Doug Lombardi: What we've done is move from textures to materials when we get down to how we build things. In the old world, we would take a 2D texture, a piece of art, apply it to some polygons, and then it would just sit there in the world unless we wanted a very specific functionality on it. A set parameter would allow you to slide a crate (or not slide a crate).

In Half-Life 2, we've moved from textures to materials so everything that used to be a texture, whether it is wood or metal or whatever, has properties assigned to it for weight, consistency, etc. that all physical objects would have. So what you have is that a bottle or a crate or a barrel sort of take on all of these properties and allows the player, and non-player characters, to manipulate these objects. Things that you thought were safe to hide under or behind are no longer so because they can be blown apart, moved, or what have you.

All objects in the world can be used for alternative gameplay solutions as well. It also keeps players on their toes because they aren't safe in places that they thought they would be.

Doug Lombardi: There will be set things that can be exploded, either by the player or by the non-player character for dramatic effect. And, it will be pre-authored as destructible or deformable, but we're not going to have this open-ended sort of "you can morph the world."

Doug Lombardi: We really wanted to bring people outside a lot more in the first game; there was the cliff scene in Half-Life 1 where you had to work your way up the side of this cliff … and all the while you were next to this big ugly 2D skybox thing. We were horribly embarrassed by that, so in this game when we take you outside we want it to be far more believable than that. We've implemented a 3D skybox to give us a believable endless horizon, while the downtown urban environments are far more detailed with what have you, like the variety of architecture and objects. The telephone cables, for example, are physically simulated, so when objects are fired at them or fly over them you can see the cables move.

As far as atmosphere, we have a scene at night where you can see the clouds moving and sort of block the moon. We're doing a variety of things to outdoor levels to make them more compelling and believable.

Doug Lombardi: You just have to be really careful when you are authoring the scenes and the attributes that you are giving to your AI characters. In many ways, friendly non-player characters are much harder to achieve than the enemy NPCs. For that very reason it requires careful programming and design work to make sure that you have solid AI and that you put those friendly AI characters in the right places with the right attributes.