Used by the routines at place_under_player, calc_2d_info and project_and_draw.

The isometric projection. The object's x, y and z (bytes 1 to 3) become a pixel x in byte $1A and a pixel y in byte $1B, counted up from the bottom of the screen:

pixel x = x + y - 128, and pixel y = (y - x + 128) / 2 + z - 104.

So a step in x moves one pixel right and half a pixel down, a step in y one pixel right and half a pixel up, and a step in z one pixel straight up -- the two-to-one slopes of the view. Bytes $12 and $13 are added on top: set by set_pixel_adj, they shift an object's picture without moving the object, for sprites whose artwork does not sit where the object stands.