convert Eigen MatrixXd to QImage
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@Christian-Ehrlicher double. Eigen::MatrixXd mean a matrix of dynamic size and double type.
The problem(@jsulm mentioned) is wrong direction.
I print the value ofsrc_row[x], this is incorrect value.// the first pixel in the loop qDebug() << src_row[x]; // output: // 1.05908e-319Eigen uses expression templates and lazy-evaluation. You're taking a pointer to data contained in an rvalue here
src.row(y).data(). Anything can be there after the object goes out of scope. -
I want to convert matrixXd of Eigen to QImage(format, grayscale)。Here is my code, but the result is wrong. I am newer in QT and Eigen.
ObjWindow::ObjWindow(QWidget *parent) : QWidget(parent), ui(new Ui::ObjWindow) { ui->setupUi(this); int height = 100; int width = 100; Eigen::MatrixXd doubleM(height, width); for (int i = 0;i<height; i++) { for (int k = 0; k<width; k++) { if (i>=50) { doubleM(i, k) = 200; }; }; }; QImage xx = Eigen2QImage(doubleM); ui->label->setPixmap(QPixmap::fromImage(xx)); } QImage ObjWindow::Eigen2QImage(const MatrixXd &src) { double scale = 255.0; double max_value = src.maxCoeff(); double min_value = src.minCoeff(); QImage dest(src.cols(), src.rows(), QImage::Format_Grayscale8); for (int y = 0; y < src.rows(); ++y) { const double *src_row = src.row(y).data(); uchar * destrow = dest.scanLine(y); for (int x = 0; x < src.cols() ; ++x) { uchar gray = (uchar)(src_row[x] / max_value * 255); destrow[x] = gray; } } return dest; }@hei6775 said in convert Eigen MatrixXd to QImage:
const double *src_row = src.row(y).data();
I think you should change this to:
const auto srcRow = src.row(y); const double *src_row = srcRow.data();To not loose temporary container
It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)
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@hei6775 said in convert Eigen MatrixXd to QImage:
const double *src_row = src.row(y).data();
I think you should change this to:
const auto srcRow = src.row(y); const double *src_row = srcRow.data();To not loose temporary container
@KroMignon said in convert Eigen MatrixXd to QImage:
I think you should change this to:
Yes, in principle, but you must not use
autowithEigen.Read and abide by the Qt Code of Conduct
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@hei6775 said in convert Eigen MatrixXd to QImage:
const double *src_row = src.row(y).data();
I think you should change this to:
const auto srcRow = src.row(y); const double *src_row = srcRow.data();To not loose temporary container
@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
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@hei6775 said in convert Eigen MatrixXd to QImage:
This doesn't seem to work
Yes, in principle, but you must not use auto with Eigen.
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@KroMignon said in convert Eigen MatrixXd to QImage:
I think you should change this to:
Yes, in principle, but you must not use
autowithEigen.@kshegunov said in convert Eigen MatrixXd to QImage:
Yes, in principle, but you must not use auto with Eigen.
What is the problem with
autoandEigen?
Can you explain more, I don't knowEigenand always pleased to complete my knowledge about C++ :)It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)
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@kshegunov said in convert Eigen MatrixXd to QImage:
Yes, in principle, but you must not use auto with Eigen.
What is the problem with
autoandEigen?
Can you explain more, I don't knowEigenand always pleased to complete my knowledge about C++ :)@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 withauto, becauseautocaptures a type which isn't a real type, but rather an "evaluation step" type.Read and abide by the Qt Code of Conduct
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@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 withauto, becauseautocaptures a type which isn't a real type, but rather an "evaluation step" type.@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();It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)
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@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();@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);Read and abide by the Qt Code of Conduct
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@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);@kshegunov Thanks, for now I never used Eigen, but when I will have to use it I will try to remember this pitfall :)
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@kshegunov Thanks, for now I never used Eigen, but when I will have to use it I will try to remember this pitfall :)
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@KroMignon when i use
eval(). It work. Thanks@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 useautobut to specify right variable type like:const Eigen::VectorXd row = src.row(y); const double *src_row = srcRow.data(); -
@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 useautobut to specify right variable type like:const Eigen::VectorXd row = src.row(y); const double *src_row = srcRow.data();@KroMignon yes, you are right! Thanks.
