With directional light sources, the degree to which a light source contributes to the color of a surface is dependent upon the angle formed between the light vector and the surface normal for the surface in question

Color values are uniform across the face of each polygon, based on the orientation of the polygon relative to the light source.

Linear (Gouraud) Shading: using the colors at each vertex normal as bounding values, color is linearly interpolated across the face each polygon.

Bilinear interpolation: linear interpolation across two dimensions

Linear shading often is supported in hardware and therefore can be very fast.

Shading Models and Interpolation of Normals

More sophisticated shading models perform more refined interpolations on surface normals in order to derive more localized shading and reflection distinctions across the surface as it is rendered. This can add significantly to the computations required for rendering.

But what about surface models?

The renderer will decompose continuous surfaces into polygonal representations of those surfaces prior to rendering.

• Basic surface properties

• Ambient (Ka) - rendered color of the surface considers only the ambient color component and the color (but not the position) of the light source
• Diffuse (Kd) - rendered color of surface considers the diffuse color component, the color of the light source, and the position of the light source relative to the surface, but not the viewer
• Specular (Ks) - "highlight color" - color of surface considers the color of the specular component, color of the light source, position of the light source, and the position of the viewer

 

* Lighting models and associated vector notation are discussed further in Watt

• Mirror, diffuse and specular reflection distributions

 

• Specular exponent controls the tightness or focus of the reflection distribution about the reflection vector

Image texture mapping

Texture coordinates are mapped into surface coordinates to position texture image colors on geometry.

• Scalar ``decal'' maps can be used to restrict the region or degree of a surface mapping effect

• Scalar transparency mapping - transparency values on a surface are varied according to values derived from an image map

• Bump mapping uses the color (usually grayscale) values from an image map to perturb surface normals to create variations in the shading of the surface to which the map is applied.

• Bump mapping
• perturbs normals according to value range in image map
• alters shading calculations to give the illusion of variations in surface geometry

 

• Displacement mapping
• displaces and retessellates geometry along normal (perpendicular to surface) according to value range in image map
• bump and displacement maps can be animated to portray undulating surfaces

• Reflection/environment mapping
• an image map is projected onto a simple geometric representation of an environment (usually a sphere or cube)
• the image environment is projected onto affected surfaces during rendering
• works best with geometry which offers some variation in normals across surfaces, as opposed to flat surfaces with no variation in normals
• careful selection or design of reflection or environment maps can offer good results without the expense of ray tracing
• maps can be animated - e.g. use the previous frame in animation as the current frame's reflection map to simulate action reflected in objects in the scene

 

• Object reflections versus mapped reflections
• true inter-object reflections are usually ray traced
• the degree of recursion in tracing reflections can be controlled by a ``trace depth'' value

 

• Transparency and Refraction

 
• refraction index determines the degree of bending of the light that passes through the refractive surfaces
• Air = 1.0
• Water = 1. 2 approx.
• Glass = 1.6 approx.

 
• refraction computations can be limited by a ``refraction depth'' which operates similarly to ``trace depth'' for reflections

• Shaders - collections of surface attributes that describe the rendering properties of a surface
• Procedural shaders - surface characteristics are derived from algorithms or expressions, rather than from explicit numerical or surface map values
• Renderman - shading language that allows shader definition of complex shaders of arbitrary complexity

 

• Special effects shaders
• Particles
• Hair

Back to 3D Color outline

This file was last modified on September 23, 2002.