@Raddi_On:
You can use for example request for json.
For example:

defmodule MyAppWeb.Controllers.ExampleController do
  def action_name(conn, %{param_name: param_value}) do
    matrix = QRCode.encode(param_value).matrix
    data = matrix |> Tuple.to_list() |> Enum.map(&Tuple.to_list/1)
    json(conn, data)
  end
end

You can also use probably Drab:

@Raddi_On: Just few steps:

0. Of course you need somehow send these data to JavaScript (let’s say in json format)
1. You need a simple function that draws qrcode - it’s just matrix of 0 (white) and 1 (black) squares. Drawing squares in canvas is one of the easiest things to do.
2. You need to use export as I already show

I will show you how it could look in Elixir:

defmodule Example do
  # @canvas ...

  def draw_image(matrix) do
    # firstly we need to have enumarable - not tuples, so:
    matrix
    |> Tuple.to_list()
    |> Enum.map(&Tuple.to_list/1)
    # then we need to have a row numbers like:
    |> Enum.with_index()
    # then for all rows:
    |> Enum.map(&draw_row/1)
  end

  defp draw_row({row, index}) do
    row
    # we need to know also column index:
    |> Enum.with_index()
    # then for all cells in row:
    |> Enum.map(&draw_cell(&1, index))
  end

  defp draw_cell({0, column}, row) do
    do_draw_cell(row, column, {255, 255, 255})
  end
  defp draw_cell({1, column}, row) do
    do_draw_cell(row, column, {0, 0, 0})
  end

  defp do_draw_cell(row, column, color) do
    # you have row, column and color to draw
    # simply calculate `x` and `y` like:
    square_size = 5
    x = column * square_size
    y = row * square_size
    # now you can call real draw function
    # with x, y, width (square_size), height (square_size) 
    # and finally fill it with color
    Canvas.draw(@canvas, x, y, square_size, square_size, color)
    # use canvas HTML5 API to export image
    # for example on download button click
  end
end

As you can see square coordinates are noting special. You just need to know in which row and column you are and simply multiply them by number of pixels that you wish to have for each cell.

@sbs: No need to implement png encoder from scratch. Just draw QR code in canvas and add simple option to export it.

For example you can draw a canvas and change it to image really easy in JavaScript:

var canvas = document.querySelector("canvas#selector.here");
var data = canvas.toDataURL("image/png");
document.write('<img src="' + data + '"/>');

It’s much more simpler and allows to simple export in much more formats without any library for this!