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Roughly, if both the source and the reference areas are darker too dark (R/G/B <= ~6), they are ignored. One caveat: SSIM calculation won't work for U/V planes, which are 128-centered and not related to luminance. But WebPPlaneDistortion() enforces the conversion to RGB, if needed. Change-Id: I586c2579c475583b8c90c5baefd766b1d5aea591