Blast damage

Explosion blast damage. Here, B and C take the same damage because they are at the same x distance, and the x distance is larger than y for both. Both take 128-80=48 units of damage. A and D take no damage since the line of sight is obstructed. (In this illustration, one pixel corresponds to one map unit.)

Blast damage attacks, also referred to as splash damage, inflict radius damage in the area surrounding their point of explosion, in addition to any damage inflicted by the direct hit from the source actor (such as a rocket). The original Doom engine uses the same code for all blasts, and even the same sound effect. In Doom II, the blast radius is either 70 or 128 map units, and any damageable object that is in that radius and exposed to the explosion will lose hit points proportional to the object's distance to the explosion. As special cases, the cyberdemon and spiderdemon do not take blast damage. Other games based on the Doom engine introduce more variability and additional mechanics to explosions.

The distance calculation is not euclidean; the calculation considers the greater of the x and y distances from between the center points of the explosion and the object, less the object's radius (such that the distance is to the edge of the object rather than the centre). As a result of this, both an object standing e.g. some distance to the north of an explosion, and another object standing with the same distance north and to the east, unintuitively take the exact same amount of damage, even though the object with additional distance to the east is further away considering euclidean distance.

To be considered exposed to the explosion, an object's centre must have a line of sight to the centre of the explosion; if the view of the object from the explosion is obstructed by a wall, no damage is inflicted. Otherwise, the damage inflicted in hit points is equal to the value of the blast radius minus the distance. If an explosion's centre is within the bounding box of the object and there is no obstruction, the object takes the maximum amount of blast damage, which is equal to the value of the blast radius.

Rockets (such as those launched from a rocket launcher or a cyberdemon) move, collide, and explode in the following order: In a tic, the rocket attempts to move from position A to position B, and if position B is obstructed, the obstruction in B is considered directly hit while the rocket explodes at position A. As a result, a direct hit from a rocket typically deals less than full blast damage due to the distance between position A and the obstruction at position B. To deal full blast damage, a rocket can be detonated on the ceiling above the object, as no vertical distance is considered.

Properties of radius attacks in Doom

Counting both the hit and the blast, a successful rocket launcher attack averages about the same damage as the super shotgun at close range (about three times the regular shotgun), but is equally effective at long range.

For arch-vile attacks, the radius is 70 units; for barrels and player or cyberdemon rockets, it is 128 units.

Cyberdemons and spiderdemons are immune to all blast damage (which does not include the tracer damage from BFG blasts). Therefore, a greater number of rockets is required to kill these bosses than might be expected, and they cannot be damaged at all by barrels. They receive only the direct damage from arch-vile attacks. The arch-vile itself is not immune to blast radius damage, and may take splash-back from its own attacks if it is too close when the detonation of its flame cloud is initiated. It is possible for arch-viles to eventually kill themselves when engaging a target with sufficient health due to this effect. Note that this does not cause arch-viles to infight with themselves, or with other arch-viles, due to the fact that arch-viles are specially excluded from being the target of monster infighting.

Extra effects in other games

Blast radii induce certain other effects in other games based on the Doom engine. In Heretic and Hexen, blast radius attacks will trigger terrain hit effects if they occur at a distance from the floor that is equal to or less than the explosion's radius. In Strife, blast radius attacks fire four invisible non-damaging tracers in the cardinal directions. These tracers enable nearby blasts to break glass screens and windows. Most code that creates blast radius attacks in Strife will additionally make a call to the function P_NoiseAlert to awaken monsters and set off the alarm, though this is done separately outside the actual blast damage code itself.

Extensions were also made in Hexen that allow explosions to be clipped to a certain z height around the projectile, have differing damage and radius measurements, and to optionally not inflict damage to the actor which originated the explosion (such as is used for Baratus's Hammer of Retribution).

In Heretic, all monsters flagged as being bosses are immune to splash damage attacks in the same manner as Doom's boss creatures. This includes the maulotaur and both forms of D'Sparil, but does not include iron liches.

In Strife, only the inquisitor is specifically immune to blast damage, though the spectres and the Entity are also immune to most blasts due to the fact that they are only allowed to take damage from the Sigil.

Non-reentrancy

The blast radius code, like virtually all of the clipping engine code in Doom, is non-reentrant, meaning that it can cause recursive calls into the clipping engine and into its own code, but will not properly tolerate such calls and will therefore malfunction. It is possible for the ordering of the blockmap list used to damage objects in explosions to be changed during the explosion calculation simply through the calls it makes to the function P_DamageMobj, which can change the position of monsters, and thereby inflict damage to some monsters more than once. If multiple explosions cause each other directly within the same gametic, the recursive explosions will interfere with previous explosions' damage, location, and blockmap iteration properties.

Technical

Explosions are triggered via the function P_RadiusAttack. It finds things in the vicinity by checking blockmap tiles that the explosion overlaps, and it runs the explosion logic on those things.

P_RadiusAttack, p_map.c:

...
bombspot = spot;
bombsource = source;
bombdamage = damage;

for (y = yl; y <= yh; y++) {
    for (x = xl; x <= xh; x++) {
        P_BlockThingsIterator(x, y, PIT_RadiusAttack);
    }
}

The explosion logic inside PIT_RadiusAttack contains the distance calculation and other mechanics related to how an explosion interacts with things.

PIT_RadiusAttack (slightly edited for clarity), p_map.c:

...
// Boss spider and cyborg take no damage from concussion.
if (thing->type == MT_CYBORG || thing->type == MT_SPIDER) return true;

dx = abs(thing->x - bombspot->x);
dy = abs(thing->y - bombspot->y);

farthest = dx > dy ? dx : dy; // The farther of the two distances.
dist = (farthest - thing->radius) >> FRACBITS;

if (dist < 0) dist = 0;
if (dist >= bombdamage) return true; // out of range

if (P_CheckSight(thing, bombspot)) { // must be in direct path
    P_DamageMobj(thing, bombspot, bombsource, bombdamage - dist);
}

return true;

This code is the site of several oddities:

• The blast damage immunity for cyberdemons and spiderdemons is hardcoded directly into the logic. For this reason, the blast damage component of rockets is ineffective so it requires more rockets than would be expected to kill them, and they are fully immune to exploding barrels.
• The distance calculation not euclidean, and instead uses the biggest of the differences in the x and y coordinates, adjusted for the thing's radius. These differences have a fractional component, and downshifting using FRACBITS removes that fractional component, essentially rounding the value to a whole number. This is used as the distance that subtracts the damage, so damage inflicted does not follow the real world intuition that euclidean distance would provide.
• To determine whether or not there is an obstruction between the explosion and the object, a line of sight check is performed (using P_CheckSight), which carries the same oddities that monsters' line of sight does; this check draws a line directly between two points to determine if there is an obstruction between them, not considering that a thing is wider than a single point. As a result, a 1 map unit thick column standing between an explosion and an object intuitively provides inadequate cover because parts of the object are exposed to the explosion, but the explosion may miss the object entirely because there was no direct line of sight to the object's center.

See also

• Arch-vile
• Barrel
• Barrel suicide
• BFG9000
• Cyberdemon
• Fléchette
• Incinerator
• Phoenix rod
• Rocket launcher
• Time Bomb of the Ancients