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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.