Question

Below is some code from a C++ procedure. The original can be found here;
https://code.ros.org/trac/opencv/browser/trunk/opencv/modules/imgproc/src/grabcut.cpp

My perspective is that I am a C# rather than C++ programmer so this code is a bit of a mystery to me even though I ran it through a code converter and it came out unchanged.

My questions are: a). How does ‘coefs’ become an array, and b). how does ‘c’ get populated?

To give a bit more information: ‘coefs’ does not seem to start out as an array and ‘c’ somehow seems to magically get at least 8 items

    // fields in class called MAT:
    // a distance between successive rows in bytes; includes the gap if any 
    size_t step;
    // pointer to the data
    uchar* data;

    // where ptr comes from:
    template<typename _Tp> _Tp* ptr(int y=0);
    template<typename _Tp> inline _Tp* Mat::ptr(int y)
    {
        CV_DbgAssert( (unsigned)y < (unsigned)rows );
        return (_Tp*)(data + step*y);
    }    
    .....

    // original question code:
    static const int componentsCount = 5;
    Mat model;
    float* coefs;
    float* mean;
    float* cov;

    coefs = model.ptr<float>(0);
    mean = coefs + componentsCount;
    cov = mean + 3*componentsCount;

    for( int ci = 0; ci < componentsCount; ci++ )
        if( coefs[ci] > 0 )
            calcInverseCovAndDeterm( ci );

    void GMM::calcInverseCovAndDeterm( int ci )
    {
        if( coefs[ci] > 0 )
        {
            float *c = cov + 9*ci;
            float dtrm =
                covDeterms[ci] = c[0]*(c[4]*c[8]-c[5]*c[7]) - c[1]*(c[3]*c[8]-c[5]*c[6]) + c[2]*(c[3]*c[7]-c[4]*c[6]);

Answer 1

When you declare an array like this:

int my_ints[10];

my_ints is an array. But in C++ my_ints can also be evaluated as a pointer to the first item in the array. So code like this:

int* the_first_int = my_ints;

…is legal and valid, not to mention very common. You can also use the_first_int as if it were also an array, like this:

int the_third_int = the_first_int[2];

…which makes it practically seamless to go between C-style arrays and pointers and back again.

In your code you declare a pointer to a float:

float* coefs;

(by the way, this should be initialized to NULLL, but that’s another story), and then you set it to some value:

coefs = model.ptr<float>(0);

I don’t know what model.ptr<float>(0) does, but it probably either allocates an array and returns a pointer to that, or returns a pointer to an array that was already set up.

Now because the coefs is a pointer to float, you can use it like it were an array, which you do in the loop:

for( int ci = 0; ci < componentsCount; ci++ )
    if( coefs[ci] > 0 )

That’s why this works.