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With the release of Half-Life 2: Episode One a number enhancments were made to the way lighting works within the Source engine. The emphasis was on global illumination in order to ground characters in the game worlds and make them seem to be truly present in their environment.
One of these enhancements was the addtion of Phong shading for players with graphic cards using ps_2_b or later pixel shaders - around 40% at the time of release. This new feature gives texture artists the the ability to include Phong terms which can be driven by specular masks and exponent textures. These integrate naturally with the existing lighting and provides greater definition, particularly to key characters such as Alyx Vance.
The Phong shading can also include a Fresnel term in order to heavily favor rim lighting cases as opposed to specular highlights. Under very bright lights, the specular rim highlights are often bright enough to bloom out in a dramatic fashion as a result of HDR post-processing.
A brief introduction to Phong Shading
The following short introduction is not necessary knowledge to use the Source Phong shader, but does provide some technical insights as to how the technique works and may be of interest to some readers.
In simple terms, Phong shading is a technique used in 3D graphics to get better resolution of specular reflections. It was developed by Dr. Bui Tuong Phong at the University of Utah in 1973. His original publications combined the interpolation part of technique with his reflection model, the term Phong shading is commonly used to refer to either just the reflection model or to the combination of the reflection model and the interpolation method.
The Phong Reflection Model
The Phong reflection model is basically a simplified method of deciding the shade of a specific point on a 3D surface.
- It doesn't take into account second-order reflections often found in raytraced or diffuse rendering. To compensate an extra ambient lighting term is added to the scene that is rendered.
- It divides the reflection from a surface into three subcomponents - specular reflection, diffuse reflection and ambient reflection.
Phong reflection is an empirical model based on informal observation. Dr. Phong observed that for very shiny surfaces the specular highlight was small and that the intensity fell off rapidly, while for duller surfaces it was larger and fell off more slowly.
Phong shading provides a better approximation to a per-pixel application of an underlying reflection model by assuming a smoothly varying surface normal vector. The Phong interpolation method works particularly well when applied to the Phong reflection model or to any reflection model that has small specular highlights.
In some modern hardware, variants of the interpolation algorithm are called "pixel shading" (this should not be confused with "pixel shaders"). Usually this means that the lighting calculations can be done per-pixel and include other lighting variables such as surface normals from a normap map which are interpolated across the polygon.
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