Bidirectional texture function (BTF) [1][2][3] is a 6-dimensional function depending on planar texture coordinates (x,y) as well as on view and illumination spherical angles. In practice this function is obtained as a set of several thousand color images of material sample taken during different camera and light positions.
The BTF is a representation of the appearance of texture as a function of viewing and illumination direction. It is an image-based representation, since the geometry of the surface is unknown and not measured. BTF is typically captured by imaging the surface at a sampling of the hemisphere of possible viewing and illumination directions. BTF measurements are collections of images. The term BTF was first introduced in [1][2] and similar terms have since been introduced including BSSRDF[4] and SBRDF (spatial BRDF). SBRDF has a very similar definition to BTF, i.e. BTF is also a spatially varying BRDF.
To cope with a massive BTF data with high redundancy, many compression methods were proposed.[3][5]
Application of the BTF is in photorealistic material rendering of objects in virtual reality systems and for visual scene analysis,[6] e.g., recognition of complex real-world materials using bidirectional feature histograms or 3D textons.
Biomedical and biometric applications of the BTF include recognition of skin texture.[7]
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