An indexed-colour picture drawn a column, a plane or a line at a time — the drawing a sector view does — and turned into RGBA through a palette at the end.

frame =
  Frame.new(320, 200)
  |> Frame.fill(0)
  |> Frame.columns(pictures, [Frame.column(x, {top, bottom}, wall, {texel_x, v, step}, light)])
  |> Frame.plane(pictures, plane, tops, bottoms)

Frame.to_raster(frame, palette)

Every drawing call takes a frame and gives one back, and the frame given back is the one to keep drawing on. Where the NIF loaded the frame is drawn on in place, so a frame must be drawn on by one process at a time, and an earlier value of it is the same frame.

Columns

column/6 describes one vertical run of pixels at screen column x from row top to row bottom, filled from the picture at {texel_x, v, step}: from picture column texel_x, row top reading the picture at row v and each row below step further on. The texel goes through light table light. A plain column wraps both its picture column and its rows, as a wall texture repeats. With masked: true neither wraps — rows outside the picture and transparent texels are skipped — which is how a sprite is drawn. With shimmer: true the column draws no texels: each pixel takes the colour of a pixel a row above or below it already on the frame, through light, for a figure seen as a blur.

Planes

plane/5 fills, column by column, the rows between tops and bottoms with a horizontal plane seen in perspective: a floor or a ceiling at height above the eye (negative below it), textured with its picture tiled across the world one texel per unit, its world x along the picture's columns and its world -y along the rows. Each pixel's light table is start - floor(fade / distance), clamped to the tables, or fixed where that is not -1. With sky: true the plane is not perspective at all: the picture is wrapped around the view, sky_columns picture columns per radian of heading, its rows from sky_mid at the horizon down by sky_step a pixel, drawn through fixed's table or table 0.

The view a plane is drawn for is x, y and angle (radians, 0 along +x, the world's +y up and to the left of the screen) with the eye at screen center_x, center_y, and focal_x, focal_y the distances in pixels at which one world unit spans one pixel across and down.

available?/0 says whether the NIF loaded; where it did not, every call draws the same picture in Elixir, many times slower.