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.
BTF is typically captured by imaging the surface at a sampling of the hemisphere of possible viewing and illumination directions.
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.