# HG changeset patch
# User Tracey Jaquith <tracey@archive.org>
# Date 1623797180 0
# Tue Jun 15 22:46:20 2021 +0000
# Node ID 1879d49fe0cf739f48287b5a38a83d3a1adab939
# Parent 5f765427c17ac8cf753967387562201cf4f78dc4
Add optional "mp4_exact_start" nginx config off/on to show video between keyframes.

archive.org has been using mod_h264_streaming with a similar "exact start" patch from me since 2013.
We just moved to nginx mp4 module and are using this patch.
The technique is to find the video 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&exact=1

Tested on linux and macosx.

(this is me: https://github.com/traceypooh )

diff -r 5f765427c17a -r 1879d49fe0cf 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 Tue Jun 15 22:46:20 2021 +0000
@@ -43,6 +43,7 @@
typedef struct {
size_t buffer_size;
size_t max_buffer_size;
+ ngx_flag_t exact_start;
} ngx_http_mp4_conf_t;


@@ -340,6 +341,13 @@
offsetof(ngx_http_mp4_conf_t, max_buffer_size),
NULL },

+ { ngx_string("mp4_exact_start"),
+ NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
+ ngx_conf_set_flag_slot,
+ NGX_HTTP_LOC_CONF_OFFSET,
+ offsetof(ngx_http_mp4_conf_t, exact_start),
+ NULL },
+
ngx_null_command
};

@@ -2156,6 +2164,83 @@


static ngx_int_t
+ngx_http_mp4_exact_start_video(ngx_http_mp4_file_t *mp4, ngx_http_mp4_trak_t *trak)
+{
+ uint32_t n, speedup_samples, current_count;
+ ngx_uint_t sample_keyframe, start_sample_exact;
+ ngx_mp4_stts_entry_t *entry, *entries_array;
+ ngx_buf_t *data;
+
+ data = trak->out[NGX_HTTP_MP4_STTS_DATA].buf;
+
+ // Find the keyframe just before the desired start time - so that we can emit an mp4
+ // where the first frame is a keyframe. We'll "speed up" the first frames to 1000x
+ // normal speed (typically), so they won't be noticed. But this way, perceptively,
+ // playback of the _video_ track can start immediately
+ // (and not have to wait until the keyframe _after_ the desired starting time frame).
+ 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);
+
+ entries_array = ngx_palloc(mp4->request->pool,
+ (1 + trak->time_to_sample_entries) * sizeof(ngx_mp4_stts_entry_t));
+ if (entries_array == NULL) {
+ return NGX_ERROR;
+ }
+ entry = &(entries_array[1]);
+ ngx_memcpy(entry, (ngx_mp4_stts_entry_t *)data->pos,
+ trak->time_to_sample_entries * sizeof(ngx_mp4_stts_entry_t));
+
+ current_count = ngx_mp4_get_32value(entry->count);
+ 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;
+ }
+
+ 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);
+ }
+
+ return NGX_OK;
+}
+
+
+static ngx_int_t
ngx_http_mp4_crop_stts_data(ngx_http_mp4_file_t *mp4,
ngx_http_mp4_trak_t *trak, ngx_uint_t start)
{
@@ -2164,6 +2249,8 @@
ngx_buf_t *data;
ngx_uint_t start_sample, entries, start_sec;
ngx_mp4_stts_entry_t *entry, *end;
+ ngx_http_mp4_conf_t *conf;
+

if (start) {
start_sec = mp4->start;
@@ -2238,6 +2325,10 @@
"start_sample:%ui, new count:%uD",
trak->start_sample, count - rest);

+ conf = ngx_http_get_module_loc_conf(mp4->request, ngx_http_mp4_module);
+ if (conf->exact_start) {
+ ngx_http_mp4_exact_start_video(mp4, trak);
+ }
} else {
ngx_mp4_set_32value(entry->count, rest);
data->last = (u_char *) (entry + 1);
@@ -3590,6 +3681,7 @@

conf->buffer_size = NGX_CONF_UNSET_SIZE;
conf->max_buffer_size = NGX_CONF_UNSET_SIZE;
+ conf->exact_start = NGX_CONF_UNSET;

return conf;
}
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