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author | Aki <please@ignore.pl> | 2021-10-01 23:46:47 +0200 |
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committer | Aki <please@ignore.pl> | 2021-10-01 23:46:47 +0200 |
commit | ddd76aa4e1571c8d5965709db5c3cd47a89c9a38 (patch) | |
tree | 4e07bfaebede6531a70984fdebc855bca52f394a /vorbis/doc/07-floor1.tex | |
parent | 3a507e08b1d4e5970b27401a7e6517570d529400 (diff) | |
parent | c2d2c3551501110fddd78674d5435bfaa70382a3 (diff) | |
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diff --git a/vorbis/doc/07-floor1.tex b/vorbis/doc/07-floor1.tex new file mode 100644 index 0000000..47ad798 --- /dev/null +++ b/vorbis/doc/07-floor1.tex @@ -0,0 +1,404 @@ +% -*- mode: latex; TeX-master: "Vorbis_I_spec"; -*- +%!TEX root = Vorbis_I_spec.tex +\section{Floor type 1 setup and decode} \label{vorbis:spec:floor1} + +\subsection{Overview} + +Vorbis floor type one uses a piecewise straight-line representation to +encode a spectral envelope curve. The representation plots this curve +mechanically on a linear frequency axis and a logarithmic (dB) +amplitude axis. The integer plotting algorithm used is similar to +Bresenham's algorithm. + + + +\subsection{Floor 1 format} + +\subsubsection{model} + +Floor type one represents a spectral curve as a series of +line segments. Synthesis constructs a floor curve using iterative +prediction in a process roughly equivalent to the following simplified +description: + +\begin{itemize} + \item the first line segment (base case) is a logical line spanning +from x_0,y_0 to x_1,y_1 where in the base case x_0=0 and x_1=[n], the +full range of the spectral floor to be computed. + +\item the induction step chooses a point x_new within an existing +logical line segment and produces a y_new value at that point computed +from the existing line's y value at x_new (as plotted by the line) and +a difference value decoded from the bitstream packet. + +\item floor computation produces two new line segments, one running from +x_0,y_0 to x_new,y_new and from x_new,y_new to x_1,y_1. This step is +performed logically even if y_new represents no change to the +amplitude value at x_new so that later refinement is additionally +bounded at x_new. + +\item the induction step repeats, using a list of x values specified in +the codec setup header at floor 1 initialization time. Computation +is completed at the end of the x value list. + +\end{itemize} + + +Consider the following example, with values chosen for ease of +understanding rather than representing typical configuration: + +For the below example, we assume a floor setup with an [n] of 128. +The list of selected X values in increasing order is +0,16,32,48,64,80,96,112 and 128. In list order, the values interleave +as 0, 128, 64, 32, 96, 16, 48, 80 and 112. The corresponding +list-order Y values as decoded from an example packet are 110, 20, -5, +-45, 0, -25, -10, 30 and -10. We compute the floor in the following +way, beginning with the first line: + +\begin{center} +\includegraphics[width=8cm]{floor1-1} +\captionof{figure}{graph of example floor} +\end{center} + +We now draw new logical lines to reflect the correction to new_Y, and +iterate for X positions 32 and 96: + +\begin{center} +\includegraphics[width=8cm]{floor1-2} +\captionof{figure}{graph of example floor} +\end{center} + +Although the new Y value at X position 96 is unchanged, it is still +used later as an endpoint for further refinement. From here on, the +pattern should be clear; we complete the floor computation as follows: + +\begin{center} +\includegraphics[width=8cm]{floor1-3} +\captionof{figure}{graph of example floor} +\end{center} + +\begin{center} +\includegraphics[width=8cm]{floor1-4} +\captionof{figure}{graph of example floor} +\end{center} + +A more