SCC: supports left-hand cursor movements on current line

src/main/python/ttconv/scc/caption_line.py

@@ -58,9 +58,9 @@ def add_text(self, text: Union[SccCaptionText, str]):
 
       if self._current_text is None:
         # Initialize a new text element if necessary
-        self._texts.append(SccCaptionText(text))
+        self._texts.append(SccCaptionText())
         self._current_text = self._texts[-1]
-        self._cursor = self._current_text.get_length()
+        self._append_text(text)
 
       else:
         remaining_text = text
@@ -71,21 +71,27 @@ def add_text(self, text: Union[SccCaptionText, str]):
           text_to_write = remaining_text[:available]
 
           # Replace current text element content
-          self._current_text.append(text_to_write)
-          self.set_cursor(self._cursor + len(text_to_write))
+          self._append_text(text_to_write)
           remaining_text = remaining_text[available:]
 
         # If some text remains on the last text element
         if len(remaining_text) > 0:
           assert self._current_text is self._texts[-1]
 
           # Replace and append to current text element content
-          self._current_text.append(remaining_text)
-          self.set_cursor(self._cursor + len(remaining_text))
+          self._append_text(remaining_text)
 
     else:
       raise ValueError("Unsupported text type for SCC caption line")
 
+  def _append_text(self, text: str):
+    """Appends text and update cursor position"""
+    self._current_text.append(text)
+    if self._cursor < 0:
+      self._cursor = 0
+
+    self.set_cursor(self._cursor + len(text))
+
   def indent(self, indent: int):
     """Indent current line"""
     self._indent += indent

src/main/python/ttconv/scc/caption_paragraph.py

@@ -167,7 +167,7 @@ def set_cursor_at(self, row: int, indent: Optional[int] = None):
     self._current_line = self._caption_lines.get(row)
 
     if indent is not None:
-      self._current_line.set_cursor(self._cursor[1] - self._current_line.get_indent())
+      self._update_current_line_cursor()
 
   def get_cursor(self) -> (int, int):
     """Returns cursor coordinates"""
@@ -181,7 +181,16 @@ def indent_cursor(self, indent: int):
       # If the current line is empty, set cursor indent as a line tabulation
       self._current_line.indent(indent)
     else:
-      self._current_line.set_cursor(self._cursor[1] - self._current_line.get_indent())
+      self._update_current_line_cursor()
+
+  def _update_current_line_cursor(self):
+    """Updates cursor position on current line"""
+    new_cursor_position = self._cursor[1] - self._current_line.get_indent()
+
+    if new_cursor_position < 0:
+      self._current_line.indent(new_cursor_position)
+
+    self._current_line.set_cursor(new_cursor_position)
 
   def get_lines(self) -> Dict[int, SccCaptionLine]:
     """Returns the paragraph lines per row"""

src/main/python/ttconv/scc/caption_text.py

@@ -79,6 +79,11 @@ def is_empty(self) -> bool:
 
   def append(self, text: str):
     """Add or replace text content at cursor position"""
+    if self._cursor < 0:
+      # Insert space characters before current text
+      self._text = ' ' * -self._cursor + self._text
+      self._cursor = 0
+
     # print("Append text: ", text, "to", self._text, "at", self._cursor)
     self._text = self._text[:self._cursor] + text + self._text[(self._cursor + len(text)):]
     self._cursor += len(text)

src/test/python/test_scc_reader.py

@@ -1192,5 +1192,43 @@ def test_scc_content_parsing_from_preamble_address_code(self):
                        "consectetur adipiscing elit.")
     self.assertEqual(region_1, p_list[0].get_region())
 
+  def test_scc_with_negative_cursor(self):
+    scc_content = """Scenarist_SCC V1.0
+00:00:01:00	94AE 94AE 9420 9420 94F8 94F8 45E5 E5E3 68A1 94F4 94F4 D3E3 61F2 79A1 942C 942C 942F 942F
+00:00:02:00	942F 942F
+"""
+    scc_disassembly_expected = """\
+00:00:01:00	{ENM}{ENM}{RCL}{RCL}{1516}{1516}Eeech!{1508}{1508}Scary!{EDM}{EDM}{EOC}{EOC}
+00:00:02:00	{EOC}{EOC}
+"""
+    scc_disassembly = to_disassembly(scc_content)
+    self.assertEqual(scc_disassembly_expected, scc_disassembly)
+
+    doc = to_model(scc_content)
+
+    self.assertIsNotNone(doc)
+
+    region_1 = doc.get_region("pop1")
+    self.assertIsNotNone(region_1)
+    self.check_region_origin(region_1, 12, 16, doc.get_cell_resolution())
+    self.check_region_extent(region_1, 14, 1, doc.get_cell_resolution())
+    self.check_element_style(region_1, StyleProperties.DisplayAlign, DisplayAlignType.before)
+    self.check_element_style(region_1, StyleProperties.ShowBackground, ShowBackgroundType.whenActive)
+
+    body = doc.get_body()
+    self.assertIsNotNone(body)
+
+    div_list = list(body)
+    self.assertEqual(1, len(div_list))
+    div = div_list[0]
+    self.assertIsNotNone(div)
+
+    p_list = list(div)
+    self.assertEqual(1, len(p_list))
+
+    self.check_caption(p_list[0], "caption1", "00:00:01:12", "00:00:02:01", "Scary!  Eeech!")
+    self.assertEqual(region_1, p_list[0].get_region())
+
+
 if __name__ == '__main__':
   unittest.main()