Wire-frame rendering

The simplest forms of rendering are wire rame renderings, which do not show surface characteristics but which are very fast. Wireframe rendering is especially useful nowadays for modelling. It is common to work with models as wireframes during the geometry modelling stages and then switch to shaded surface rendering techniques.

Hidden-line rendering

The next step up from see-through wire frame rendering is hidden line rendering, which takes into account that objects have surfaces and removes the lines that are hidden by surfaces.

Shading algorithms

For virtual worlds, shaded surface rendering is what we are most interested. Modern hardware supports high-quality rendering in hardware enabling models to be presented with complex surface characteristics and lighting effects.

The most common forms of shading in use are flat shading, gouraud shading, phong shading, and ray tracing. Flat shading is not used much any more since modern 3D hardware usually offers at least support for gouraud shading. While real-time ray tracing is possible using extremely simple scenes on high-performance hardware, it is currently not an option for real-time virtual worlds. Ray tracing is most useful for creating high-quality video animation.

Flat (or faceted/Lambert)

A flat shaded surface is coloured uniformly according to its orientation towards light sources and has a faceted appearance.

Gouraud

Each vertex is assigned a colour according to the orientation of its vertex normal towards light sources. The colour of intermediate points is a blend of the surrounding vertex colours. The result is a smooth shaded surface. The result can be too smooth (can smooth away edges that are meant to be there) if vertex normals are not specifically defined but are left to the rendering algorithm to compute based on angles between surfaces.

Most 3D modelling packages for 3D animation and virtual worlds (surface modelling) compute normals and include them in the 3D geometry data. Most solid modelling CAD systems do not include vertex normals in the geometry data.

Phong

Phong shading gives results similar to Gauraud shading but also renders highlights that provide an impression of a surface's shininess ('specular reflection').

Radiosity

The radiosity shading model gives high quality results, which takes into account direct/indirect light, reflections and shadows. While current computer hardware technology is too slow to do useful high-quality real-time ray-tracing with dyanamic lights and shadows, it is possible to precompute radiosity to add greater lighting realism to a virtual environment (we will look at an example of this later in the section on pre-coloured indexed face sets).