Frenet to Cartesian Coordinates

[YashRunwal]

Hello,

I have some doubts in the FRT2Cart function in MATLAB.

    nNextRefPoint = 0;

    while ((i_fS > i_faRefRunLength(nNextRefPoint+1)) && (nNextRefPoint < length(i_faRefRunLength)))

        nNextRefPoint = nNextRefPoint + 1;
    end

    nPrevRefPoint = nNextRefPoint-1;

Can you please explain me what these variables mean? And what exactly is happening here?

[fjp]

Hi @YashRunwal and sorry for the late reply. I agree the variable names can seem strange but these follow some MISRA C style guide. For example nNexRefPoint means that it is a variable of type int and i_faRefRunLength denotes a function input (hence i_ ... and fa refers to a float array:

n: int
f: float
fa: array of floats (or float array)
strName: string
...

nNextRefPoint and nPrevRefPoint specify two consecutive indices of the reference path. nNextRefPoint starts at zero and is increased until the current value of the running length i_fS is reached, which is checked with the first condition of the while loop: (i_fS > i_faRefRunLength(nNextRefPoint+1)). The second condition makes sure that the index nNextRefPoint doesn't exceed the length or last index of the array i_faRefRunLength, which stores the running length values of the reference path. Hope this helps?

[fjp]

To give another interpretation in case you or someone else needs it: In this function we try to locate the given input running length i_fS (in frenet coordinates) on the reference path, which consists of sampled points and has itself a running length (obtained by i_faRefRunLength(nNextRefPoint+1)). To do this we use the two points nPrevRefPoint and nNextRefPoint to locate an interval on the reference path into which the input running length i_fS falls. Given this interval we can compute the heading of the vehicle and use i_fD to compute the desired Cartesian output coordiantes o_fX and o_fY.