On 02/15/2021 06:38 AM, Chris Angelico wrote:
On Mon, Feb 15, 2021 at 9:21 PM Mark Filipak (ffmpeg)
<markfili...@bog.us> wrote:
On 02/15/2021 01:56 AM, Chris Angelico wrote:
On Mon, Feb 15, 2021 at 5:32 PM Mark Filipak (ffmpeg)
<markfili...@bog.us> wrote:
frame->pts = (
(s->start_time == AV_NOPTS_VALUE) ? 0 : s->start_time) +
av_rescale(outlink->frame_count_in, s->ts_unit.num, s->ts_unit.den);
I don't know what this: 'frame->pts', means. ...
struct FrameyThingyWhatsit {
int foo;
int bar;
void *quux;
int pts;
const char *flurble;
};
"frame" is a variable, declared to be a pointer to a
FrameyThingyWhatsit, and assigned to something. It might be something
like this:
struct FrameyThingyWhatsit *frame;
frame = malloc(sizeof(FrameyThingyWhatsit));
That'll allocate enough memory for the entire structure, and then
"frame" is a pointer to the start of that structure. Just like with
arrays ...
"Thunk"! ...The sound of pieces falling in place.
So this:
struct FrameyThingyWhatsit {...int pts;...} ...a 'C' structure referenced by
'FrameyThingyWhatsit->pts'
is equivalent to this:
FrameyThingyWhatsit = [...pts,...] ...an array referenced by 'FrameyThingyWhatsit[3]' (except that
mixed datatypes in arrays are verboten)
or maybe better analogy, to this:
FrameyThingyWhatsit = {...pts,...} ...an object referenced by
'FrameyThingyWhatsit.pts'
... you hang onto a pointer to the base of it, and then you index
into it; the compiler knows the layout of the structure, so when you
say "frame->pts", it knows exactly how many bytes above the base of
the structure to find that member.
Ah, so there can be
struct thisframe {...int pts;...}
struct thatframe {...int pts;...}
struct otherframe {...int pts;...}
referenced by 'thisframe->pts', 'thatframe->pts', and 'otherframe->pts', and the 3 'pts's are
independent of one another, sort of like 3 namespaces.
I think I have it right, or do I?
But I think Carl's point was that you can't simply look at an
expression and decode it as algebra. To understand this line of code,
you not only have to interpret the syntax of this exact line, but -
and probably more importantly - you have to understand what the
av_rescale function is doing.
Well, I think I know what 'av_rescale()' is doing (at least for the
'minterpolate=fps=48/1.001' &
'telecine' filters). I ran some tests. Look:
Uhh..... cool. This is the part where I'm the one who has no clue
what's going on, so I'm just going to do the whole "smile and nod"
thing, and trust that you know what's going on :)
Well, here's the thing: At times PTSs are perfect [1], or sometimes they're bollixed but the video
nonetheless plays properly [2], or sometimes it doesn't play properly [3].
[1] 60/1.001fps example: MPV shows "FPS: 59.940 (specified) 59.940 (estimated)"
[2] 60/1.001fps example: MPV shows "FPS: 60.025 (specified) 59.940 (estimated)"
[3] 60/1.001fps example: MPV shows "FPS: 59.940 (specified) ...wanders...
(estimated)"
That playback behavior is the 1st clue I got that PTSs are key. Why are there MPV differences
remains a mystery -- I assume that it indicates that the PTSs are (may be) correct but the video's
metadata is wrong. Of course, my inference may be wrong -- ffmpeg investigation is like reading a
murder mystery.
Glad I could help! You're on your own for the next part, but if I've
been of any value at all, that's awesome. All the best!
ChrisA
You did well, Chris, and you did good. Thanks so much. I feel I know what 'C' structures are and how
the '->' operator works -- before you, I didn't even know that '"struct "+label+"{ "+type+" "+var"
}"' and 'label+"->"+var"' were related. The bottom line is this: A struct is the 'C'-way to
instantiate an object -- Yes, I'm aware of C++. It's sort of a 'C' language architecture hack, isn't
it?
--
Any journey, no matter how long, is just a series of small steps.
"Government is the problem!" -- 1982 and onward.
"_______ is the enemy of the people!" -- 2016 and onward.
"You have to fight like hell or you're not going to have a country!" -- Jan 6,
2021.
It isn't the distance that's important, it's the direction.
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