CODEWITHSUNDEEP
c++matlabfloating-pointprecision
Ander Biguri
Ander Biguri

Reputation: 35525

Why does Matlab gives different value than C++

I am creating a huge program in c++ for computer vision, and for debugging purposes I use Matlab. After time fighting with it I realised that some simple arithmetic equation's answer is different when using Matlab and c++ (Visual studio 9 compiler BTW). why is that?

Here is the arithmetic operation:

Matlab function:

function [x,y]=shape_fun(p,shape,Ax,Ay)

x=p(1)+Ax+shape(1)+Ax.*shape(3)+Ay.*shape(4)+Ax.*Ax.*shape(5)/2+Ay.*Ay.*shape(6)/2+Ax.*Ay.*shape(7);
y=p(2)+Ay+shape(2)+Ax.*shape(8)+Ay.*shape(9)+Ax.*Ax.*shape(10)/2+Ay.*Ay.*shape(11)/2+Ax.*Ay.*shape(12);

end

C++ function

cv::Point2d deformed(const double shape_fun[12],const cv::Point2d p,const double Ax,const double Ay){
    cv::Point2d result;

    result.x=(p.x+Ax)+shape_fun[0]+shape_fun[2]*Ax+shape_fun[3]*Ay+shape_fun[4]*Ax*Ax/2*shape_fun[5]*Ay*Ay/2+shape_fun[6]*Ax*Ay;
    result.y=(p.y+Ay)+shape_fun[1]+shape_fun[7]*Ax+shape_fun[8]*Ay+shape_fun[9]*Ax*Ax/2*shape_fun[10]*Ay*Ay/2+shape_fun[11]*Ax*Ay;
    return result;

}

Data:

Note: Matlab values are copy-pasted from debbuger c++ values, they are EXACTLY the same.

Ax=-12
Ay=-12
p=[468,683];
shape=[
     63.178114688537441
     36.536135487588474

    -0.038695673779030673
    -0.045313362559036965

     0.016469896824803026
     0.0017122284868442948
    -0.0030285669997117204

    -0.067902655024060773
     0.17995980761526389

     0.012716871878336870
    -0.036890386929202310
    -0.00081243692842574420
];

results:

Matlab: 

x =
     3.947029931219995e+02
y =
     7.043339656551383e+02

C++:

result  {x=393.54007007383439 y=703.64248713855773 }    cv::Point_<double>

Note: I am not tagging the question as OpenCV on purpose. While evidently I am using OpenCV in C++, cv::Point2d is just a struct with two double variables, x and y, and I don't think this question is about OpenCV at all.

Upvotes: 0

Views: 249

Answers (1)

john
john

Reputation: 8027

result.x=
    (p.x+Ax)+
    shape_fun[0]+
    shape_fun[2]*Ax+
    shape_fun[3]*Ay+
    shape_fun[4]*Ax*Ax/2*
    shape_fun[5]*Ay*Ay/2+
    shape_fun[6]*Ax*Ay;
result.y=
    (p.y+Ay)+
    shape_fun[1]+
    shape_fun[7]*Ax+
    shape_fun[8]*Ay+
    shape_fun[9]*Ax*Ax/2*
    shape_fun[10]*Ay*Ay/2+
    shape_fun[11]*Ax*Ay;

should be (presumably)

result.x=
    (p.x+Ax)+
    shape_fun[0]+
    shape_fun[2]*Ax+
    shape_fun[3]*Ay+
    shape_fun[4]*Ax*Ax/2+ /** change here **/
    shape_fun[5]*Ay*Ay/2+
    shape_fun[6]*Ax*Ay;
result.y=
    (p.y+Ay)+
    shape_fun[1]+
    shape_fun[7]*Ax+
    shape_fun[8]*Ay+
    shape_fun[9]*Ax*Ax/2+ /** change here **/
    shape_fun[10]*Ay*Ay/2+
    shape_fun[11]*Ax*Ay;

Upvotes: 10

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