
Models for visual rendering 349
compress this data. This step should maintain the characteristics of the BTF as much
as possible and offer a quick decompression.
A large number of compression methods consider a BTF as a collection of discrete
textures or an “apparent BRDF varying spatially’’ (ABRDF Wong et al., 1997). Note
that in this approach, the ABRDF are not physically plausible as non reciprocal, since
they consider the scattering of other parts of the surface.
The most natural compression is to approach the BTF in each pixel by a BRDF
model.
BTF(x, ρ
r
, ρ
i
) ≈ f
r,x
(ω
i
→ ω
r
) (15.2)
The BRDF models f
r
(ω
i
→ ω
r
) must be, however, quick enough in assessment ...