//current way to setup the options

std::map<std::string, size_t> options; //public data member

options["num_images"] = 2;
options["row_size"] = 321;
options["col_size"] = 481;
options["rgbd"] = 0;
options["shrink"] = 2;

//if we change it to member function, we could
//1 : set std::map<std::string, size_t> options as private data member or store each option as a data member, your choice.
//I do not think these options will be the performance bottleneck.I prefer map in this case, because we can reduce the codes
//the codes we need to change yet make the api easier to use
//2 : let user read, set the options by member function

void NumImages(size_t value){ options["num_images"] = value; };
void RowSize(size_t value){ options["row_size"] = value; };
void ColSize(size_t value){ options["col_size"] = value; };

//or
void NumImages(size_t value){ numImages = value; };
void RowSize(size_t value){ rowSize = value; };
void ColSize(size_t value){ colSize = value; };