= Visual contrast
{title2=$C=(L-L_0)/L_0$}
For the toy models here, <visual contrast> means brightness relative to a positive uniform background $L_0$: $C(x,y)=(L(x,y)-L_0)/L_0$. Positive values mean brighter than background, negative values darker, and zero unchanged. This convention separates spatial structure from the background level. A <sinusoid> $C=a\cos(\cdots)$ with $0\leq a\leq1$ keeps $L=L_0(1+C)$ nonnegative. Different experimental conventions for <visual contrast> require their own definitions; the toy values below do not specify a historical stimulus intensity.
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