From: Ronald S. Bultje Date: Tue, 24 Jul 2012 22:46:49 +0000 (-0700) Subject: Add comment for use of shift/mul instead of div. X-Git-Tag: v1.2.0~131 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=8967fcaf845ae956c5f59baebd917a5e3da53e56;p=libvpx Add comment for use of shift/mul instead of div. Change-Id: Ia6a0090da2097ca9337f8179a6d7b6a09e12f2e6 --- diff --git a/vp8/common/quant_common.c b/vp8/common/quant_common.c index a0b212cc4..05f921070 100644 --- a/vp8/common/quant_common.c +++ b/vp8/common/quant_common.c @@ -109,6 +109,9 @@ int vp8_ac2quant(int QIndex, int Delta) else if (QIndex < 0) QIndex = 0; + /* For all x in [0..284], x*155/100 is bitwise equal to (x*101581) >> 16. + * The smallest precision for that is '(x*6349) >> 12' but 16 is a good + * word size. */ retval = (ac_qlookup[ QIndex ] * 101581) >> 16; if (retval < 8)