Re: [eigen] RotationBase times DiagonalMatrix

[Hauke Heibel <hauke.heibel@xxxxxxxxxxxxxx>]

Hi Benoit,

welcome back. :)

On Tue, Aug 9, 2011 at 7:25 AM, Benoit Jacob <jacob.benoit.1@xxxxxxxxx> wrote:
>> inline Transform<Scalar,Dim,AffineCompact> operator*(const
>> DiagonalMatrix<Scalar,Dim>& s) const
>> { return Transform<Scalar,Dim,AffineCompact>(*this) * s; }
>
> My problem with this solution is that the product of a rotation times
> a diagonal matrix is still a linear transformation, so why return a
> Transform which is specifically an affine (not linear) transformation?
> I'm in favor of using plain matrices everytime that a plain, arbitrary
> linear transformation is meant.

I see what you mean and when Affine transformations were allowed to be
implicitly constructed from any Dim x Dim matrix that were a perfect
solution.

> Ah OK, I see. It doesn't compile with a) because the assignment in the
> declaration is interpreted as construction, so it tries to use the
> constructor (taking EigenBase) instead of operator= and fails as it's
> an explicit constructor.

Exactly.

> That's really stupid :-/ C++ is able to convert "T a = b;" into "T
> a(b);" only to fail when the constructor here is explicit. But if one
> writes T a(b); or T a; a=b;  then the error goes away.

I think that behavior is perfectly fine. The question is why at all allow

T a; a=b;

while declaring the ctor explicit!? That's a little bit of a
contradiction. From what I understand the reason Gael implemented it
like this is to prevent the creation of hidden temporaries but I am
not sure anymore whether this preemptive optimization step is useful.
Is there at all a measurable performance penalty for these little
stack objects?

> I'd say that's a problem with C++ itself and I see only two approaches:
>  - either live with that and tell Eigen users to use A a(b) instead of
> A a  = b when the class A has an explicit constructor

The syntax becomes really ugly. My colleagues and me are working since
4-5 weeks extensively with the Geometry parts of Eigen and just one
example is writing and using functions that take Eigen::Transform<...>
as an input parameter. You cannot pass an Eigen::Translation, you
cannot pass Eigen::Scaling, all due to the explicit constructors. In
these cases, I really want would love to have the explicit conversion
capability which would make the code much more readable at the cost of
a few extra bytes wasted when converting an Eigen::Translation into an
Eigen::Transform.

>  - or stop making constructors 'explicit', consider that C++ language
> feature flawed: it doesn't work nicely with C++'s
> convert-assignment-to-construction rule

I still think explicit does exactly what it is meant to do. The only
question is whether we really want it in this case.

Regards,
Hauke