Add hacky NALU completion handling (#39)

* Add hacky NALU completion handling

* Add explanation for new AU start detection

README.md

@@ -22,7 +22,7 @@ The package can be installed by adding `membrane_h264_plugin` to your list of de
 ```elixir
 def deps do
   [
-	  {:membrane_h264_plugin, "~> 0.5.0"}
+	  {:membrane_h264_plugin, "~> 0.5.1"}
   ]
 end
 ```
@@ -39,7 +39,7 @@ defmodule Decoding.Pipeline do
 
   @impl true
   def handle_init(_ctx, _opts) do
-    structure = 
+    structure =
       child(:source, %File.Source{location: "test/fixtures/input-10-720p-main.h264"})
       |> child(:parser, H264.Parser)
       |> child(:decoder, H264.FFmpeg.Decoder)

lib/membrane_h264_plugin/parser/au_splitter.ex

@@ -169,6 +169,19 @@ defmodule Membrane.H264.Parser.AUSplitter do
     {state.nalus_acc, %{state | nalus_acc: []}}
   end
 
+  # Reference source for the behaviour below:
+  # https://github.com/GStreamer/gst-plugins-bad/blob/ca8068c6d793d7aaa6f2e2cc6324fdedfe2f33fa/gst/videoparsers/gsth264parse.c#L1183C45-L1185C49
+  #
+  # NOTE: The following check is not a part of the original H264 specification unlike the other checks below.
+  #
+  # It happens that some streams have broken frame numbers (that are either non-monotically
+  # increasing or just reset on a key frame) but the `first_mb_in_slice` set to zero can mean that
+  # we are dealin with a new AU (given a proper `nal_unit_type`). It seems that it is sufficient
+  # condition to check for `first_mb_in_slice` set to zero to detect a new AU.
+  defguardp first_mb_in_slice_zero(a)
+            when a.first_mb_in_slice == 0 and
+                   a.nal_unit_type in [1, 2, 5]
+
   defguardp frame_num_differs(a, b) when a.frame_num != b.frame_num
 
   defguardp pic_parameter_set_id_differs(a, b)
@@ -210,6 +223,7 @@ defmodule Membrane.H264.Parser.AUSplitter do
   # Conditions based on 7.4.1.2.4 "Detection of the first VCL NAL unit of a primary coded picture"
   # of the "ITU-T Rec. H.264 (01/2012)"
   defp is_new_primary_coded_vcl_nalu(%{parsed_fields: nalu}, %{parsed_fields: last_nalu})
+       when first_mb_in_slice_zero(nalu)
        when frame_num_differs(nalu, last_nalu)
        when pic_parameter_set_id_differs(nalu, last_nalu)
        when field_pic_flag_differs(nalu, last_nalu)
@@ -223,5 +237,7 @@ defmodule Membrane.H264.Parser.AUSplitter do
     true
   end
 
-  defp is_new_primary_coded_vcl_nalu(_nalu, _last_nalu), do: false
+  defp is_new_primary_coded_vcl_nalu(_nalu, _last_nalu) do
+    false
+  end
 end

mix.exs

@@ -1,7 +1,7 @@
 defmodule Membrane.H264.Plugin.Mixfile do
   use Mix.Project
 
-  @version "0.5.0"
+  @version "0.5.1"
   @github_url "https://github.com/membraneframework-labs/membrane_h264_plugin"
 
   def project do