# Accessing current tensor value indexes in defn

Hello,

I’ve implemented a very basic “ray tracing” algorithm in Elixir, and given that it involves float arithmetic over large 2-dimensional data I thought that I’d try to port it to Nx to give it a performance boost. However being new to this kind of things I’m not sure that it’s actually feasible.

The algorithm is the following: I have a 2d height map, which is a two dimensional list with float values (for example `[[2.0, 7.2], [1.3, 2.4]`). I also have a ray tracing direction vector `{dx, dy, dz}`. For every point in my height map, I’ll trace a ray starting from that point, moving along this vector until I’m out of the map, or the ray bumps into an obstacle (meaning it’s height `z` is below the value of the height map at this point).

The elixir implementation looks like this:

``````for {row, i} <- Enum.with_index(height_map) do
for {z, j} <- Enum.with_index(row) do
is_in_the_shade?(to_map(height_map), i, j, z, sun_x, sun_y, sun_z)
end
end

def is_in_the_shade?(height_map, x, y, z, sun_x, sun_y, sun_z) do
case Map.fetch(height_map, {trunc(x), trunc(y)}) do
:error -> # we've reached the end of the height_map without meeting any obstacle
0
{:ok, height} when height > z -> # we've met an obstacle
1
{:ok, height } -> # continue ray tracing along sun_v
is_in_the_shade?(height_map, x + sun_x, y + sun_y, z + sun_z, sun_x, sun_y, sun_z)
end
end
``````

I’ve tried to port it to Nx like this:

``````
shade(hm, sun_v: {sun_x, sun_y, sun_z}, height_map: hm)

defn shade({x, y, z}, opts \\ []) do
{sun_x, sun_y, sun_z} = keyword!(opts, [:sun_v])
height_map = keyword!(opts, [:height_map])
while {res = 0, height_map, x, y, z, sun_x, sun_y, sun_z},
res == 0 and x <= 0 and x > 256 and y <= 0 and y > 256 do
if height_map[x][y] > z do
{ 1, height_map, x, y, z, sun_x, sun_y, sun_z }
else
{ 0, height_map, x + sun_x, y + sun_y, z + sun_z }
end
end
end
``````

The problem is that I can’t figure out how to get access to `x` and `y` here, I’ve tried to define my tensor as a list of tuples, like `[{0, 0, 2.0}, {0, 1, 3.2}, {1, 0, 3.1}, {1, 1, 3.0}]` but that doesn’t seem possible (I’m getting the error `invalid value given to Nx.tensor/1, got: {0, 0, 38.75}`, which makes sense tensors values need to be ints or floats).

Is there a way to make this work with Nx, or should I be looking at a C port to improve performance?