The Lighting Engine

The engine uses static and dynamic light sources for its lighting effects. Static light sources create light and shadow maps on level surfaces, and are compiled into precomputed radiance volumes (PRV) that affect the ambient lighting of entities. Dynamic light sources affect the diffuse, specular, and emissive lighting of entities, and generate entity shadows. Static lights are placed in the level with WED; dynamic light can be emitted by entities at run time, or (with version A7) also by static light sources.

The PRV equally lights the whole entity mesh depending on light and shadow maps along the floor plane. Dynamic light only affects the entity parts in its range, regardless of shadows and floor planes. Static light has no influence on the rendering time, while many dynamic lights can reduce the frame rate. If you want an entity to be affected by dynamic light sources only, set its UNLIT flag, but do not assign the mat_unlit material. If you want an entity to be affected by the static PRV only, assign the mat_unlit material but do not set the UNLIT flag.

Normal engines support only 8 dynamic light sources because that's the limit of today's 3D hardware. The A7 engine however contains a light manager that can render a theoretically unlimited number of dynamic light sources at the same time. For this, during rendering of any mesh or object the 8 closest light sources are automatically activated and all other light sources are switched off. This way the 3D hardware limit is still kept, despite hundreds of light sources can be visible at the same time.

The light sorting, although very fast, is an additional rendering process that can affect the frame rate, depending on the number and ranges of active lights. Levels and map entities must be compiled in mesh mode (Create Mesh activated). Terrain should be chunked; the smaller the chunks, the less visible are light swapping effects when more than 8 lights move in or out of the range of a chunk.

The light formula

Both static and dynamic lights offer enough flexibility to solve a wide range of lighting situations even without using shaders. Shaders can be used to program a customized lighting engine; otherwise the total amount of light of every vertex is computed using the following equation:

Total Light = Ambient Light + Diffuse Light + Specular Light + Emissive Light

Ambient Light

Ambient light provides constant lighting for a scene. It lights all object vertices the same and is not dependent on any other lighting factors such as vertex normals, light direction, or camera position. The ambient lighting for a scene is described by the following equation.

Where:

Precomuted Radiance Volumes (PRV) contain the precomputed light values for a grid of positions in the level, and are generated by the map compiler from static light sources and level geometry. The light values are stored in radiance planes along the shaded level surfaces. Every entity gets its radiance color value from the nearest radiance plane below its origin. This way, entities are automatically lit in bright regions and dark in shadows. Note that you'll get wrong PRV lights when placing the entities' origin below the floor or into a solid block. For debugging purposes the current PRV values of the watched entity are indicated on screen.

Diffuse Light

Diffuse Light depends on both the light direction and the object surface normal. It varies across the surface of an object as a result of the changing light direction and the changing surface normal vector. It shades objects using a Gouraud shading algorithm and gives them three-dimensional (3-D) depth.

After adjusting the light intensity for any attenuation effects, the lighting engine computes how much of the remaining light reflects from a vertex, given the angle of the vertex normal and the direction of the incident light. This step is skipped for sun light because it does not attenuate over distance. The system considers two reflection types, diffuse and specular, and uses a different formula to determine how much light is reflected for each. After calculating the amounts of light reflected, the engine applies these new values to the diffuse and specular reflectance properties of the current material. The resulting color values are the diffuse and specular components that the rasterizer uses to produce Gouraud shading and specular highlighting.

The sun light is multiplied with the PRV brightness value. This way, reflected sun light is dim indoors or within static shadows, and bright outside.

Diffuse lighting is described by the following equation.

Diffuse Light = DiffuseMtl * (SunRGB * (N . SunDir) * (PRV + AlbedoMtl)
                                + Sum( Light[n] * (N . Ldir[n]) * Atten[n] * Spot[n] ))

Specular Light

Specular Lighting identifies the bright specular highlights that occur when light hits an object surface and reflects back toward the camera. It is more intense than diffuse light and falls off more rapidly across the object surface. It takes longer to calculate specular lighting than diffuse lighting, however the benefit of using it is that it adds significant detail to a surface.

Modeling specular reflection requires that the system not only know in what direction light is traveling, but also the direction to the camera. The system uses a simplified version of the Phong specular-reflection model, which employs a halfway vector to approximate the intensity of specular reflection. Specular Lighting is described by the following equation.

Specular Light = SpecularMtl * (SunRGB * (N . H)^p * (PRV + AlbedoMtl)
                                       + Sum( Light[n] * (N . H)^p * Atten[n] * Spot[n] )

Emissive Light

Emissive Light looks as if emitted by the entity; for example, a glow (it does however not influence nearby surfaces like dynamic light that is really emitted!) It is set solely by the emissive material color,

Emissive Light = EmissiveMtl