POLYTYPE_ATEX man page
POLYTYPE_ATEX — Polygon rendering mode type Allegro game programming library.
An affine texture mapped polygon. This stretches the texture across the polygon with a simple 2d linear interpolation, which is fast but not mathematically correct. It can look OK if the polygon is fairly small or flat-on to the camera, but because it doesn't deal with perspective foreshortening, it can produce strange warping artifacts. To see what this means, run Allegro's test program and see what happens to the polygon3d() test when you zoom in very close to the cube.
ex3d(3), POLYTYPE_ATEX_LIT(3), POLYTYPE_ATEX_MASK(3), POLYTYPE_PTEX(3).