Re: [eigen] Rotations and unit vectors

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 07.05.2013 18:45, Dick Lyon wrote:
> How about this:  compute the norm of the unit vector, and compare it to 1.
>   If it's not very close, don't fail out, but normalize the vector.

I guess, normalizing every time would be more consistent then. I assume 
an inverse sqrt is more expensive than a comparison and a branch, but as 
both need to be done quite often, it could even be cheaper on average.

> Or wrap existing functions with new versions that check and correct norms
> of unit vectors, so the programmer can use one or the other; or have a mode
> flag that controls whether to normalize; you could even have a mode to fail
> out, but people who want a strong check.
>
> Maybe a CHECK_UNIT_VECTOR_NORM(vec, tolerance) that would compile to
> nothing, or to an assert, or to a re-normalization, depending on mode.

That would again be another flag which changes global behavior.
And, especially if you switch from auto-normalization to do-nothing, you 
can get very subtle bugs (both versions work, but the latter gets more 
and more inaccurate over time).

I would vote for just asserting accuracy (which will be disabled in 
NDEBUG mode), possibly with a customizable tolerance. Most importantly, 
a program which runs fine in debug mode should give the same results in 
release mode. And a program that fails in debug mode shall be allowed to 
give undefined results in release mode.

As for asserting or throwing:
It is possible to change eigen_assert to throw instead of assert and so 
far eigen_assert is the only way Eigen indicates non-recoverable errors 
(though mostly these are dimension mismatches etc)

Christoph





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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
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