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Normal-Mapping: Nearly ever surface in Half-Life 2 has been treated with some sort of normal-map. Normal-maps add extremely detailed surface variations to otherwise flat textures. For example, adding a normal-map to a brick wall texture gives the final product a more realistic surface, with actual grooves between bricks where mortar is. Normal Mapping can also be used to deform geometry and to display proper lighting, shadowing, and shading on textures in-game in real-time 3D. Using Half-Life 2's material system, a programmer or artist can a normal-map to all instances of a texture in the entire game.
Specularity: In the real world, most smooth surfaces are shiny, or at least have a sheen appearance and feel, so Half-Life 2 adds specular shine to normal-mapped surfaces that would be shiny in the real world, like tiles. Like all other materials(normal-mapping. sound properties, shadow properties, and light properties), specularity is controlled by the engine on a texture-by-texture basis.
Texture Variety: Every common gamer has all played entire games where the same boring wall and floor textures are repeated throughout the game, correct? Well, in Half-Life 2 there are at least two textures assigned to almost every surface in the game. One is the basic texture, used for video cards with 64MB of less VRAM, and the other is the best-case texture, which adds extra detail over the basic texture, AKA detailed texture. If the basic texture is of a brick wall, the best-case texture might have a vine over the brick wall.
Reflective & Refractive water: Reflective water is no longer anything to get excited about, but refractive water--which can "bend' incoming light, is new and cool. By making the water reflective when viewwed at by certain angle and refractive at lesser angles, water appears more realistic. The effect is difficult to appreciate in a mere low-resolution aliased screenshot, but it's the small subtleties that add to a greater overall experience and impression in a real-time full 3D interactive game running in real-time, increasing the realism level.
Normal-Mapped Displacement Maps: Half-Life 2 takes a basic, low polygon-count 3D mesh, and then adds more detail by combining it with a displacement map--a small grayscale bitmap--to describe how the verticles in the mesh should be deformed. At that point, the surface can be textured and normal-mapped like any other surface in the game. The really nice thing about displacement maps is that they make it easy to change the level of detail for terrain by scaling the resolution of displacement map up or down. In the game, displacement maps are used for terrain and some cave walls.
Volumetric Effects: Unless you live in San Franciso, fog usually isn't something that approaches in giant, flat walls that reach from the ground to the sky. In the real world, fog is usually patchy and uneven. By using volumetric, procedural textures to describe fog, smoke, and clouds, Half-Life 2's atmospheric effects look more realistic than I've seen before.
Level of Detail: Getting a bleeding-edge top-of-the-line maximum quality and optimized Direct-X 9 game running well on graphics hardware that's more than 4 years old is no minor feat. The secret is to ensure that every object in the game can be scaled down to lower levels of detail. If you run Half-Life 2 on older 3D graphics cards, however, the engine just draws models and extra apparent/background details to a lower quailty and variety all the time, to increase performance for a decent frame-rate.
Detail Props: Detail props set pieces that add realism to Half-Life 2's environment, but aren't manually placed. Instead, desginers associate a few prop models with a particular texture, and let the engine handle placement of the actual models whenever that texture is prsent. Most of the detail props should be props like grass and rocks in Half-Life 2.
Static Props: Once you've nailed down your basic environments, you need to fill in all those big empty spasces with deritus of life: desks, chairs, barrels, creates, rusting hulls of old ships, etc. These props are manually created and placed by the designer, and may or may not be interactive. In half-life 2, the player can pick up and toss around most ofthe smaller static props, but don't expect to move any giant props such as ships.
Particle Effects: If a bullet hits awter, it makes a splash. If a bullet hits metal, it makes sparks. If a bullet hits a brick, chuncks of the brick blast off. Thanks to Half-Life 2's material system, the source engine knows what types of particles to create and when any object impacts a surface.
Fresnel Reflection: If you walk towars a pool of water, the way it reflects light will gradually change. When you're far away, the water is fully reflective, but as you get closer, the water looses its reflective properties, and you start to see what lies beneath the surface. When you stand right next to the water and look down, you can see directly into it. Fresnel reflections describe how much light is reflected and refracted whenever light crosses from one material to another. The Source Engine implements Frensel reflections for water--if your video card has the guts to draw it.
High Dynamic Range(HDR): For this complex yet highly dynamic, and realistic complicated "special effect", which is oftenly mistaken as the different "light blooms", you'll need a DirectX 9 graphics card due to required high-precision FP16/32 blending, AKA 128-bit studio precision quality. Anyway, HDR is technically speaking just a nifty, neat term that requires studio quality floating point precision(only provided by DX9 cards) that heavily increases and stores lighting, colour, and general image based rendering values greater than the usual 0.0f and 1.0f values used in graphics rendering in games.
In other words, this allows things like rendering based on real-world elements like radiance, area lighting, the way the human eye adapts to it's lighting environment indoors and outdoors, and more as close to the real world as possible.
And HDR does this a lot better than fake light blooms. HL2 takes great advantage of HDR based rendering.
Dynamic Lights and Shadows: This is one of the graphical areas where Half-Life 2 isn't anything special. All half-life 2 uses is a mix of static lightmaps, vertex lightmaps, and dynamic lightmaps, along with shadowmaps for high-resolution world shadows, and render-to-texture basis soft, fuzzy-edged cheated shadows with self-shadowing, for accurately casted shadows by all characters and most game objects. Half-Life 2 makes it's world have some-what good lighting and shadowing capabilities, but nothing DOOM 3ish, apparently photo-realistic lighting and shadowing wasn't particulary Valve Software's target in Half-Life 2, but the lighting and shadowing in Half-Life 2 is definitley decent enough.
Alright, now you've seen what the Source engine is basically capable of graphically. Now the question is: What will it look like on your computer? Valve designed Half-Life 2 to run well on just about any videocard, from ancient two-pipeline DirectX 6 graphics cards to modern DX(DirectX)9 graphics cards with eight-pipelines and advanced programmable shaders. Consider that in 1998, when DirectX6 graphics cards were cutting-edge, shader programs were only used by Hollywood CGI animators to render CGI powered visual style movies. This technology was never intendted to run in real-time on lowly PCS. Yet it does today. In Half-Life 2, powered by the Microsoft® DirectX® 9 API.
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