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Vertical autoaim[]
Since you can't look up or down in Doom or Doom II, the system compensates for you. Accuracy is not affected by the differences in the Z-axis co-ordinates of the player and target. However, vertical autoaim will only work against targets that are within a certain distance from the player; if the target is too far away, vertical autoaim will not take effect.
Horizontal autoaim[]
The player's projectile weapons have a limited ability to adjust their direction if the nearest target is not exactly straight ahead. From P_SpawnPlayerMissile in p_mobj.c:
// see which target is to be aimed at
an = source->angle;
slope = P_AimLineAttack (source, an, 16*64*FRACUNIT);
if (!linetarget)
{
an += 1<<26;
slope = P_AimLineAttack (source, an, 16*64*FRACUNIT);
if (!linetarget)
{
an -= 2<<26;
slope = P_AimLineAttack (source, an, 16*64*FRACUNIT);
}
if (!linetarget)
{
an = source->angle;
slope = 0;
}
}
An angle in the Doom engine is 32 bits, so that 1<<26 = 360° × (226 ÷ 232) ≈ 5.6°.