efficient algorithm with carefully defined integer rounding +behavior is used for actual decode, as described later. The actual +algorithm splits Y value computation and line plotting into two steps +with modifications to the above algorithm to eliminate noise +accumulation through integer roundoff/truncation. + + + +\subsubsection{header decode} + +A list of floor X values is stored in the packet header in interleaved +format (used in list order during packet decode and synthesis). This +list is split into partitions, and each partition is assigned to a +partition class. X positions 0 and [n] are implicit and do not belong +to an explicit partition or partition class. + +A partition class consists of a representation vector width (the +number of Y values which the partition class encodes at once), a +'subclass' value representing the number of alternate entropy books +the partition class may use in representing Y values, the list of +[subclass] books and a master book used to encode which alternate +books were chosen for representation in a given packet. The +master/subclass mechanism is meant to be used as a flexible +representation cascade while still using codebooks only in a scalar +context. + +\begin{Verbatim}[commandchars=\\\{\}] + + 1) [floor1\_partitions] = read 5 bits as unsigned integer + 2) [maximum\_class] = -1 + 3) iterate [i] over the range 0 ... [floor1\_partitions]-1 \{ + + 4) vector [floor1\_partition\_class\_list] element [i] = read 4 bits as unsigned integer + + \} + + 5) [maximum\_class] = largest integer scalar value in vector [floor1\_partition\_class\_list] + 6) iterate [i] over the range 0 ... [maximum\_class] \{ + + 7) vector [floor1\_class\_dimensions] element [i] = read 3 bits as unsigned integer and add 1 + 8) vector [floor1\_class\_subclasses] element [i] = read 2 bits as unsigned integer + 9) if ( vector [floor1\_class\_subclasses] element [i] is nonzero ) \{ + + 10) vector [floor1\_class\_masterbooks] element [i] = read 8 bits as unsigned integer + + \} + + 11) iterate [j] over the range 0 ... (2 exponent [floor1\_class\_subclasses] element [i]) - 1 \{ + + 12) array [floor1\_subclass\_books] element [i],[j] = + read 8 bits as unsigned integer and subtract one + \} + \} + + 13) [floor1\_multiplier] = read 2 bits as unsigned integer and add one + 14) [rangebits] = read 4 bits as unsigned integer + 15) vector [floor1\_X\_list] element [0] = 0 + 16) vector [floor1\_X\_list] element [1] = 2 exponent [rangebits]; + 17) [floor1\_values] = 2 + 18) iterate [i] over the range 0 ... [floor1\_partitions]-1 \{ + + 19) [current\_class\_number] = vector [floor1\_partition\_class\_list] element [i] + 20) iterate [j] over the range 0 ... ([floor1\_class\_dimensions] element [current\_class\_number])-1 \{ + 21) vector [floor1\_X\_list] element ([floor1\_values]) = + read [rangebits] bits as unsigned integer + 22) increment [floor1\_values] by one + \} + \} + + 23) done +\end{Verbatim} + +An end-of-packet condition while reading any aspect of a floor 1 +configuration during setup renders a stream undecodable. In addition, +a \varname{[floor1\_class\_masterbooks]} or +\varname{[floor1\_subclass\_books]} scalar element greater than the +highest numbered codebook configured in this stream is an error +condition that renders the stream undecodable. Vector +[floor1\_x\_list] is limited to a maximum length of 65 elements; a +setup indicating more than 65 total elements (including elements 0 and +1 set prior to the read loop) renders the stream undecodable. All +vector [floor1\_x\_list] element values must be unique within the +vector; a non-unique value renders the stream undecodable. + +\subsubsection{packet decode} \label{vorbis:spec:floor1-decode} + +Packet decode begins by checking the \varname{[nonzero]} flag: + +\begin{Verbatim}[commandchars=\\\{\}] + 1) [nonzero] = read 1 bit as boolean +\end{Verbatim} + +If \varname{[nonzero]} is unset, that indicates this channel contained +no audio energy in this frame. Decode immediately returns a status +indicating this floor curve (and thus this channel) is unused this +frame. (A return status of 'unused' is different from decoding a +floor that has all points set to minimum representation amplitude, +which happens to be approximately -140dB). + + +Assuming \varname{[nonzero]} is set, decode proceeds as follows: + +\begin{Verbatim}[commandchars=\\\{\}] + 1) [range] = vector \{ 256, 128, 86, 64 \} element ([floor1\_multiplier]-1) + 2) vector [floor1\_Y] element [0] = read \link{vorbis:spec:ilog}{ilog}([range]-1) bits as unsigned integer + 3) vector [floor1\_Y] element [1] = read \link{vorbis:spec:ilog}{ilog}([range]-1) bits as unsigned integer + 4) [offset] = 2; + 5) iterate [i] over the range 0 ... [floor1\_partitions]-1 \{ + + 6) [class] = vector [floor1\_partition\_class] element [i] + 7) [cdim] = vector [floor1\_class\_dimensions] element [class] + 8) [cbits] = vector [floor1\_class\_subclasses] element [class] + 9) [csub] = (2 exponent [cbits])-1 + 10) [cval] = 0 + 11) if ( [cbits] is greater than zero ) \{ + + 12) [cval] = read from packet using codebook number + (vector [floor1\_class\_masterbooks] element [class]) in scalar context + \} + + 13) iterate [j] over the range 0 ... [cdim]-1 \{ + + 14) [book] = array [floor1\_subclass\_books] element [class],([cval] bitwise AND [csub]) + 15) [cval] = [cval] right shifted [cbits] bits + 16) if ( [book] is not less than zero ) \{ + + 17) vector [floor1\_Y] element ([j]+[offset]) = read from packet using codebook + [book] in scalar context + + \} else [book] is less than zero \{ + + 18) vector [floor1\_Y] element ([j]+[offset]) = 0 + + \} + \} + + 19) [offset] = [offset] + [cdim] + + \} + + 20) done +\end{Verbatim} + +An end-of-packet condition during curve decode should be considered a +nominal occurrence; if end-of-packet is reached during any read +operation above, floor decode is to return 'unused' status as if the +\varname{[nonzero]} flag had been unset at the beginning of decode. + + +Vector \varname{[floor1\_Y]} contains the values from packet decode +needed for floor 1 synthesis. + + + +\subsubsection{curve computation} \label{vorbis:spec:floor1-synth} + +Curve computation is split into two logical steps; the first step +derives final Y amplitude values from the encoded, wrapped difference +values taken from the bitstream. The second step plots the curve +lines. Also, although zero-difference values are used in the +iterative prediction to find final Y values, these points are +conditionally skipped during final line computation in step two. +Skipping zero-difference values allows a smoother line fit. + +Although some aspects of the below algorithm look like inconsequential +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 \varname{[floor1\_final\_Y]} values in +the prediction loop and at the end of step 1 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] + +Unwrap the always-positive-or-zero values read from the packet into ++/- difference values, then apply to line prediction. + +\begin{Verbatim}[commandchars=\\\{\}] + 1) [range] = vector \{ 256, 128, 86, 64 \} element ([floor1\_multiplier]-1) + 2) vector [floor1\_step2\_flag] element [0] = set + 3) vector [floor1\_step2\_flag] element [1] = set + 4) vector [floor1\_final\_Y] element [0] = vector [floor1\_Y] element [0] + 5) vector [floor1\_final\_Y] element [1] = vector [floor1\_Y] element [1] + 6) iterate [i] over the range 2 ... [floor1\_values]-1 \{ + + 7) [low\_neighbor\_offset] = \link{vorbis:spec:low:neighbor}{low\_neighbor}([floor1\_X\_list],[i]) + 