Game Development Reference
a heterogeneous material, which is more likely to be encountered in reality, requires
defining a variation of the BRDF on the surface.
This variation can be represented in different manners. The first and the most
trivial approach consists of explicitly representing this variation in the properties of
materials by segmenting the geometry. This approach significantly increases the cost
of geometric processing which becomes prohibitive as soon as we try to represent
mesostructures on a relatively homogenous material (for example, a knitted woollen
sweater). These properties are modelled efficiently by a 2D information card called
texture , very effective for real time rendering and constructed on a parameterisation
of the surface of objects.
When we talk of texture, the first representation we think of is a variation of colour
along the surface. This way, a texture can be seen as a colour chart defined on the
surface of the object. This definition, introduced by Catmull in 1974, makes it possible
to concisely represent the exact details of colours and offers two important advantages
in virtual reality regarding the creation and use of this model. Represent details of
colours in a compact
To make it suitable to use in a virtual reality application, every model of the
appearance should be easy to create using real information. In this respect, the 2D
textures represent a model most suitable for creating the contents. In fact, a simple
photograph of a real object helps defining a texture to be applied on a virtual object.
Corrections necessary to improve the final appearance like equalizing the luminance,
accentuating the contrasts or any other corrective filter are available in the standard
process of processing 2D images. If the appearance that we want tomodel does not exist
in reality, it can be created from any part using standard designing and painting tools.
Bidirectional texture functions
Diffuse textures, though widely used and visually convincing, do not give the best
perceptual quality possible for the images in virtual reality. This approximation results
directly from the concept of BRDF discussed earlier and is approximated by a constant
function in case of diffuse textures. If one wishes to increase the realism of the images
produced, it is necessary to increase the complexity of information transferred by the
texture in order to best represent all the subtleties of the materials. In this manner,
bidirectional texture functions (BTF) introduced by Dana in 1999 (Dana et al., 1999)
are 6D textures which extend the standard representation of a 2D texture by adding
the factor of dependency on directions of vision and light. These functions thus define
a BRDF that varies with texel and make it possible to represent a number of subtle
effects of light on the surface of objects and to significantly improve the quality of
images (figure 15.4).
Modelling the appearance by BTF method is thus very similar to the approach by
diffuse texture and is based on the following steps: