Tracey Jaquith
June 13, 2021 05:54AM
Roman - genius that works! :)

I’ll formalize this towards using an nginx config var exact setting and some minor cleanup, but here’s the new smaller patch, logically, with your idea:

Sample item working with all sort of clip testing, and a start/end with prior patch and this patch yielded bitwise identical emitted mp4 :)


Method: ngx_http_mp4_crop_stts_data()

if (start) {
ngx_mp4_set_32value(entry->count, count - rest);
data->pos = (u_char *) entry;
trak->time_to_sample_entries = entries;
trak->start_sample = start_sample;

ngx_log_debug2(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
"start_sample:%ui, new count:%uD",
trak->start_sample, count - rest);

+ // xxx only do this if nginx mp4 config exact setting is set
+ uint32_t n, speedup_samples;
+ ngx_uint_t sample_keyframe, start_sample_exact;
+
+ start_sample_exact = trak->start_sample;
+ for (n = 0; n < trak->sync_samples_entries; n++) {
+ // each element of array is the sample number of a keyframe
+ // sync samples starts from 1 -- so subtract 1
+ sample_keyframe = ngx_mp4_get_32value(trak->stss_data_buf.pos + (n * 4)) - 1;
+ if (sample_keyframe <= trak->start_sample) {
+ start_sample_exact = sample_keyframe;
+ }
+ if (sample_keyframe >= trak->start_sample) {
+ break;
+ }
+ }
+
+ if (start_sample_exact < trak->start_sample) {
+ // We're going to prepend an entry with duration=1 for the frames we want to "not see".
+ // MOST of the time (eg: constant video framerate),
+ // we're taking a single element entry array and making it two.
+ speedup_samples = trak->start_sample - start_sample_exact;
+
+ ngx_log_debug3(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
+ "exact trak start_sample move %l to %l (speed up %d samples)\n",
+ trak->start_sample, start_sample_exact, speedup_samples);
+
+ uint32_t current_count = ngx_mp4_get_32value(entry->count);
+ ngx_mp4_stts_entry_t* entries_array = ngx_palloc(mp4->request->pool,
+ (1 + entries) * sizeof(ngx_mp4_stts_entry_t));
+ if (entries_array == NULL) {
+ return NGX_ERROR;
+ }
+ ngx_copy(&(entries_array[1]), entry, entries * sizeof(ngx_mp4_stts_entry_t));
+
+ ngx_log_debug1(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
+ "exact split in 2 video STTS entry from count:%d", current_count);
+
+ if (current_count <= speedup_samples) {
+ return NGX_ERROR;
+ }
+
+ entry = &(entries_array[1]);
+ ngx_mp4_set_32value(entry->count, current_count - speedup_samples);
+ ngx_log_debug2(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
+ "exact split new[1]: count:%d duration:%d",
+ ngx_mp4_get_32value(entry->count),
+ ngx_mp4_get_32value(entry->duration));
+ entry--;
+ ngx_mp4_set_32value(entry->count, speedup_samples);
+ ngx_mp4_set_32value(entry->duration, 1);
+ ngx_log_debug1(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
+ "exact split new[0]: count:%d duration:1",
+ ngx_mp4_get_32value(entry->count));
+
+ data->pos = (u_char *) entry;
+ trak->time_to_sample_entries++;
+ trak->start_sample = start_sample_exact;
+ data->last = (u_char *) (entry + trak->time_to_sample_entries);
+ }
} else {
ngx_mp4_set_32value(entry->count, rest);
data->last = (u_char *) (entry + 1);
trak->time_to_sample_entries -= entries - 1;
trak->end_sample = trak->start_sample + start_sample;

ngx_log_debug2(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
"end_sample:%ui, new count:%uD",
trak->end_sample, rest);
}