8) [high\_neighbor\_offset] = \link{vorbis:spec:high:neighbor}{high\_neighbor}([floor1\_X\_list],[i]) + + 9) [predicted] = \link{vorbis:spec:render:point}{render\_point}( vector [floor1\_X\_list] element [low\_neighbor\_offset], + vector [floor1\_final\_Y] element [low\_neighbor\_offset], + vector [floor1\_X\_list] element [high\_neighbor\_offset], + vector [floor1\_final\_Y] element [high\_neighbor\_offset], + vector [floor1\_X\_list] element [i] ) + + 10) [val] = vector [floor1\_Y] element [i] + 11) [highroom] = [range] - [predicted] + 12) [lowroom] = [predicted] + 13) if ( [highroom] is less than [lowroom] ) \{ + + 14) [room] = [highroom] * 2 + + \} else [highroom] is not less than [lowroom] \{ + + 15) [room] = [lowroom] * 2 + + \} + + 16) if ( [val] is nonzero ) \{ + + 17) vector [floor1\_step2\_flag] element [low\_neighbor\_offset] = set + 18) vector [floor1\_step2\_flag] element [high\_neighbor\_offset] = set + 19) vector [floor1\_step2\_flag] element [i] = set + 20) if ( [val] is greater than or equal to [room] ) \{ + + 21) if ( [highroom] is greater than [lowroom] ) \{ + + 22) vector [floor1\_final\_Y] element [i] = [val] - [lowroom] + [predicted] + + \} else [highroom] is not greater than [lowroom] \{ + + 23) vector [floor1\_final\_Y] element [i] = [predicted] - [val] + [highroom] - 1 + + \} + + \} else [val] is less than [room] \{ + + 24) if ([val] is odd) \{ + + 25) vector [floor1\_final\_Y] element [i] = + [predicted] - (([val] + 1) divided by 2 using integer division) + + \} else [val] is even \{ + + 26) vector [floor1\_final\_Y] element [i] = + [predicted] + ([val] / 2 using integer division) + + \} + + \} + + \} else [val] is zero \{ + + 27) vector [floor1\_step2\_flag] element [i] = unset + 28) vector [floor1\_final\_Y] element [i] = [predicted] + + \} + + \} + + 29) done + +\end{Verbatim} + + + +\item[step 2: curve synthesis] + +Curve synthesis generates a return vector \varname{[floor]} of length +\varname{[n]} (where \varname{[n]} is provided by the decode process +calling to floor decode). Floor 1 curve synthesis makes use of the +\varname{[floor1\_X\_list]}, \varname{[floor1\_final\_Y]} and +\varname{[floor1\_step2\_flag]} vectors, as well as [floor1\_multiplier] +and [floor1\_values] values. + +Decode begins by sorting the scalars from vectors +\varname{[floor1\_X\_list]}, \varname{[floor1\_final\_Y]} and +\varname{[floor1\_step2\_flag]} together into new vectors +\varname{[floor1\_X\_list]'}, \varname{[floor1\_final\_Y]'} and +\varname{[floor1\_step2\_flag]'} according to ascending sort order of the +values in \varname{[floor1\_X\_list]}. That is, sort the values of +\varname{[floor1\_X\_list]} and then apply the same permutation to +elements of the other two vectors so that the X, Y and step2\_flag +values still match. + +Then compute the final curve in one pass: + +\begin{Verbatim}[commandchars=\\\{\}] + 1) [hx] = 0 + 2) [lx] = 0 + 3) [ly] = vector [floor1\_final\_Y]' element [0] * [floor1\_multiplier] + 4) iterate [i] over the range 1 ... [floor1\_values]-1 \{ + + 5) if ( [floor1\_step2\_flag]' element [i] is set ) \{ + + 6) [hy] = [floor1\_final\_Y]' element [i] * [floor1\_multiplier] + 7) [hx] = [floor1\_X\_list]' element [i] + 8) \link{vorbis:spec:render:line}{render\_line}( [lx], [ly], [hx], [hy], [floor] ) + 9) [lx] = [hx] + 10) [ly] = [hy] + \} + \} + + 11) if ( [hx] is less than [n] ) \{ + + 12) \link{vorbis:spec:render:line}{render\_line}( [hx], [hy], [n], [hy], [floor] ) + + \} + + 13) if ( [hx] is greater than [n] ) \{ + + 14) truncate vector [floor] to [n] elements + + \} + + 15) for each scalar in vector [floor], perform a lookup substitution using + the scalar value from [floor] as an offset into the vector \link{vorbis:spec:floor1:inverse:dB:table}{[floor1\_inverse\_dB\_static\_table]} + + 16) done + +\end{Verbatim} + +\end{description} |