Long ago, in <a href="https://graphics.stanford.edu/~boulos/papers/brdftog.pdf" rel="nofollow noreferrer">https://graphics.stanford.edu/~boulos/papers/brdftog.pdf</a> we tried to trade off energy conservation and data reproduction at the cost of reciprocity.<p>In the end, nobody actually cared about automatically fitting material data from BRDFs or if they do, they actually prefer a symmetric and conserving-enough BSDF, which is what most future research improved. In particular, breaking symmetry means making bidirectional path tracing and other techniques kinda weird.