Thanks for that great insight!
-Tracey



> On Jun 12, 2021, at 4:01 PM, Tracey Jaquith <tracey@archive.org> wrote:
>
> Hello Roman,
>
>> On Jun 9, 2021, at 5:10 AM, Roman Arutyunyan <arut@nginx.com <mailto:arut@nginx.com>> wrote:
>>
>> Hello Tracey.
>>
>> Thanks for your patch, it looks very interesting.
>
> Yay! :)
>
>>
>> On Thu, Jun 03, 2021 at 07:54:49PM +0000, Tracey Jaquith wrote:
>>> # HG changeset patch
>>> # User Tracey Jaquith <tracey@archive.org <mailto:tracey@archive.org>>
>>> # Date 1622678642 0
>>> # Thu Jun 03 00:04:02 2021 +0000
>>> # Node ID 5da9c62fa61016600f2c59982ae184e2811be427
>>> # Parent 5f765427c17ac8cf753967387562201cf4f78dc4
>>> Add optional "&exact=1" CGI arg to show video between keyframes.
>>
>> I think this can be a directive (like mp4_buffer_size/mp4_max_buffer_size)
>> rather than an argument. Is it possible than we need both exact and non-exact
>> in the same location? It feels like we don't need to give clients control over
>> this. The exact mode introduces some overhead and should only be used when
>> needed.
>
> That’s a fine idea.
> So far, we’ve been using “exact clipping” at archive.org http://archive.org/ both ways — but there’s some interest from our TV team to just always do the “exact” mode, anyway.
> So that could work for us.
> That something I should look into for a revised / 2nd patch, sounds like?
>
>>
>>> archive.org http://archive.org/ has been using mod_h264_streaming with an "&exact=1" patch from me since 2013.
>>> We just moved to nginx mp4 module and are using this patch.
>>> The technique is to find the keyframe just before the desired "start" time, and send
>>> that down the wire so video playback can start immediately.
>>> Next calculate how many video samples are between the keyframe and desired "start" time
>>> and update the STTS atom where those samples move the duration from (typically) 1001 to 1.
>>> This way, initial unwanted video frames play at ~1/30,000s -- so visually the
>>> video & audio start playing immediately.
>>>
>>> You can see an example before/after here (nginx binary built with mp4 module + patch):
>>>
>>> https://pi.archive.org/0/items/CSPAN_20160425_022500_2011_White_House_Correspondents_Dinner.mp4?start=12&end=30 https://pi.archive.org/0/items/CSPAN_20160425_022500_2011_White_House_Correspondents_Dinner.mp4?start=12&end=30
>>> https://pi.archive.org/0/items/CSPAN_20160425_022500_2011_White_House_Correspondents_Dinner.mp4?start=12&end=30&exact=1
>>>
>>> Tested on linux and macosx.
>>>
>>> I realize two var declarations should style-wise get moved "up" in the lower hooked in method.
>>> However, to minimize the changeset/patch, I figured we should at least start here and see what you folks think.
>>
>> Syle-wise there are a few minor issues. We'll figure these out later.
>
> Perfect thanks. I figured _at least_ those var decls but am sure there are probably some more since I’m stepping into your house :)
>
>>
>> Below are my comments on ways to simplify the code.
>>
>>> (this is me: https://github.com/traceypooh https://github.com/traceypooh )
>>>
>>> diff -r 5f765427c17a -r 5da9c62fa610 src/http/modules/ngx_http_mp4_module.c
>>> --- a/src/http/modules/ngx_http_mp4_module.c Tue Jun 01 17:37:51 2021 +0300
>>> +++ b/src/http/modules/ngx_http_mp4_module.c Thu Jun 03 00:04:02 2021 +0000
>>> @@ -2045,6 +2045,109 @@
>>> u_char duration[4];
>>> } ngx_mp4_stts_entry_t;
>>>
>>> +typedef struct {
>>> + uint32_t speedup_samples;
>>> + ngx_uint_t speedup_seconds;
>>> +} ngx_mp4_exact_t;
>>> +
