[FFmpeg-devel] [PATCH 4/9] avfilter/af_flanger: use rint instead of floor hack

Ganesh Ajjanagadde gajjanagadde at gmail.com
Wed Dec 2 13:29:26 CET 2015

On Wed, Dec 2, 2015 at 1:18 AM, Paul B Mahol <onemda at gmail.com> wrote:
> On 12/2/15, Ganesh Ajjanagadde <gajjanagadde at gmail.com> wrote:
>> Signed-off-by: Ganesh Ajjanagadde <gajjanagadde at gmail.com>
>> ---
>>  libavfilter/af_flanger.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> diff --git a/libavfilter/af_flanger.c b/libavfilter/af_flanger.c
>> index f8ec830..a92367c 100644
>> --- a/libavfilter/af_flanger.c
>> +++ b/libavfilter/af_flanger.c
>> @@ -130,7 +130,7 @@ static int config_input(AVFilterLink *inlink)
>>          return AVERROR(ENOMEM);
>>      ff_generate_wave_table(s->wave_shape, AV_SAMPLE_FMT_FLT, s->lfo,
>> s->lfo_length,
>> -                           floor(s->delay_min * inlink->sample_rate + 0.5),
>> +                           rint(s->delay_min * inlink->sample_rate),
>>                             s->max_samples - 2., 3 * M_PI_2);
>>      return av_samples_alloc_array_and_samples(&s->delay_buffer, NULL,
>> --
>> 2.6.2
>> _______________________________________________
>> ffmpeg-devel mailing list
>> ffmpeg-devel at ffmpeg.org
>> http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
> Have you checked that output is same?

Well if is not, rint is more accurate than floor, this is the whole
point of the patch. What I can tell is that FATE passes.

One can craft input so that floor(x + 0.5) is not identical to
rint(x), and that is the point of these patches - to be more accurate
when we can be. A simple example: what happens at half-integers, i.e
1.5? Then, floor always returns the next up, e.g 2.0, while rint(x)
rounds to the nearest even integer in accord with IEEE-754. This is
done to reduce rounding biases on floating point numbers - think of a
large number of half integer samples, the floor hack results in
consistent upward bias, the rint (or llrint, lrint more generally)
avoids this.

I care about technical purity of filters; you seem to care about
copying it over from some other place and matching another filter
exactly, regardless of the quality of such filters. In that case, I
think FFmpeg's monolithic filter design needs to be reconsidered; we
should allow seamless integration of external filters. These two goals
are at odds with each other, and I will always personally prefer the
first, since it actually allows greater flexibility for improvements.
Ultimately, I am not a maintainer for these things and I have no say
on the matter or personal interest in it.

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