[FFmpeg-devel] [PATCH v1] filter select - avoid 2 times rounding

[Ulf Zibis]

Am 19.07.19 um 19:23 schrieb Nicolas George:
> Ulf Zibis (12019-07-19):
>> So please give an alternative theory than a rounding issue, why I
>> actually experience a different result.
> Code with doubles is very fragile, changing anything in the structure of
> your formula will give slightly different results. You were just lucky
> that this change made your use case work.

May be, but I believe single rounding over twice rounding is less fragile.

If I see right, the expression evaluation in libavutil/eval.c only
rounds once, so the chance of equal results against one rounding in
filter select should be higher.

>> In the current code there is a 1st rounding with "(double)(time_base.nom
>> / time_base.den)" and then a 2nd with multipliying this value with
>> "frame->pts". In my code there is only 1 rounding after division of
>> (int64_t)(frame->pts * time_base.nom) with "(double)time_base.den".
> There is no rounding, all is done in double.
Try this:
    printf("single rounding: %.40f\n", 130560LL * 1 / (double)12800);
    printf("twice rounding:  %.40f\n", 130560LL * (double)(1 /
(double)12800));
Result:
single rounding: 10.1999999999999992894572642398998141288757
twice rounding:  10.2000000000000010658141036401502788066864
The 1st result is closer to 10.2 as the 2nd.