>>> +static void
>>> +exact_video_adjustment(ngx_http_mp4_file_t *mp4, ngx_http_mp4_trak_t *trak, ngx_mp4_exact_t *exact)
>>> +{
>>> + // will parse STTS -- time-to-sample atom
>>> + ngx_str_t value;
>>> + ngx_buf_t *stts_data;
>>> + ngx_buf_t *atom;
>>> + ngx_mp4_stts_entry_t *stts_entry, *stts_end;
>>> + uint32_t count, duration, j, n, sample_keyframe, sample_num;
>>> + uint64_t sample_time, seconds, start_seconds_closest_keyframe;
>>> + uint8_t is_keyframe;
>>> +
>>> + exact->speedup_samples = 0;
>>> + exact->speedup_seconds = 0;
>>> +
>>> + // if '&exact=1' CGI arg isn't present, do nothing
>>> + if (!(ngx_http_arg(mp4->request, (u_char *) "exact", 5, &value) == NGX_OK)) {
>>> + return;
>>> + }
>>> +
>>> + if (!trak->sync_samples_entries) {
>>> + // Highly unlikely video STSS got parsed and _every_ sample is a keyframe.
>>> + // However, if the case, we don't need to adjust the video at all.
>>> + return;
>>> + }
>>> +
>>> + // check HDLR atom to see if this trak is video or audio
>>> + atom = trak->out[NGX_HTTP_MP4_HDLR_ATOM].buf;
>>> + // 'vide' or 'soun'
>>> + if (!(atom->pos[16] == 'v' &&
>>> + atom->pos[17] == 'i' &&
>>> + atom->pos[18] == 'd' &&
>>> + atom->pos[19] == 'e')) {
>>> + return; // do nothing if not video
>>> + }
>>
>> Do we really need to check this? If a track has an stss atom, this seems
>> enough to do the work.
>
> With the .mp4 that we make at archive.org http://archive.org/, I am seeing STSS consistently for the audio track.
> That’s pointing out the start times of the audio samples, so that sort of makes sense to me.
> So I found we needed a way to do nothing for any audio tracks.
>
> I guess… we _could_ maybe run the same logic on any audio track(s) since, presumably,
> each audio sample is a “keyframe” (and so theoretically we should do nothing below)?
> If you’d prefer something like that, I’d need to do a little digging/testing.
>
>
>>
>>> + stts_data = trak->out[NGX_HTTP_MP4_STTS_DATA].buf;
>>> + stts_entry = (ngx_mp4_stts_entry_t *) stts_data->pos;
>>> + stts_end = (ngx_mp4_stts_entry_t *) stts_data->last;
>>> +
>>> + sample_num = 0; // they start at one <shrug>
>>> + sample_time = 0;
>>> + start_seconds_closest_keyframe = 0;
>>> + while (stts_entry < stts_end) {
>>> + // STTS === time-to-sample atom
>>> + // each entry is 4B and [sample count][sample duration] (since durations can vary)
>>> + count = ngx_mp4_get_32value(stts_entry->count);
>>> + duration = ngx_mp4_get_32value(stts_entry->duration);
>>> +
>>> + for (j = 0; j < count; j++) {
>>> + sample_num++;
>>> +
>>> + // search STSS sync sample entries to see if this sample is a keyframe
>>> + is_keyframe = (trak->sync_samples_entries ? 0 : 1);
>>
>> Considering the above, trak->sync_samples_entries is always non-zero here.
>
> Oh, excellent eyes, thanks!
> I will update that to
> is_keyframe = 0;
>
>
>>> + for (n = 0; n < trak->sync_samples_entries; n++) {
>>> + // each one of this these are a video sample number keyframe
>>> + sample_keyframe = ngx_mp4_get_32value(trak->stss_data_buf.pos + (n * 4));
>>> + if (sample_keyframe == sample_num) {
>>> + is_keyframe = 1;
>>> + break;
>>> + }
>>> + if (sample_keyframe > sample_num) {
>>> + break;
>>> + }
>>> + }
>>> +
>>> + seconds = sample_time * 1000 / trak->timescale;
>>> + sample_time += duration;
>>> +
>>> + if (seconds > mp4->start) {
>>> + goto found;
>>> + }
>>> +
>>> + if (is_keyframe) {
>>> + start_seconds_closest_keyframe = seconds;
>>> + exact->speedup_samples = 0;
>>> + } else {
>>> + exact->speedup_samples++;
>>> + }
