imsc reader: tolerate bad time-clock frames values

#446

Author: palemieux

src/main/python/ttconv/imsc/attributes.py

@@ -431,13 +431,9 @@ def extract(context: TemporalAttributeParsingContext, xml_element) -> typing.Opt
     begin_raw = xml_element.attrib.get(BeginAttribute.qn)
 
     try:
-
-      return utils.parse_time_expression(context.tick_rate, context.frame_rate, begin_raw) if begin_raw is not None else None
-
+      return utils.parse_time_expression(context.tick_rate, context.frame_rate, begin_raw, False) if begin_raw is not None else None
     except ValueError:
-
-      LOGGER.error("bad begin value")
-
+      LOGGER.error("Bad begin attribute value: %s", begin_raw)
       return None
 
   @staticmethod
@@ -458,13 +454,9 @@ def extract(context: TemporalAttributeParsingContext, xml_element) -> typing.Opt
     end_raw = xml_element.attrib.get(EndAttribute.qn)
 
     try:
-
-      return utils.parse_time_expression(context.tick_rate, context.frame_rate, end_raw) if end_raw is not None else None
-
+      return utils.parse_time_expression(context.tick_rate, context.frame_rate, end_raw, False) if end_raw is not None else None
     except ValueError:
-
-      LOGGER.error("bad end value")
-
+      LOGGER.error("Bad end attribute value: %s", end_raw)
       return None
 
   @staticmethod
@@ -483,13 +475,9 @@ def extract(context: TemporalAttributeParsingContext, xml_element) -> typing.Opt
     dur_raw = xml_element.attrib.get(DurAttribute.qn)
 
     try:
-
-      return utils.parse_time_expression(context.tick_rate, context.frame_rate, dur_raw) if dur_raw is not None else None
-
+      return utils.parse_time_expression(context.tick_rate, context.frame_rate, dur_raw, False) if dur_raw is not None else None
     except ValueError:
-
-      LOGGER.error("bad dur value")
-
+      LOGGER.error("Bad dur attribute value: %s", dur_raw)
       return None
 
   @staticmethod

src/main/python/ttconv/imsc/utils.py

@@ -25,11 +25,14 @@
 
 '''IMSC style properties'''
 
+import logging
 import re
 import typing
 from fractions import Fraction
 import ttconv.style_properties as styles
 
+LOGGER = logging.getLogger(__name__)
+
 _LENGTH_RE = re.compile(r"^((?:\+|\-)?\d*(?:\.\d+)?)(px|em|c|%|rh|rw)$")
 
 _CLOCK_TIME_FRACTION_RE = re.compile(r"^(\d{2,}):(\d\d):(\d\d(?:\.\d+)?)$")
@@ -104,7 +107,7 @@ def _serialize_one_family(family):
   return ", ".join(map(_serialize_one_family, font_family))
 
 
-def parse_time_expression(tick_rate: typing.Optional[int], frame_rate: typing.Optional[Fraction], time_expr: str) -> Fraction:
+def parse_time_expression(tick_rate: typing.Optional[int], frame_rate: typing.Optional[Fraction], time_expr: str, strict: bool = True) -> Fraction:
   '''Parse a TTML time expression in a fractional number in seconds
   '''
 
@@ -144,14 +147,18 @@ def parse_time_expression(tick_rate: typing.Optional[int], frame_rate: typing.Op
     return Fraction(m.group(1)) * 3600 + \
             Fraction(m.group(2)) * 60 + \
             Fraction(m.group(3))
-  
+
   m = _CLOCK_TIME_FRAMES_RE.match(time_expr)
 
   if m and frame_rate is not None:
     frames = Fraction(m.group(4)) if m.group(4) else 0
 
     if frames >= frame_rate:
-      raise ValueError("Frame cound exceeds frame rate")
+      if strict:
+        raise ValueError("Frame count exceeds frame rate")
+      else:
+        LOGGER.error("Frame count %s exceeds frame rate %s, rounding to frame rate minus 1", frames, frame_rate)
+        frames = round(frame_rate) - 1
 
     return Fraction(m.group(1)) * 3600 + \
             Fraction(m.group(2)) * 60 + \

src/test/python/test_imsc_time_expressions.py

@@ -59,6 +59,9 @@ def test_bad_frame_count(self):
     with self.assertRaises(ValueError):
       parse_time_expression(None, 24, "100:00:00:100")
 
+  def test_bad_frame_count_non_strict(self):
+    self.assertEqual(parse_time_expression(None, 24, "100:00:00:100", False), 360000 + 23/24) 
+
   def test_bad_syntax(self):
     with self.assertRaises(ValueError):
       parse_time_expression(60, 24, "100:00:00;01")