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

[Ulf Zibis]

Am 19.07.19 um 20:01 schrieb Nicolas George:
> Ulf Zibis (12019-07-19):
>> May be, but I believe single rounding over twice rounding is less fragile.
> Yes. And two plastic bags will slow your fall more than one when you
> jump from a plane.
>
>> 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.
> By a completely negligible difference. 
The wing beat of a butterfly can trigger a hurricane.
> And that is assuming it would hold for other values of 10.2, which I am not convinced.
Then try this:
#define TIME_BASE 12800 // typical time base for 25 fps video
    int false_single = 0, false_twice = 0;
    for (int t = 0; t < 100 * 100; t += 4) {
        char expr_str[16], end, *end_p = &end;
        sprintf(expr_str, "%6g", t/(float)100); // simulate string from
command line
        double expr_parsed = strtod(expr_str, &end_p); //    ... to parse
        int64_t pts = t * TIME_BASE / 100;
        double t_single = pts * 1 / (double)TIME_BASE;
        double t_twice = pts * (double)(1 / (double)TIME_BASE);
        int r_single = expr_parsed == t_single;
        int r_twice = expr_parsed == t_twice;
/*
        printf("time:%6s, pts:%8ld, parsed:%22.18f, t_single:%22.18f=%s,
t_twice:%22.18f=%s\n",
                expr_str, pts, expr_parsed, t_single, r_single ? "true "
: "false", t_twice, r_twice ? "true " : "false");
*/
        false_single += !r_single;
        false_twice += !r_twice;
    }

    printf("single rounding fails: %d, twice rounding fails: %d, in %d
frames\n", false_single, false_twice, 100 * 100 / 4);

Result:
single rounding fails: 0, twice rounding fails: 327, in 2500 frames

> You are wasting your time and making the code less readable for no
> actual benefit.

Finding out, why the select filter doesn't work as I expected, wasted
enough time, so some time more doesn't matter ;-)

-Ulf