>>> + }
>>> +
>>> + stts_entry++;
>>> + }
>>> +
>>> + found:
>>> +
>>> + ngx_log_debug2(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
>>> + "exact_video_adjustment() new keyframe start: %d, speedup first %d samples",
>>> + start_seconds_closest_keyframe,
>>> + exact->speedup_samples);
>>> +
>>> + // NOTE: begin 1 start position before keyframe to ensure first video frame emitted is always
>>> + // a keyframe
>>> + exact->speedup_seconds = mp4->start - start_seconds_closest_keyframe
>>> + - (start_seconds_closest_keyframe ? 1 : 0);
>>> +}
>>> +
>>>
>>> static ngx_int_t
>>> ngx_http_mp4_read_stts_atom(ngx_http_mp4_file_t *mp4, uint64_t atom_data_size)
>>> @@ -2164,10 +2267,14 @@
>>> ngx_buf_t *data;
>>> ngx_uint_t start_sample, entries, start_sec;
>>> ngx_mp4_stts_entry_t *entry, *end;
>>> + ngx_mp4_exact_t exact;
>>>
>>> if (start) {
>>> start_sec = mp4->start;
>>>
>>> + exact_video_adjustment(mp4, trak, &exact);
>>> + start_sec -= exact.speedup_seconds;
>>
>> Here you find the closest key frame from the past and adjust start_sec to
>> match it. But what if we don't do this here, but continue with the original
>> start_sec. When we find the right (probably non-key) frame, we'll have its
>> number in start_sample. All we'll need to do at that point is to search for
>> the closest key frame to that sample number - just a simple loop. When we
>> find the number of samples into the past until the most recent key frame,
>> we'll just add one entry to the stts array with (this number, 1) and then
>> decrease start_sample accordingly. Sounds simpler.
>
> Ah, ooh, that’s very interesting! I like the sound of it.
> So we’d use
> trak->start_sample
> Sounds like, and go… either forwards or backwards in a simple loop over the
> trak->sync_samples_entries
> until we find the the keyframe entry number that is just less than or equal to the
> trak->start_sample
> Does that sound right?
>
> And I think we could be adding up to 300 STTS entries compared to the non-exact method
> (depending on video, but assuming up to 10s between keyframes and 30fps).
>
>
>>
>>> +
>>> ngx_log_debug1(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
>>> "mp4 stts crop start_time:%ui", start_sec);
>>>
>>> @@ -2230,6 +2337,42 @@
>>>
>>> if (start) {
>>> ngx_mp4_set_32value(entry->count, count - rest);
>>> +
>>> + if (exact.speedup_samples) {
>>> + // We're going to prepend an entry with duration=1 for the frames we want to "not see".
>>> + // MOST of the time, we're taking a single element entry array and making it two.
>>> + uint32_t current_count = ngx_mp4_get_32value(entry->count);
>>> + ngx_mp4_stts_entry_t* entries_array = ngx_palloc(mp4->request->pool,
>>> + (1 + entries) * sizeof(ngx_mp4_stts_entry_t));
>>> + if (entries_array == NULL) {
>>> + return NGX_ERROR;
>>> + }
>>
>> I believe there are situations when there's an entry from the past in the old
>> array, that can be reused.
>
> So this is indeed a _very_ interesting point!
> Most of the time, certainly for our created .mp4, we’re talking about constant frame rates for the video.
> (The audio is indeed often a huge list of entries w/ minor variance in their durations).
> So for the video, I’m mostly only ever seeing a single entry list with duration “1001”
> (For the 29.97 fps typical TV we see in US where it’s 30000 / 1001 => 29.97).
>
> I did start trying to make the logical list split anywhere it might be found for 2+ entries,
> but it _really_ was making my head hurt since there were so many computations to do
