= Chromatic null plane
{title2=$\mathbf w^{\mathsf T}\mathbf c=0$}
If a nonzero weight vector combines cone contrasts as $r=\mathbf w^{\mathsf T}\mathbf c$, its zero-response contrasts satisfy one linear constraint and form a two-dimensional plane through the adapted origin in three-dimensional contrast space. Any two independent vectors in that plane constrain the direction of the normal $\mathbf w$, up to scale. Nonlinearities and response noise limit empirical identification.
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