On Tue, Aug 24, 2021 at 6:06 PM amindfv--- via Mlt-devel < mlt-devel@lists.sourceforge.net> wrote:
> Thank you, this is exactly the type of clear answer I was hoping for! > > A couple follow-up questions: > > - I seem to be able to, with the calculation on line 371 linked below, > get frame-inaccurate results, i.e. dropped or added frames. For example, > with the simple timecode "01:01:00.1" at 23.976 fps: I think the un-rounded > value should be 87754.5576, so I'd expect the rounded value to be either > 87754 or 87755. However, it's 87753! [0] I think this may be caused by > using "floor" for the hours and minutes, in combination with the fractional > frame rate. > > 23.976 is the rounded form of 24000/1001, which is the more typical value. In any case, from here you can see unit tests that exercise this: https://github.com/mltframework/mlt/blob/master/src/tests/test_properties/test_properties.cpp#L188 A few of these cycle through 9999999 frames (over 11 hours at 240 fps) at some common frame rates to ensure that when you convert from frame number to a clock time value and back that you get the same frame number. But I do not think every frame rate is going to work with the limited precision of milliseconds. > - It seems `lrint` determines its rounding direction depending on values > in the current environment. That seems to mean a user can also lose sample > accuracy if, for example, `lrint` rounds one way on my machine and then a > friend reads my .mlt file with a different rounding direction. Is this true? > > Not to my understanding. The rounding mode can be set programmatically using fesetround() where the "e" refers to an "environment" in the documentation, but that is not the same thing as an environment variable. It refers to the process runtime state and mentions no environment variables. MLT does not call this, and while it is possible an application has that seems rather rare. Thanks, > Tom > > [0] My code: > > #include <stdio.h> > #include <math.h> > > void main(void) { > double fps = 23.976; > int hours = 1; > int minutes = 1; > double seconds = 0.1; > printf("%f\n", floor( fps * hours * 3600 ) + floor( fps * minutes * 60 > ) + lrint( fps * seconds )); > } > > > On Sat, Aug 21, 2021 at 02:25:43AM +0000, Brian Matherly wrote: > > > Am I correct that MLT translates from these clock times to frame > numbers and uses frame numbers for everything internally? > > Yes. "frame number" is often referred to as "position" in the code. I > think the two terms are used interchangably. > > > How does rounding work? > > The code is here: > https://github.com/mltframework/mlt/blob/master/src/framework/mlt_property.c#L371 > > It uses lrint() for rounding the seconds part. > > ~Brian > > > > On Thursday, August 19, 2021, 11:45:00 PM CDT, amindfv--- via > Mlt-devel <mlt-devel@lists.sourceforge.net> wrote: > > > > From reading the "time properties" doc ( > https://www.mltframework.org/blog/time_properties), it seems any number > with a radix is interpreted as a clock time - e.g. "00:00:00.5" means half > a second. > > > > A couple questions: > > > > - Am I correct that MLT translates from these clock times to frame > numbers and uses frame numbers for everything internally? > > - How does rounding work? If I'm working at 24fps, frame number 8 has > clock time 0.3333... and frame 9 has clock time 0.375. If I say > "00:00:00.334", is that identical to saying "00:00:00.374"? And thus > "00:00:00.333" would refer to the earlier frame 7? (I.e. the frame number > is always rounded down?) Or is there some other simple and consistent > translation from these clock times to frame numbers? > > > > Thanks, > > Tom > > > > > > > > _______________________________________________ > > Mlt-devel mailing list > > Mlt-devel@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/mlt-devel > > > > > _______________________________________________ > Mlt-devel mailing list > Mlt-devel@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/mlt-devel >
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