Re: [eigen] RotationBase times DiagonalMatrix

[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
>>
>
>
>