Re: [eigen] Rotations and unit vectors

Just to summarize. I think in general, we agree that we should try to cover at least the most basic cases in which the user can create invalid objects. So far we have two reasonable options

a) use asserts enabled ifndef DNDEBUG

b) use NaNs to signal a wrong usage

Personally I am still for option a), despite Benoit's reasoning. The main question remaining for both versions is how to choose a good threshold. Here, we also have two options on the table

a) Eigen's dummy precision (I think something like 10e-15 for double and 1-e-5 for float)

b) sqrt( numeric_limits<Scalar>::epsilon )

a) probably provides better support for user defined scalars and b) is less strict than a) and is more unlikely to raise asserts when they are not necessary

Regards,

Hauke

On Wed, May 8, 2013 at 1:42 PM, Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

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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