What does gstreamer's h264parse really do?

Question

I'm confused with what h264parse really does.

I tested with this commands:

with h264parse:

gst-launch-1.0 videotestsrc num-buffers=10 ! x264enc ! h264parse ! avdec_h264 ! videoconvert ! autovideosink

without h264parse:

gst-launch-1.0 videotestsrc num-buffers=10 ! x264enc ! avdec_h264 ! videoconvert ! autovideosink

it makes no difference and both work fine.

Then I tried to save h264 into file and then open it.

save it into file:

gst-launch-1.0  videotestsrc num-buffers=10 ! x264enc ! filesink location=videotestsrc.h264

Open it with h264parse, it works ok and I can see the video:

gst-launch-1.0 filesrc location=videotestsrc.h264 ! h264parse ! avdec_h264 ! videoconvert ! autovideosink

But if I open it without h264parse:

gst-launch-1.0 filesrc location=videotestsrc.h264 ! avdec_h264 ! videoconvert ! autovideosink

it does not work and the error message is like this:

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
ERROR: from element /GstPipeline:pipeline0/avdec_h264:avdec_h264-0: GStreamer error: negotiation problem.
Additional debug info:
gstvideodecoder.c(2448): gst_video_decoder_chain (): /GstPipeline:pipeline0/avdec_h264:avdec_h264-0:
decoder not initialized
ERROR: pipeline doesn't want to preroll.
Setting pipeline to NULL ...
Freeing pipeline ...

I don't know how to understand it.

Thanks

Answer 1

According to the documentation of avdec_h264, his sink expects parsed format of h264 stream. And what h264parse does, it just parses bytes of h264 in a way that avdec_h264 could understand.

# avdec_h264 sink
video/x-h264:
      alignment: au
      stream-format: { (string)avc, (string)byte-stream }
      video/x-h264:
      alignment: nal
      stream-format: byte-stream

# h264parse src
video/x-h264:
      parsed: true
      stream-format: { (string)avc, (string)avc3, (string)byte-stream }
      alignment: { (string)au, (string)nal }

So, h264parse doesn't decode h264 bytes into the raw video stream, it just assembles bytes in some form.