> while trying to figure out where all the frames were, and where we might be splitting.
>
> In the end, I found I have almost entirely single-entry arrays, splitting into two.
> (Though the current patch doesn’t care what the number of array elements is — it just
> prepends a new entry (of duration 1 instead of typical 1001) via a logical `realloc()`)
>
> If you’d prefer something for 2+ entries case, that tries to see if we’re dropping enough entries from
> the front — and to just reuse one (and not realloc — though we’d <still> need a realloc case for
> corner cases anyway, _i think_) then perhaps I could use some help or thoughts on that since
> it was getting super complicated when I was trying to diagram it out and was fretting and
> nearly paralyzed with how to proceed cleanly and safely :-p
>
> Thanks so much for the detailed feedback and time for your thoughts!
> Super appreciated and I’m very upbeat about this all.
>
> -Tracey
>
>>
>>> + ngx_copy(&(entries_array[1]), entry, entries * sizeof(ngx_mp4_stts_entry_t));
>>> +
>>> + ngx_log_debug1(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
>>> + "exact split in 2 video STTS entry from count:%d", current_count);
>>> +
>>> + if (current_count <= exact.speedup_samples)
>>> + return NGX_ERROR;
>>> +
>>> + entry = &(entries_array[1]);
>>> + ngx_mp4_set_32value(entry->count, current_count - exact.speedup_samples);
>>> + ngx_log_debug2(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
>>> + "exact split new[1]: count:%d duration:%d",
>>> + ngx_mp4_get_32value(entry->count),
>>> + ngx_mp4_get_32value(entry->duration));
>>> + entry--;
>>> + ngx_mp4_set_32value(entry->count, exact.speedup_samples);
>>> + ngx_mp4_set_32value(entry->duration, 1);
>>> + ngx_log_debug1(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
>>> + "exact split new[0]: count:%d duration:1",
>>> + ngx_mp4_get_32value(entry->count));
>>> +
>>> + data->last = (u_char *) (entry + 2);
>>> +
>>> + entries++;
>>> + }
>>> +
>>> data->pos = (u_char *) entry;
>>> trak->time_to_sample_entries = entries;
>>> trak->start_sample = start_sample;
>>> _______________________________________________
>>> nginx-devel mailing list
>>> nginx-devel@nginx.org <mailto:nginx-devel@nginx.org>
>>> http://mailman.nginx.org/mailman/listinfo/nginx-devel
>>
>> --
>> Roman Arutyunyan
>> _______________________________________________
>> nginx-devel mailing list
>> nginx-devel@nginx.org <mailto:nginx-devel@nginx.org>
>> http://mailman.nginx.org/mailman/listinfo/nginx-devel
>
> -Tracey
> @tracey_pooh
> TV Architect https://archive.org/tv https://archive.org/tv
-Tracey
@tracey_pooh
TV Architect https://archive.org/tv https://archive.org/tv





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Subject Author Views Posted

[PATCH] Add optional "&exact=1" CGI arg to show video between keyframes

Tracey Jaquith 204 June 03, 2021 03:56PM

Re: [PATCH] Add optional "&exact=1" CGI arg to show video between keyframes

Maxim Dounin 40 June 04, 2021 01:40PM

Re: [PATCH] Add optional "&exact=1" CGI arg to show video between keyframes

Tracey Jaquith 50 June 04, 2021 10:20PM

Re: [PATCH] Add optional "&exact=1" CGI arg to show video between keyframes

Roman Arutyunyan 45 June 09, 2021 08:12AM

Re: [PATCH] Add optional "&exact=1" CGI arg to show video between keyframes

Tracey Jaquith 46 June 12, 2021 11:46PM

Re: [PATCH] Add optional "&exact=1" CGI arg to show video between keyframes

Tracey Jaquith 55 June 13, 2021 05:54AM



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