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ExternalLibs/libvorbis-1.3.1/doc/09-helper.tex

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+% -*- mode: latex; TeX-master: "Vorbis_I_spec"; -*-
+%!TEX root = Vorbis_I_spec.tex
+% $Id$
+\section{Helper equations} \label{vorbis:spec:helper}
+
+\subsection{Overview}
+
+The equations below are used in multiple places by the Vorbis codec
+specification.  Rather than cluttering up the main specification
+documents, they are defined here and referenced where appropriate.
+
+
+\subsection{Functions}
+
+\subsubsection{ilog} \label{vorbis:spec:ilog}
+
+The "ilog(x)" function returns the position number (1 through n) of the highest set bit in the two's complement integer value
+\varname{[x]}.  Values of \varname{[x]} less than zero are defined to return zero.
+
+\begin{programlisting}
+  1) [return_value] = 0;
+  2) if ( [x] is greater than zero ) {
+
+       3) increment [return_value];
+       4) logical shift [x] one bit to the right, padding the MSb with zero
+       5) repeat at step 2)
+
+     }
+
+   6) done
+\end{programlisting}
+
+Examples:
+
+\begin{itemize}
+ \item ilog(0) = 0;
+ \item ilog(1) = 1;
+ \item ilog(2) = 2;
+ \item ilog(3) = 2;
+ \item ilog(4) = 3;
+ \item ilog(7) = 3;
+ \item ilog(negative number) = 0;
+\end{itemize}
+
+
+
+
+\subsubsection{float32_unpack} \label{vorbis:spec:float32:unpack}
+
+"float32_unpack(x)" is intended to translate the packed binary
+representation of a Vorbis codebook float value into the
+representation used by the decoder for floating point numbers.  For
+purposes of this example, we will unpack a Vorbis float32 into a
+host-native floating point number.
+
+\begin{programlisting}
+  1) [mantissa] = [x] bitwise AND 0x1fffff (unsigned result)
+  2) [sign] = [x] bitwise AND 0x80000000 (unsigned result)
+  3) [exponent] = ( [x] bitwise AND 0x7fe00000) shifted right 21 bits (unsigned result)
+  4) if ( [sign] is nonzero ) then negate [mantissa]
+  5) return [mantissa] * ( 2 ^ ( [exponent] - 788 ) )
+\end{programlisting}
+
+
+
+\subsubsection{lookup1_values} \label{vorbis:spec:lookup1:values}
+
+"lookup1_values(codebook_entries,codebook_dimensions)" is used to
+compute the correct length of the value index for a codebook VQ lookup
+table of lookup type 1.  The values on this list are permuted to
+construct the VQ vector lookup table of size
+\varname{[codebook_entries]}.
+
+The return value for this function is defined to be 'the greatest
+integer value for which \varname{[return_value]} to the power of
+\varname{[codebook_dimensions]} is less than or equal to
+\varname{[codebook_entries]}'.
+
+
+
+\subsubsection{low_neighbor} \label{vorbis:spec:low:neighbor}
+
+"low_neighbor(v,x)" finds the position \varname{n} in vector \varname{[v]} of
+the greatest value scalar element for which \varname{n} is less than
+\varname{[x]} and vector \varname{[v]} element \varname{n} is less
+than vector \varname{[v]} element \varname{[x]}.
+
+\subsubsection{high_neighbor} \label{vorbis:spec:high:neighbor}
+
+"high_neighbor(v,x)" finds the position \varname{n} in vector [v] of
+the lowest value scalar element for which \varname{n} is less than
+\varname{[x]} and vector \varname{[v]} element \varname{n} is greater
+than vector \varname{[v]} element \varname{[x]}.
+
+
+
+\subsubsection{render_point} \label{vorbis:spec:render:point}
+
+"render_point(x0,y0,x1,y1,X)" is used to find the Y value at point X
+along the line specified by x0, x1, y0 and y1.  This function uses an
+integer algorithm to solve for the point directly without calculating
+intervening values along the line.
+
+\begin{programlisting}
+  1)  [dy] = [y1] - [y0]
+  2) [adx] = [x1] - [x0]
+  3) [ady] = absolute value of [dy]
+  4) [err] = [ady] * ([X] - [x0])
+  5) [off] = [err] / [adx] using integer division
+  6) if ( [dy] is less than zero ) {
+
+       7) [Y] = [y0] - [off]
+
+     } else {
+
+       8) [Y] = [y0] + [off]
+
+     }
+
+  9) done
+\end{programlisting}
+
+
+
+\subsubsection{render_line} \label{vorbis:spec:render:line}
+
+Floor decode type one uses the integer line drawing algorithm of
+"render_line(x0, y0, x1, y1, v)" to construct an integer floor
+curve for contiguous piecewise line segments. Note that it has not
+been relevant elsewhere, but here we must define integer division as
+rounding division of both positive and negative numbers toward zero.
+
+
+\begin{programlisting}
+  1)   [dy] = [y1] - [y0]
+  2)  [adx] = [x1] - [x0]
+  3)  [ady] = absolute value of [dy]
+  4) [base] = [dy] / [adx] using integer division
+  5)    [x] = [x0]
+  6)    [y] = [y0]
+  7)  [err] = 0
+
+  8) if ( [dy] is less than 0 ) {
+
+        9) [sy] = [base] - 1
+
+     } else {
+
+       10) [sy] = [base] + 1
+
+     }
+
+ 11) [ady] = [ady] - (absolute value of [base]) * [adx]
+ 12) vector [v] element [x] = [y]
+
+ 13) iterate [x] over the range [x0]+1 ... [x1]-1 {
+
+       14) [err] = [err] + [ady];
+       15) if ( [err] >= [adx] ) {
+
+             16) [err] = [err] - [adx]
+             17)   [y] = [y] + [sy]
+
+           } else {
+
+             18) [y] = [y] + [base]
+
+           }
+
+       19) vector [v] element [x] = [y]
+
+     }
+\end{programlisting}
+
+
+
+
+
+
+
+