[FFmpeg-devel] [PATCH] AAC decoder round 8

[Michael Niedermayer]

On Sun, Aug 17, 2008 at 11:12:55PM +0100, Robert Swain wrote:
> 2008/8/15 Robert Swain <robert.swain at gmail.com>:
> > 2008/8/15 Michael Niedermayer <michaelni at gmx.at>:
> >> On Fri, Aug 15, 2008 at 09:04:24AM +0100, Robert Swain wrote:
> 
> [...]
> 
> >>> Index: libavcodec/aac.c
> >>> ===================================================================
> >>> --- libavcodec/aac.c  (revision 14774)
> >>> +++ libavcodec/aac.c  (working copy)
> >> [...]
> >>> +
> >>> +                for (i = 0; i < tns->order[w][filt]; i++)
> >>> +                    tns->coef[w][filt][i] = get_bits(gb, coef_len);
> >>
> >> tns->coef is only used to index into tmp2_map thus
> >> tns->coef could already contain the values from tmp2_map
> >> this also would make the tmp2_map field unneeded in the struct
> >
> > OK, I'll look into this.
> 
> See attached. I assume this is what you mean.
> 
> [...]
> 
> >>> Index: libavcodec/aacdectab.h
> >>> ===================================================================
> >>> --- libavcodec/aacdectab.h    (revision 14767)
> >>> +++ libavcodec/aacdectab.h    (working copy)
> >
> >>> +
> >>> +/* @name tns_tmp2_map
> >>> + * Tables of the tmp2[] arrays of LPC coefficients used for TNS.
> >>> + * The suffix _M_N[] indicate the values of coef_compress and coef_res
> >>> + * respectively.
> >>> + * @{
> >>> + */
> >>> +static const float tns_tmp2_map_1_3[TNS_MAX_ORDER] = {
> >>> +     0.00000000,  0.43388373, -0.64278758, -0.34202015,
> >>> +     0.97492790,  0.78183150, -0.64278758, -0.34202015,
> >>> +    -0.43388373, -0.78183150, -0.64278758, -0.34202015,
> >>> +    -0.78183150, -0.43388373, -0.64278758, -0.34202015,
> >>> +     0.78183150,  0.97492790, -0.64278758, -0.34202015
> >>> +};
> >>> +
> >>> +static const float tns_tmp2_map_0_3[TNS_MAX_ORDER] = {
> >>> +     0.00000000,  0.43388373,  0.78183150,  0.97492790,
> >>> +    -0.98480773, -0.86602539, -0.64278758, -0.34202015,
> >>> +    -0.43388373, -0.78183150, -0.97492790, -0.97492790,
> >>> +    -0.98480773, -0.86602539, -0.64278758, -0.34202015,
> >>> +     0.78183150,  0.97492790,  0.97492790,  0.78183150
> >>> +};
> >>> +
> >>> +static const float tns_tmp2_map_1_4[TNS_MAX_ORDER] = {
> >>> +     0.00000000,  0.20791170,  0.40673664,  0.58778524,
> >>> +    -0.67369562, -0.52643216, -0.36124167, -0.18374951,
> >>> +     0.99452192,  0.95105648,  0.86602539,  0.74314481,
> >>> +    -0.67369562, -0.52643216, -0.36124167, -0.18374951,
> >>> +    -0.20791176, -0.40673670, -0.58778530, -0.74314487
> >>> +};
> >>> +
> >>> +static const float tns_tmp2_map_0_4[TNS_MAX_ORDER] = {
> >>> +     0.00000000,  0.20791170,  0.40673664,  0.58778524,
> >>> +     0.74314481,  0.86602539,  0.95105654,  0.99452192,
> >>> +    -0.99573416, -0.96182561, -0.89516330, -0.79801720,
> >>> +    -0.67369562, -0.52643216, -0.36124167, -0.18374951,
> >>> +    -0.20791176, -0.40673670, -0.58778530, -0.74314487
> >>> +};
> >>
> >> iam not sure if the code is correct but i think several of these
> >> elements can never be accessed
> >
> > I'll check the code.
> 
> The current code only accesses elements 0-15 by my reckoning. 

actually i think it accesses fewer

lets just look at the table 3rd table, to access this implicates that
tmp2_idx=2
which implicates
coef_res=0
coef_compress=1

and
coef_len = coef_res + 3 - coef_compress;
which makes it 2
thus only 4 elements of the 3rd table could be accssed
unless i missed something
also the tables contain plenty of repeated elementw which makes no
sense at all in relation to the code as several indexes would be
redundant ...




> I'm
> working through the code for tns_decode_coef() in the spec to see if
> it matches what we have here.

patch ok

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Many that live deserve death. And some that die deserve life. Can you give
it to them? Then do not be too eager to deal out death in judgement. For
even the very wise cannot see all ends. -- Gandalf
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