VTT reader does not display cues with negative line attribute values (#476)

#475

Author: pcochenn-harmonicinc

src/main/python/ttconv/vtt/reader.py

@@ -292,9 +292,9 @@ def _get_or_make_region(
       line_num = parse_vtt_int(value[0])
       if line_num is not None:
         if writing_mode in (styles.WritingModeType.rltb, styles.WritingModeType.lrtb):
-          line_offset = 100 * line_num/_DEFAULT_ROWS if line_num > 0 else 100 - 100 * line_num/_DEFAULT_ROWS
+          line_offset = 100 * line_num/_DEFAULT_ROWS if line_num >= 0 else 100 + 100 * line_num/_DEFAULT_ROWS
         else:
-          line_offset = 100 * line_num/_DEFAULT_COLS if line_num > 0 else 100 - 100 * line_num/_DEFAULT_COLS
+          line_offset = 100 * line_num/_DEFAULT_COLS if line_num >= 0 else 100 + 100 * line_num/_DEFAULT_COLS
 
     if line_offset is not None:
       if line_align == "center":

src/test/python/test_vtt_reader.py

@@ -319,6 +319,40 @@ def test_ruby(self):
     self.assertIsNotNone(to_model(f))
 
 
+  def test_line_origin_extent(self):
+    f = io.StringIO(r"""WEBVTT
+
+1
+00:00:00.000 --> 00:00:02.000 line:0
+Line 0 starting from top
+
+2
+00:00:03.000 --> 00:00:05.000 line:5
+Line 5 starting from top
+
+3
+00:00:06.000 --> 00:00:08.000 line:45%
+Line in percentage
+
+4
+00:00:09.000 --> 00:00:11.000 line:-1
+Line 1 starting from bottom
+""")
+    doc = to_model(f)
+    regions = list(doc.iter_regions())
+    # line 0 starting from top
+    self.assertEqual(round(regions[0].get_style(styles.StyleProperties.Origin).y.value), 0)
+    self.assertEqual(round(regions[0].get_style(styles.StyleProperties.Extent).height.value), 100)
+    # line 5 starting from top
+    self.assertEqual(round(regions[1].get_style(styles.StyleProperties.Origin).y.value), 22)
+    self.assertEqual(round(regions[1].get_style(styles.StyleProperties.Extent).height.value), 78)
+    # line in percentage
+    self.assertEqual(round(regions[2].get_style(styles.StyleProperties.Origin).y.value), 45)
+    self.assertEqual(round(regions[2].get_style(styles.StyleProperties.Extent).height.value), 55)
+    # line 1 starting from bottom
+    self.assertEqual(round(regions[3].get_style(styles.StyleProperties.Origin).y.value), 96)
+    self.assertEqual(round(regions[3].get_style(styles.StyleProperties.Extent).height.value), 4)
+
 
 
 if __name__ == '__main__':