Covering Melee CornersPosted: 2012-05-02
Traditional PnP tactical cover rules have some gray areas when it comes to complex 3-dimensional topography and large creatures. Mostly these are swept under the generalizations from the “flatness” of tradition PnP play. However, the Ikosa Framework must calculate cover within predictable constraints and more highly variable conditions.
For instance, melee cover (which can only occur when attacking into an adjacent cell) generally requires all “corners” of the source cell to have unhindered line of effect to the target cell. Assuming an attacking creature stands on a block of solid ground 5 foot higher than an adjacent target creature (such that the target creature is “diagonally-down” from the attacker), then lines from the back of the attacker must be blocked by the solid ground to attack the target; in exactly the same way as when attacking around a corner. It is unlikely that a game-master would rule the lower target had cover from the attacker standing on a higher platform, especially since attacking from higher ground typically gives a +1 to attack.
The Ikosa Framework has strategies to deal with this situation (and some processing optimizations).
Optimization first: When calculating cover (melee or reach/ranged) Ikosa must check lines between “corners” of the source (cell or point) and target (cell or cells) for terrain blockage or cover inducing effects in the cells being traversed by the lines.
To reduce processing, Ikosa only examines points that will reach from source to target without going through the source or target (thus culling certain points from consideration). This can be determined by looking at the relative integer cell coordinates between source and target cells to determine which faces are exposed to each other. When two cells are face to face, 4 points from the source and 4 points from the target are considered. When edge to edge, 6 and 6, and when corner to corner, 7 points each. Also, for reach/ranged attacks, there are additional target points possible (since for larger creatures the entire creature is being targeted, which spans many cells), but also additional culling (so that only points exposed to the source point) are considered.
For the downward attack mentioned above, Ikosa detects the gravity direction of the attacker and whether the terrain supports the attacker (thus creatures in flight above the surface are excluded from this test) and then excludes those faces from consideration if they would otherwise be needed. As a result, only attacker points that face-outward toward the target on the non gravitationally bound attacker face are used. In this case, only 4 points are considered (if the target is adjacent and down) or 6 (if target is diagonal and down).
If the attacker were in a cell 5 feet lower, however, the gravitationally bound face isn’t facing the target anyway, and since the all the target’s points are still usable, the higher target would have cover.