[Vorbis-dev] Error in floor0 decode specification

[Роман Донченко]

Hello,

It seems there is an error in the algorithm for floor0 decode in section
6.2.2 of the Vorbis specification.

Here's the relevant part:

3         3) [coefficients] is an empty, zero length vector
4         4) [booknumber] = read an unsigned integer of ilog(
   [floor0_number_of_books] ) bits
5         5) if ( [booknumber] is greater than the highest number decode
   codebook ) then packet is undecodable
6         6) [last] = zero;
7         7) vector [temp_vector] = read vector from bitstream using
   codebook number [floor0_book_list] element [booknumber] in VQ context.
8         8) add the scalar value [last] to each scalar in vector
   [temp_vector]
9         9) [last] = the value of the last scalar in vector [temp_vector]
10        10) concatenate [temp_vector] onto the end of the [coefficients]
   vector
11        11) if (length of vector [coefficients] is less than
   [floor0_order], continue at step 6

If implemented as written, it would mean that [last] does nothing, since
the only time it is used it always equals to zero. I've looked at the
corresponding code in libvorbis (which seems to be floor0_inverse1 in
floor0.c), and it does this:

         float last=0.f;

         /* ... */

         for(j=0;j<look->m;){
           for(k=0;k<b->dim;k++,j++)lsp[j]+=last;
           last=lsp[j-1];
         }

which suggests that step 11 in the algorithm should say "continue at step
7" (skipping the initialization of [last]).

Coincidentally, there's also a cosmetic error in the same algorithm - step
7 is too long and is truncated in the PDF version.

Roman.