rotation

[4ngry]

It seems that original ceres::UnitQuaternionRotatePoint is based on idea: convert to rotation matrix, then rotate. It seems quite inefficient, since obtained rotation matrix is not cached anywhere and had to be recomputed each time UnitQuaternionRotatePoint is called.

Updated code uses ideas from here:
https://fgiesen.wordpress.com/2019/02/09/rotating-a-single-vector-using-a-quaternion/
The same code also implemented in Eigen v.3.3.7 , under Eigen::Quaternion::_transformVector().

The code was tested under context of Structure-From-Motion tasks inside bundle adjustment cost functions with COLMAP code base, and behaves quite well.

I did not measure the execution time, but when by counting multiply-adds original ceres::UnitQuaternionRotatePoint has around 24(21 if not counting negations) multiplications and 18 additions, while proposed one has 15 multiplications and 15 additions.

[sandwichmaker]

@4ngry thank you for the patch. Two things.

1. We do not accept patches on github yet. Please follow the instructions at http://ceres-solver.org/contributing.html
2. Faster has to be backed up with a benchmark :)

[4ngry]

1. Followed instructions with Gerrrit
2. Will come with syntetic benchmark for unit quaternion rotation of vector soon.

[4ngry]

Benchmark code and visualization is here https://quick-bench.com/q/RgVitjKbGLZMszyG9ivMFzvo0CE
Dummy times used for random quaternion and vector generation should be subtracted from total time, to compare existent function with proposed one, but it is clearly that in case of double performance is around 2x better. For float not that much, but still valuable. I will post the benchmark code via Gerrit system also, as Samir suggested.

[4ngry]

Seems that merged now.

[sandwichmaker]

Yes I wanted to wait till after 2.0.

…

On Wed, Nov 18, 2020, 11:11 PM Mykyta Kozlov ***@***.***> wrote: Seems that merged now. — You are receiving this because you commented. Reply to this email directly, view it on GitHub <#625 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AANCABO6LIRN2PREJ7XZ6HTSQTAJ3ANCNFSM4RYA5EQA> .