[FFmpeg-devel] [PATCH 2/3] avcodec/aacsbr: Add comment about possibly optimization in sbr_dequant()

[Andreas Cadhalpun]

On 11.12.2015 17:21, Ganesh Ajjanagadde wrote:
> On Fri, Dec 11, 2015 at 11:16 AM, Andreas Cadhalpun
> <andreas.cadhalpun at googlemail.com> wrote:
>> On 19.11.2015 14:17, Michael Niedermayer wrote:
>>> From: Michael Niedermayer <michael at niedermayer.cc>
>>>
>>> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>>> ---
>>>  libavcodec/aacsbr.c |    1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/libavcodec/aacsbr.c b/libavcodec/aacsbr.c
>>> index d1e3a91..e014646 100644
>>> --- a/libavcodec/aacsbr.c
>>> +++ b/libavcodec/aacsbr.c
>>> @@ -73,6 +73,7 @@ static void sbr_dequant(SpectralBandReplication *sbr, int id_aac)
>>>  {
>>>      int k, e;
>>>      int ch;
>>> +    //TODO: Replace exp2f(0.5*x) by a LUT, the inputs are all integer and have a small range
>>>
>>>      if (id_aac == TYPE_CPE && sbr->bs_coupling) {
>>>          float alpha      = sbr->data[0].bs_amp_res ?  1.0f :  0.5f;
>>>
>>
>> This shouldn't hurt, with or without the clarification requested by Ganesh.
> 
> I am doing related work cleaning up and optimizing usages of slow libm
> functions such as pow and exp2. Do you know the exact possible range
> of the inputs x, and if so, can it be added to the comment? That will
> be very helpful for me to come up with a patch. Thanks.

The exp2f expressions are:
exp2f(sbr->data[0].env_facs_q[e][k] * alpha + 7.0f);
exp2f((pan_offset - sbr->data[1].env_facs_q[e][k]) * alpha);
exp2f(NOISE_FLOOR_OFFSET - sbr->data[0].noise_facs_q[e][k] + 1);
exp2f(12 - sbr->data[1].noise_facs_q[e][k]);
exp2f(alpha * sbr->data[ch].env_facs_q[e][k] + 6.0f);
exp2f(NOISE_FLOOR_OFFSET - sbr->data[ch].noise_facs_q[e][k]);

Here alpha is 1 or 0.5, pan_offset 12 or 24 and NOISE_FLOOR_OFFSET is 6.
After patch 3 of this series, env_facs_q is in the range from 0 to 127 and
noise_facs_q is already limited to the range from 0 to 30.

So x should always be in the range -300..300, or so.

Best regards,
Andreas