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Support input with non-monotonic dts

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This fixes issue #1.

It appears there are inputs where the dts fluctuates. This is apparently
inputs that are not well formed. However, I discovered that ffmpeg
itself allows for this. If it detects this happening, it updates the
pts/dts of the packet. To do this, it tracks the last output dts. This
ensures the output dts is monotonic even if the input is not.

I use essentially the same approach as in ffmpeg.
This commit is contained in:
Will Storey
2017-03-14 12:00:07 -07:00
parent 0b32184301
commit 734ccb6d6b
2 changed files with 55 additions and 4 deletions
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48
videostreamer.c
View File

@@ -239,6 +239,8 @@ vs_open_output(const char * const output_format_name,
av_dict_free(&opts);
output->last_dts = 0;
return output;
}
@@ -319,8 +321,7 @@ vs_read_packet(const struct VSInput * input, AVPacket * const pkt,
// 1 if we wrote the packet
int
vs_write_packet(const struct VSInput * const input,
const struct VSOutput * const output, AVPacket * const pkt,
const bool verbose)
struct VSOutput * const output, AVPacket * const pkt, const bool verbose)
{
if (!input || !output || !pkt) {
printf("%s\n", strerror(EINVAL));
@@ -357,6 +358,45 @@ vs_write_packet(const struct VSInput * const input,
return -1;
}
// It is possible that the input is not well formed. Its dts (decompression
// timestamp) may fluctuate. av_write_frame() says that the dts must be
// strictly increasing.
//
// Packets from such inputs might look like:
//
// in: pts:18750 pts_time:0.208333 dts:18750 dts_time:0.208333 duration:3750 duration_time:0.0416667 stream_index:1
// in: pts:0 pts_time:0 dts:0 dts_time:0 duration:3750 duration_time:0.0416667 stream_index:1
//
// dts here is 18750 and then 0.
//
// If we try to write the second packet as is, we'll see this error:
// [mp4 @ 0x10f1ae0] Application provided invalid, non monotonically increasing dts to muxer in stream 1: 18750 >= 0
//
// This is apparently a fairly common problem. In ffmpeg.c (as of ffmpeg
// 3.2.4 at least) there is logic to rewrite the dts and warn if it happens.
// Let's do the same. Note my logic is a little different here.
if (pkt->dts != AV_NOPTS_VALUE && output->last_dts != AV_NOPTS_VALUE &&
pkt->dts < output->last_dts) {
int64_t const next_dts = output->last_dts+1;
if (verbose) {
printf("Warning: Non-monotonous DTS in input stream. Previous: %" PRId64 " current: %" PRId64 ". changing to %" PRId64 ".\n",
output->last_dts, pkt->dts, next_dts);
}
// We also apparently (ffmpeg.c does this too) need to update the pts.
// Otherwise we see an error like:
//
// [mp4 @ 0x555e6825ea60] pts (3780) < dts (22531) in stream 0
if (pkt->pts != AV_NOPTS_VALUE && pkt->pts >= pkt->dts) {
pkt->pts = FFMAX(pkt->pts, next_dts);
}
pkt->dts = next_dts;
}
// Set pts/dts if not set. Otherwise we will receive warnings like
//
// [mp4 @ 0x55688397bc40] Timestamps are unset in a packet for stream 0. This
@@ -388,6 +428,10 @@ vs_write_packet(const struct VSInput * const input,
}
// Track last dts we see (see where we use it for why).
output->last_dts = pkt->dts;
// Write encoded frame (as a packet).
// av_interleaved_write_frame() works too, but I don't think it is needed.

11
videostreamer.h
View File

@@ -3,6 +3,7 @@
#include <libavformat/avformat.h>
#include <stdbool.h>
#include <stdint.h>
struct VSInput {
AVFormatContext * format_ctx;
@@ -11,6 +12,13 @@ struct VSInput {
struct VSOutput {
AVFormatContext * format_ctx;
// Track the last dts we output. We use it to double check that dts is
// monotonic.
//
// I am not sure if it is available anywhere already. I tried
// AVStream->info->last_dts and that is apparently not set.
int64_t last_dts;
};
void
@@ -37,7 +45,6 @@ vs_read_packet(const struct VSInput *, AVPacket * const,
int
vs_write_packet(const struct VSInput * const,
const struct VSOutput * const, AVPacket * const,
const bool);
struct VSOutput * const, AVPacket * const, const bool);
#endif