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
* prevent 64bit overflow by controlling the 32b->64b conversions
and preventively descaling by 8bit before the final multiply
* adjust the threshold constants C1 and C2 to de-emphasis the dark
areas
* use a hat-like filter instead of box-filtering to avoid blockiness
during averaging
SSIM distortion calc is actually *faster* now in SSE2, because of the
unrolling during the function rewrite.
The C-version is quite slower because still un-optimized.
Change-Id: I96e2715827f79d26faae354cc28c7406c6800c90
using -ssim -o will trigger SSIM map calculation instead of add-diff map.
-gray converts the error map to intensity instead of having each channels' error separated.
Change-Id: I4bdb88880a252e5562aa4e0e3c2353ad93aef20e
