From a9396f251eb5df590709bfa1ad710b0abe530d88 Mon Sep 17 00:00:00 2001 From: Monty Date: Fri, 3 Feb 2012 19:49:57 +0000 Subject: [PATCH] Add a clarification to the floor1 decode spec, since I had to re-determine some of what it says for myself after not thinking about it for 15-ish years. Also, fix an indentation bug that occurred due to TABs sneaking in svn path=/trunk/vorbis/; revision=18181 --- doc/07-floor1.tex | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/doc/07-floor1.tex b/doc/07-floor1.tex index fd36314..76fd453 100644 --- a/doc/07-floor1.tex +++ b/doc/07-floor1.tex @@ -252,6 +252,16 @@ optimizations, implementors are warned to follow the details closely. Deviation from implementing a strictly equivalent algorithm can result in serious decoding errors. +{\em Additional note:} Although predicted values in the prediction +loop and at the end of step 1 (that is, the values in vector +\varname{[floor1\_final\_Y]} are inherently limited by the prediction +algorithm to \[0, \varname{[range]}\), it is possible to abuse the +setup and codebook machinery to produce negative or over-range +results. We suggest that decoder implementations guard the values in +vector \varname{[floor1\_final\_Y]} by clamping each element to \[0, +\varname{[range]}\) after step 1. Variants of this suggestion are +acceptable as valid floor1 setups cannot produce out of range values. + \begin{description} \item[step 1: amplitude value synthesis] @@ -307,7 +317,7 @@ Unwrap the always-positive-or-zero values read from the packet into \} else [val] is less than [room] \{ - 24) if ([val] is odd) \{ + 24) if ([val] is odd) \{ 25) vector [floor1\_final\_Y] element [i] = [predicted] - (([val] + 1) divided by 2 using integer division) -- 2.7.4