Self-shadowed normal mapping. That got you sitting up in your seats didn't it? Forget zombie hordes for a second, put the intricate and sophisticated animation system to one side, and focus on what's going to make Left 4 Dead special: self-shadowed normal mapping.
What's self-shadowed normal mapping? We had no idea, so we picked up the phone and asked Valve.
"The flashlights are really tied into the gameplay," begins Valve's technical superbrain Jason Mitchell, paving the way for the incoming jargon.
"In our past games they were attached to the player, but in Left 4 Dead they're attached to the weapons.
When you reload or use a shove attack, your flashlight tracks that weapon as it moves and points off towards the ceiling or somewhere.
So you can't see for a certain amount of time because your light's not pointing straight ahead."
This is an effect greatly enhanced by the darkness of L4D's urban levels, where a group's ability to cast light into menacing corners could make the difference between life and being strangled by a prehensile tongue.
"Shadows are an important visual cue too," continues Mitchell. "They help you tell where things are in 3D space. So as you're strafing around you're seeing those shadows move and change in a natural and believable way - you're able to quickly perceive that space."
On paper, and even in screenshots, it'll seem like nothing more than a visually pleasing effect - and one, we hasten to add, that's been seen in other PC titles.
In motion though, when surrounded by 30 infected maniacs, the shadows give a staggering degree of unconscious feedback, and spatial awareness. Now here comes the science.
"In self-shadowed normal mapping," begins Mitchell, having taken a deep breath, "instead of doing all your light calculations on strictly flat polygons, you have a texture map that contains normal information mapped to those polygons, and locally, you're changing the lighting."
We'll cut to the final product of this tech-wizardry, which is a subtle but striking lighting effect that goes way beyond your average bump mapping.
If you've ever placed a torch along a brick wall and seen how the light picks out and exaggerates every detail of the surface, that's what's happening in L4D.
Combined with the other visual effects Valve are bolting on to the Source engine it makes a serious difference.
A number of per frame post-processing effects are being employed to ensure L4D will be nothing less than a feat of visual engineering (see Horrifying box).
Talk in colour
"The filmic post-processing is, from a stylistic standpoint, useful for evoking the style of a classic horror movie," explains Mitchell. "We also view post-processing as another communication channel with our players.
"There are subtle ways of interacting with the player, think of the soundtrack to a feature film for instance. We'll use music in that way, but we'll also use the post-processing to communicate game state, for example.
"So when the player is injured we'll change our colour correction operation to desaturate the environment.
"The stuff they did in Grindhouse was similar," smiles Mitchell, talking about the double-feature version seen in the US, "they did colour grading as well, though they were trying more for the '70s look.
They were also going for a damaged film look, so they'd have scratches and areas of overexposure - we're not going for that exact look, but we're certainly in the same vein."