Generating tensors with elements based on position in xtensor - xtensor

Question

I am trying to build a data structure to represent an RGB image in xtensor (a 3D matrix, with shape in the form of (WIDTH, HEIGHT, 3).
Each "pixel" contains data collected by a function of the pixel coordinates. Basically, I want to replicate what this code does in python:
image = [[cell_info(x, y) for x in range(WIDTH)]
for y in range(HEIGHT)]
where cell info returns a 3 elements list representing the color channels.
I suppose the proper way to do this should be using an xgenerator, but to be honest I cannot understand how to use that class.

Answer 1

I found a solution:
I changed cell_info to accept a int channel parameter, so that it returns an integer instead of an array. Then I wrote this:
class img_generator_fn {
public:
using value_type = int;
img_generator_fn(const Map *map, const Position &center, shared_ptr<const Player> player,
const unsigned int field_radius)
: m_map(map), m_center(center), m_player(player), m_translation(-(field_radius + 1)) {}
~img_generator_fn() { m_map = nullptr; }
inline auto operator()(const unsigned int x, const unsigned int y, const unsigned int channel) const {
return m_map->at(m_center + Position(x, y))->cell_info(m_player, channel);
}
template <class It> inline auto element(It, It end) const {
return m_map->at(m_center + Position(*(end - 2) + m_translation, (*(end - 3)) + m_translation))
->cell_info(m_player, *(end - 1));
}
private:
const Map *m_map;
const Position &m_center;
shared_ptr<const Player> m_player;
const unsigned int m_translation;
};
template <unsigned int field_side> auto field(const Position &center, shared_ptr<const Player> player) const {
const array<unsigned int, 3> shape = {field_side, field_side, 3};
auto gen = xt::detail::make_xgenerator(img_generator_fn(this, center, player, (field_side - 1) / 2), shape);
return xt::xtensor_fixed<int, xt::xshape<field_side, field_side, 3>>(gen);
}
Here Map represents a 2D matrix, it is the structure which contains, togheter with player, the informations I want to store in an image. The function at picks up the map cell at the specified position (the map cell will be converted into a pixel). The function field generates an image centered around center from a given Map, using an xgenerator.