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As mentioned above, there is a backwards-compatible error metric called "contrast-v22". This is the metric that was used in RenderMan 22 and older: a relative contrast, with the contrast in dark pixels optionally reduced with "darkfalloff".  (Note that the new error metric does not use "darkfalloff" since the gamma-weighted absolute differences have a similar effect.)There is also "variance-v22" that uses the variance metrics from RenderMan 22 to match previous renders if needed.

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