convert Eigen MatrixXd to QImage

[kshegunov]

@KroMignon said in convert Eigen MatrixXd to QImage:

I think you should change this to:

Yes, in principle, but you must not use auto with Eigen.

[hei6775]

@KroMignon said in convert Eigen MatrixXd to QImage:

const auto srcRow = src.row(y);
const double *src_row = srcRow.data();

This doesn't seem to work

[Christian Ehrlicher]

@hei6775 said in convert Eigen MatrixXd to QImage:

This doesn't seem to work

@kshegunov

Yes, in principle, but you must not use auto with Eigen.

[KroMignon]

@kshegunov said in convert Eigen MatrixXd to QImage:

Yes, in principle, but you must not use auto with Eigen.

What is the problem with auto and Eigen?
Can you explain more, I don't know Eigen and always pleased to complete my knowledge about C++ :)

[kshegunov]

@KroMignon said in convert Eigen MatrixXd to QImage:

What is the problem with auto and Eigen?

I'm going to spare you my personal opinion on auto, but this is it:

@kshegunov said in convert Eigen MatrixXd to QImage:

Eigen uses expression templates and lazy-evaluation.

And here's a reference if you want to read even more: https://eigen.tuxfamily.org/dox/TopicPitfalls.html
In a nutshell Eigen builds up a sort of a parse tree from its methods and delays evaluation to the last possible moment. It has the advantage that it's really efficient to pack up complex expressions in a single pass, but doesn't play well with auto, because auto captures a type which isn't a real type, but rather an "evaluation step" type.

[KroMignon]

@kshegunov said in convert Eigen MatrixXd to QImage:

In a nutshell Eigen builds up a sort of a parse tree from its methods and delays evaluation to the last possible moment. It has the advantage that it's really efficient to pack up complex expressions in a single pass, but doesn't play well with auto, because auto captures a type which isn't a real type, or rather a "evaluation step" type.

Thanks for the link, so if I understand it right, there should be a call to eval() to ensure getting right type:
So this should then work?

const auto srcRow = src.row(y).eval();
const double *src_row = srcRow.data();

[kshegunov]

@KroMignon said in convert Eigen MatrixXd to QImage:

Thanks for the link, so if I understand it right, there should be a call to eval() to ensure getting right type:

Not really. Instead of bending backwards to enforce compile-time type deduction, you just declare the type explicitly:

Eigen::VectorXd row = src.row(y);

[KroMignon]

@kshegunov Thanks, for now I never used Eigen, but when I will have to use it I will try to remember this pitfall :)

[hei6775]

@KroMignon when i use eval(). It work. Thanks

[KroMignon]

@hei6775 said in convert Eigen MatrixXd to QImage:

when i use eval(). It work. Thanks

Yes it works, but as @kshegunov has written and as you can read it in documentation (https://eigen.tuxfamily.org/dox/TopicPitfalls.html), this not be the best way to do with Eigen.
It would be better to not use auto but to specify right variable type like:

const Eigen::VectorXd row = src.row(y);
const double *src_row = srcRow.data();

[hei6775]

@KroMignon yes, you are right! Thanks.