AAF Adapter: Mob transcription heuristics (#1249)

* Look for composition mob first if it's available

When we describe our AAF into OTIO space, we apply the following heuristic:

1) First look for top level mobs and if found use that to transcribe.

2) If we don't have top level mobs, look for composition mobs and use them to
transcribe.

3) Lastly if we don't have either, try to use master mobs to transcribe.

If we don't find any Mobs, just tell the user and do transcrption on an empty
list (to generate some 'empty-level' OTIO structure)

This heuristic is based on 'real-world' examples. There may still be some
corner cases / open questions (like could there be metadata on both
a composition mob and master mob? And if so, who would 'win'?)

In any way, this heuristic satisfies the current set of AAFs we are using
in our test-environment.

contrib/opentimelineio_contrib/adapters/advanced_authoring_format.py

@@ -1454,6 +1454,50 @@ def _contains_something_valuable(thing):
     return True
 
 
+def _get_mobs_for_transcription(storage):
+    """
+    When we describe our AAF into OTIO space, we apply the following heuristic:
+
+    1) First look for top level mobs and if found use that to transcribe.
+
+    2) If we don't have top level mobs, look for composition mobs and use them to
+    transcribe.
+
+    3) Lastly if we don't have either, try to use master mobs to transcribe.
+
+    If we don't find any Mobs, just tell the user and do transcrption on an empty
+    list (to generate some 'empty-level' OTIO structure)
+
+    This heuristic is based on 'real-world' examples. There may still be some
+    corner cases / open questions (like could there be metadata on both
+    a composition mob and master mob? And if so, who would 'win'?)
+
+    In any way, this heuristic satisfies the current set of AAFs we are using
+    in our test-environment.
+
+    """
+
+    top_level_mobs = list(storage.toplevel())
+
+    if len(top_level_mobs) > 0:
+        _transcribe_log("---\nTranscribing top level mobs\n---")
+        return top_level_mobs
+
+    composition_mobs = list(storage.compositionmobs())
+    if len(composition_mobs) > 0:
+        _transcribe_log("---\nTranscribing composition mobs\n---")
+        return composition_mobs
+
+    master_mobs = list(storage.mastermobs())
+    if len(master_mobs) > 0:
+        _transcribe_log("---\nTranscribing master mobs\n---")
+        return master_mobs
+
+    _transcribe_log("---\nNo mobs found to transcribe\n---")
+
+    return []
+
+
 def read_from_file(
     filepath,
     simplify=True,
@@ -1484,25 +1528,20 @@ def read_from_file(
     _BAKE_KEYFRAMED_PROPERTIES_VALUES = bake_keyframed_properties
 
     with aaf2.open(filepath) as aaf_file:
-
-        storage = aaf_file.content
-
-        # Note: We're skipping: f.header
+        # Note: We're skipping: aaf_file.header
         # Is there something valuable in there?
-        _transcribe_log("---\nTranscribing top level mobs\n---", 0)
 
-        # Get just the top-level MOBS from the AAF
-        top = list(storage.toplevel())
+        storage = aaf_file.content
+        mobs_to_transcribe = _get_mobs_for_transcription(storage)
 
-        # Transcribe just the top-level mobs
-        result = _transcribe(top, parents=list(), edit_rate=None)
+        result = _transcribe(mobs_to_transcribe, parents=list(), edit_rate=None)
 
     # Attach marker to the appropriate clip, gap etc.
     if attach_markers:
         result = _attach_markers(result)
 
     # AAF is typically more deeply nested than OTIO.
-    # Lets try to simplify the structure by collapsing or removing
+    # Let's try to simplify the structure by collapsing or removing
     # unnecessary stuff.
     if simplify:
         result = _simplify(result)