Re: [eigen] RotationBase times DiagonalMatrix

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To be clear... if *anyone* can beat me to it, you're more than
welcome. I've been away from Eigen development for 6 months now :-(


2011/9/22 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> Oops, sorry I forgot again :-( I'm at a Khronos meeting this week, but
> I will try to review it asap... unless Gael beats me to it.
> Benoit
> 2011/9/22 Hauke Heibel <hauke.heibel@xxxxxxxxxxxxxx>:
>> Sorry guys,
>> but I wanted to bump this thread since it seems to fall asleep and it
>> would be great to get some feedback.
>> If you are too busy to make such reviews I will simply start creating
>> a fork, from which you then may pull ...
>> I am even fine, if we come to a negative conclusion regarding my
>> proposals as long as we manage to get to some decision.
>> Regards,
>> Hauke
>> On Tue, Aug 9, 2011 at 8:48 AM, Hauke Heibel
>> <hauke.heibel@xxxxxxxxxxxxxx> wrote:
>>> 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

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