rotations live in planes — 6 in 4-D = n(n−1)/2:
x∧y · x∧z · y∧z · x∧t · y∧t · z∧t
DOUBLE = one angle applied to two orthogonal planes (a self-dual pair) — a motion with no 3-D counterpart.
4-D PERSP: eye at (0,0,0,D) on the t-axis, f = D/(D−t) — the near cube really foreshortens.
2:1 PAR: (x+t/2, y+t/2, z+t/2) — shears, but never foreshortens.
FLAT-87: plots (x,y) only, exactly as TESSERAC.BAS did — that's why its two cubes never distorted, the "further work" Lt. Christopher Biow USN (VF-41) flagged on 28 Feb 1987.