Talk:Bidirectional texture function

According to the sources I have read, the BTF is a 6-dimensional function (and not 7-dimensional). Refer e.g. to A Sparse Parametric Mixture Model for BTF Compression, Editing and Rendering: "The bidirectional texture function (BTF) was introduced by Dana et al. [DvGNK99] to capture the appearance of complex materials. It is essentially a 6D function parameterized by position, lighting direction as well as view direction". Could anyone explain why the article mentions 7 dimensions? --192.67.207.123 (talk) 20:46, 4 November 2013 (UTC)