Specifications

MAXIMUMPC JANUARY 200546
HALF-LIFE 2
HALF-LIFE 2
HALF-LIFE 2
Once upon a time, there was an ambitious game that
promised to make physics interactions an integral part of
gameplay. That game, Trespasser, is now regularly derid-
ed as the worst game to ever grace the PC. In Trespasser,
“physics-based gameplay” amounted to using a freak-
ish, disembodied arm to stack improbably light boxes so
you could jump over artificial barriers into new areas of
the game. Its unrealistic, half-baked physics model and
hybrid hardware/software 3D renderer made it unplayable
and un-fun. Consequently, the title was savaged by the
gaming press. Given the big ol’ egg that Trespasser laid,
it’s not difficult to see why developers have eschewed
physics-based gameplay in favor of simple ragdoll char-
acter animation and one-shot physics gimmicks—a la
Max Payne 2.
That is, until Half-Life 2. With its latest game, Valve
reintroduces physics puzzles and to our utter delight, the
experiences are fun without being tedious, and feature
physics-based behaviors that remain consistent through-
out the game.
Making Physics Phun
PHYSICS AND YOU: REALISTIC OBJECT MOVEMENT = INFINITE POSSIBILITIES
In Half-Life 2, each level contains hundreds of physics objects that you can use in any way you desire. For example, you
might pile up objects in order to climb into an otherwise inaccessible area and avoid a difficult fight, while your friend
might use a piece of sheet metal as a bulletproof shield and advance under cover. Here’s a more focused look at how these
numerous gameplay possibilities present themselves throughout the game.
Anything that is literally not bolted down to
the ground can be picked up with the Gravity
Gun, including tools, tables, chairs, and yes,
even toilets.
Half-Life 2 owes its realistic physics to the tight integra-
tion of the Source engine’s materials system and the
third-party Havok physics engine. The materials system
associates textures with certain physical properties, such
as density and friction coefficient. When an artist applies
a texture to a model, the model receives the attributes of
the material the texture represents.
With this information, the Source engine then uses the
density of the material and the volume of the object to
calculate properties like mass and buoyancy. The engine
then passes that info on to the Havok physics engine,
which performs the grunt work of calculating forces—
everything from the effects of friction on a rolling barrel
to the air resistance exerted upon a grenade in mid-
flight—and making sure the objects behave in a manner
that is a reasonable approximation of the real world.
Here’s the upshot: The Source engine makes it really
easy for designers to add physics objects to their maps.
In other games, the designers have to manually specify
the weight, density, volume, and friction coefficients for
every single object in the game. By using the materials
engine, Valve was able to quickly and easily place hun-
dreds of consistent physics objects into every level.
This refined process also means that creating physics-
based puzzles is just as easy. The artists create the objects
they want to use in the puzzle, and then place them into
the game. The Source engine takes care of everything
else, meaning there’s no need to script puzzles.
It’s a Material World
The ability to pick up and toss hundreds of
objects opens the door to some interesting
gameplay choices. For example, should
you use the wheelbarrow as a bulletproof
shield or chuck it at your opponent? We like
doing both.
Once you obtain the Gravity Gun, it’s easy
to amuse yourself by coming up with new
and gruesome ways to slaughter enemies.
Does pulling saw blades out of walls and
using them to slice zombies in half ever
get old? Frankly, no.