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A gaming and technology blog by TWHL admins Penguinboy and Ant. A music blog by TWHL users Ant and Hugh.

Beheading Zombies

By Jonathan 'Teh_Freak' Smith

After hearing that the zombies in HL2 will have the headcrabs jump off them when it's killed, I decided to try it out, and it's not hard either...

First you'll need to get the modified zombie model here. It's not the HD one, just the good 'ol normal zombie. Got it? OK, now go into MSVC++ 6.0 (or whatever you use) and open zombie.cpp.

The Code

At the top there should be these #defines:

//=========================================================
// Monster's Anim Events Go Here
//=========================================================

#define     ZOMBIE_AE_ATTACK_RIGHT          0x01
#define     ZOMBIE_AE_ATTACK_LEFT          0x02
#define     ZOMBIE_AE_ATTACK_BOTH          0x03

Just add these ones:

// headcrab jumps from zombie
#define     ZOMBIE_AE_CRAB1          0x04
#define     ZOMBIE_AE_CRAB2          0x05
#define     ZOMBIE_AE_CRAB3          0x06
// headcrab jumps from zombie

Those just define the animation events used in the model for this, now just a few lines lower (after ZOMBIE_FLINCH_DELAY), add this:

#define ZOMBIE_CRAB          "monster_headcrab" // headcrab jumps from zombie

That defines the ZOMBIE_CRAB, which will be used later to spawn a headcrab of all things.

Now add this line in the public definitions (right under int IgnoreConditions):

     void SpawnCrab( void ); // headcrab jumps from zombie

Now plop this piece of code at the end of the file:

//=========================================================
// Spawn Headcrab - headcrab jumps from zombie
//=========================================================

void CZombie :: SpawnCrab( void )
{
     CBaseEntity *pCrab = CBaseEntity::Create( ZOMBIE_CRAB, pev->origin, pev->angles, edict() ); // create the crab

     pCrab->pev->spawnflags |= SF_MONSTER_FALL_TO_GROUND; // make the crab fall
}

That top line just spawns a crab (notice the ZOMBIE_CRAB), and that second line makes the crab fall to the ground.

Almost done, now go to HandleAnimEvents function, at the bottom of it (before the default:)add this:

// headcrab jumps from zombie
          case ZOMBIE_AE_CRAB1:
          {
               ALERT( at_console, "Crab1 spawned!\n" );
               pev->body = 1; // set the head to a skull
               SpawnCrab(); // spawn a headcrab
          }
          break;

          case ZOMBIE_AE_CRAB2:
          {
               ALERT( at_console, "Crab2 spawned!\n" );
               pev->body = 1; // set the head to a skull
               SpawnCrab(); // spawn a headcrab
          }
          break;

          case ZOMBIE_AE_CRAB3:
          {
               ALERT( at_console, "Crab3 spawned!\n" );
               pev->body = 1; // set the head to a skull
               SpawnCrab(); // spawn a headcrab
          }
          break;
// headcrab jumps from zombie

The 'pev->body = 1' line sets the body to the skull found in the modified zombie model, and the 'SpawnCrab()' simply calls that function.

Now you may be thinking,"Why do I have three events that are the exact same?". Well that is so you can add different effects for the headcrab when it spawns (like the TE_IMPLOSION effect).

Finally, go to the Precache area of the code and add this:

     UTIL_PrecacheOther( ZOMBIE_CRAB ); // headcrab jumps from zombie

Now compile it and run a test map, you may notice that the crab will only spawn when it isn't a headshot kill, makes the effect more realistic. Well, enjoy beheading those zombies!

The Model

For those of you that want to find out what's different about the model (besides a new head), read on, however, I'm not going to explain how to actually do the modeling part, this is about the .QC file.

When I made the zombie model I simply added new events to the .QC file, I'll explain a little about what does what in the .QC.

$bodygroup body
{
studio "Zom3_Template_Biped(White_Suit)1"
studio "skullhead"
}

If you open up normal zombies .QC file, you'll notice this bit isn't there. What this does it make the two bodygroups that the new zombie has. The new one here is skullhead, which 'pev->body = 1' is linked to, while 'pev->body = 0' would be the White_Suit.


$sequence dieheadshot "dieheadshot" X fps 20 ACT_DIE_HEADSHOT 1 { event 2001 19 }
$sequence dieheadshot2 "dieheadshot2" fps 22 ACT_DIE_GUTSHOT 1 { event 2001 18 }
$sequence diesimple "diesimple" X fps 20 ACT_DIESIMPLE 1 { event 2001 13 } { event 4 8 }
$sequence diebackward "diebackward" fps 18 ACT_DIEBACKWARD 1 { event 2001 10 } { event 5 2 }
$sequence dieforward "dieforward" X fps 18 ACT_DIEFORWARD 1 { event 2001 21 } { event 6 18 }

Those are the zombie's death animations. The only relevent parts here are the { event ## ##}.
The first number calls the event that cooresponds to the number in the code, while the second one tells it what frame to do it on.

For example:
------------QC file------------
$sequence diesimple "diesimple" X fps 20 ACT_DIESIMPLE 1 { event 2001 13 } { event 4 8 }

------------cpp file------------
#define     ZOMBIE_AE_CRAB1          0x04

As you can see, event 4 is called on frame 8. Event 4 is refered to in zombie.cpp as 0x04 with the name ZOMBIE_AE_CRAB1 (where the code is called).


$sequence dieheadshot "dieheadshot" X fps 20 ACT_DIE_HEADSHOT 1 { event 2001 19 }
$sequence dieheadshot2 "dieheadshot2" fps 22 ACT_DIE_GUTSHOT 1 { event 2001 18 }

As you can see here, there are no new events for these two, this is what makes it so the crab won't jump off when its a headshot or a gutshot.

Well, now you know how the model is important for this effect as well as learning about the relationship between model events and the code.