[FFmpeg-devel] [PATCH] atrac decoder

[Benjamin Larsson]

Michael Niedermayer wrote:

> if the groups are a power of 2 you can do
> 0123456789ABCDEF
> ^3
> 32107654...
> 
> thats just a ^C in the index and can be omited when filling whole
> blocks of size C with zeros
> 

When implemented this code

pos = su->bsm[band_num] ? bfu_start_short[bfu_num] :
bfu_start_long[bfu_num];

for (i=0 ; i<num_specs ; i++) {
/* read in a quantized spec and convert it to signed and then inverse
quantization */
   spec[pos + i] = get_sbits(gb, word_len) *
                      sf_tab[su->idsfs[bfu_num]] * max_quant;


becomes this code

if (su->bsm[band_num]) {
 /* get the position of the 1st spec according to the block size mode */
    pos = bfu_start_short[bfu_num];
    for (i=0 ; i<num_specs ; i++) {
        int j = band_num ? ((pos&~31)+((pos+i)^31)) : pos +i;
        spec[j] = get_sbits(gb, word_len) *
                            sf_tab[su->idsfs[bfu_num]] * max_quant;
    }
} else {
 /* get the position of the 1st spec according to the block size mode */
    pos = bfu_start_long[bfu_num];
    for (i=0 ; i<num_specs ; i++) {
        int j = band_num ? 511 - (pos +i) : pos + i;
        /* read in a quantized spec and convert it to signed and then
inverse quantization */
        spec[j] = get_sbits(gb, word_len) *
                            sf_tab[su->idsfs[bfu_num]] * max_quant;
    }
}


I have some bug in there but the code would look something like that. Do
you prefer this solution ?

MvH
Benjamin Larsson