Simplified 'combat at different levels' for size

[Anthony]

The rules for Combat at Different Levels in DF are greatly simplified from vanilla GURPS, which I appreciate, but then don't address different SMs. Here's a rule that is, I think, simple enough (and not 100% compatible with existing rules, but close enough):
Combat at Different Levels
In any situation where two melee combatants are at different levels, the higher one gets a benefit (due to gravity and positioning).

If your target is above your head, you get a -2 to hit, or -4 if it's at least 45° above your head (this replaces the active defense bonus). For a humanoid, vitals height is roughly (SM-1), torso height is roughly (SM-2); quadrupeds will generally have head, vitals, and torso all at (SM-2). This does generally favor attacking people's legs if you're below them.

If an attack comes from above your head, you get a -1 to defend, or -2 if it's from at least 45° above your head. Assume a melee attack is launched from vitals height; thus, on flat ground this usually only applies with a 2 or more point SM difference.

Regardless of angle, you cannot attack a target you cannot reach. Use common sense here, most cases that are large enough for this to matter are also multi-hex creatures and you'll also have to consider the creature's shape.

[Rasputin]

I had started a blog post related to this, so I’ll cut and paste:

For foes of differing size, Kromm in “Combat Writ Large” (Pyramid #3/77, p. 4) says to use Combat at Different Levels (p. B402). For this, however, you need to translate everything first into feet. Furthermore, that table is assuming human height for one of the fighters. So, I’m trying to make this more generic to work for all sizes, and easier to remember in play.

Let’s assume a human (SM 0) is striking other humanoids of differing sizes, and rewrite the table. I’m only worrying about the human to sidestep arm length for the moment.

SM +2 (Hill giant) or bigger: The human can only strike at the giant’s Legs and Feet, with no special modifier (the Legs would be at -0 and Feet at -2 from normal math). The giant gets +3 to defend against all the human’s strikes.
SM +1 (Ogre): The human can try to strike anywhere on the ogre, but is at an extra -2 to hit the Head (after relative SM and normal Hit Location penalties, Face -6, Skull -8, Eyes -10). If he has a Reach 2 weapon, he can ignore the extra penalty to hit the Head. The human does get an extra +2 to hit the Legs and Feet (so, after relative SM and normal Hit Location penalties, he is +1 to hit the Legs and -2 to hit the Feet). The ogre is +1 to defend against all the human’s strikes.
SM -1 (Gnome): The human can try to hit the gnome anywhere, but is -2 to hit the Legs or Feet and +1 to hit the Head (thus, Legs -5, Feet -7, but same chance for the Head locations as against a human). The human may kick the gnome with no special bonus or penalty.
SM -2 (Halfling): The human can try to hit the halfling anywhere, but is -2 to hit the Legs or Feet and +1 to hit the Head (thus, Legs -6, Feet -8, Face -6, Skull -8, Eyes -10). With a Reach 2 weapon, the human may strike the Legs and Feet with no penalty. The human may kick the halfling with no special bonus or penalty. The halfling defends against the human’s strikes at -1.
SM -3 (Kobold): The human can try to hit the kobold anywhere but the Legs or Feet, and gets +1 to hit the Head (thus, Face -7, Skull -9, Eyes -11). With a Reach 2 weapon, the human may strike the Legs and Feet with no penalty. The human may kick the kobold with no special bonus or penalty. The kobold defends against the human’s strikes at -2.
SM -4 (Gremlin): The human can try to hit the gremlin anywhere but the Legs or Feet, and gets +1 to hit the Head (thus, Face -8, Skull -10, Eyes -12). With a Reach 2 weapon, the human may strike the Legs and Feet with no penalty. The human may kick the gremlin with no special bonus or penalty. The gremlin defends against the human’s strikes at -3.
SM -5 (Brownie): The human may only strike the brownie's Head, Neck, Arms, or Torso, with no special bonus. With a Reach 2 weapon, the human may strike the Legs and Feet with no penalty. The human may kick the brownie with no special bonus or penalty. The brownie defends against the human’s strikes at -3.

So, for the difference in effective size (lower SM by 1 for Horizontal, by 2 for Horizontal and No Legs), we ignore a difference of 1. For a difference of 2, the higher fighter gets +1 to defend, the lower gets -1. The pattern of SM difference minus one stops after an SM difference of 4, which seems fair since that’s the limit on the bonus to hit a bigger foe stops at 4. That’s for below SM 0.

It does get hairy if we look at trying to have parallel structure to this:

SM +2 (Hill giant): The hill giant may only strike the human's Head, Neck, Arms, or Torso, with no special bonus. With a Reach 3 weapon, the hill giant may strike the Legs and Feet at -2, or at no penalty with a Reach 4 weapon. The hill giant may kick the human with no special bonus or penalty. The human defends against the hill giant’s strikes at -3.
SM -2 (Halfling): The human can try to hit the halfling anywhere, but is -2 to hit the Legs or Feet and +1 to hit the Head (thus, Legs -6, Feet -8, Face -6, Skull -8, Eyes -10). With a Reach 2 weapon, the human may strike the Legs and Feet with no penalty. The human may kick the halfling with no special bonus or penalty. The halfling defends against the human’s strikes at -1.

This suggests a SM difference of 1 gets a defense bonus/penalty of 1, and 2 or more gets 3 when going up, but goes 0 and 1 going down. Looking at it this way does bring out the fact that arm length in GURPS is normalized at SM 0.

Personally, I like the algorithm of SM difference less 1, capped at 3, since it’s easy to remember and is usually advantageous to the PCs. In a simplified game like DFRPG, that’s the best. If you’re caring about Hit Location, you can use the version in Campaigns, or just give a +/-2 to the Head or Legs/Feet at any difference, and only hitting the Head/Torso or Legs/Feet at a defense bonus/penalty of +/-3 or more.

[Anthony]

Actually, I realize the simplest model is to not use SM in melee combat at all, figuring that the effects of superior angle and SM cancel out.