Keep background color by default in LCD

Add color syntax examples

README.md

@@ -75,7 +75,7 @@ tt convert -i <input .scc file> -o <output .ttml file>
 
 Example:
 
-`tt convert -i <.scc file> -o <.ttml file> --itype SCC --otype TTML --filter lcd --config '{"general": {"progress_bar":false, "log_level":"WARN"}, "lcd": {"bg_color": "blue"}}'`
+`tt convert -i <.scc file> -o <.ttml file> --itype SCC --otype TTML --filter lcd --config '{"general": {"progress_bar":false, "log_level":"WARN"}, "lcd": {"bg_color": "transparent", "color": "#FF0000"}}'`
 
 ### General configuration (`"general"`)
 
@@ -230,17 +230,23 @@ Default: `10`
 
 `"color" : <TTML color> | null`
 
-If not `null`, overrides text color
+If not `null`, overrides text color. The syntax of `TTML color` is
+specified at <https://www.w3.org/TR/ttml2/#style-value-color>.
 
 Default: `null`
 
+Examples: `"#FFFFFF"` (white), `"white"`
+
 #### bg_color
 
-`"color" : <TTML color>`
+`"bg_color" : <TTML color>`
+
+If not `null`, overrides the background color. The syntax of `TTML color` is
+specified at <https://www.w3.org/TR/ttml2/#style-value-color>.
 
-Specifies the background color of text areas
+Default: `null`
 
-Default: `"black"`
+Examples: `"#FF0000"` (red), `"transparent"`, `"black`
 
 ## Library
 

src/main/python/ttconv/filters/doc/lcd.py

@@ -78,8 +78,8 @@ def name(cls):
  # overrides the text color
  color: typing.Optional[ColorType] = field(default=None, metadata={"decoder": ttconv.utils.parse_color})
 
- # specifies the paragraph background color
- bg_color: typing.Optional[ColorType] = field(default=NamedColors.black.value, metadata={"decoder": ttconv.utils.parse_color})
+ # overrides the background color
+ bg_color: typing.Optional[ColorType] = field(default=None, metadata={"decoder": ttconv.utils.parse_color})
 
 class LCDDocFilter(DocumentFilter):
  """Merges regions and removes all text formatting with the exception of color
@@ -109,6 +109,9 @@ def process(self, doc: ContentDocument) -> ContentDocument:
  if self.config.color is None:
  supported_styles.update({StyleProperties.Color: []})
 
+ if self.config.bg_color is None:
+ supported_styles.update({StyleProperties.BackgroundColor: []})
+
  style_filter = SupportedStylePropertiesFilter(supported_styles)
 
  style_filter.process_initial_values(doc)
@@ -236,12 +239,13 @@ def process(self, doc: ContentDocument) -> ContentDocument:
  doc.remove_region(region.get_id())
 
  # apply background color
- if doc.get_body() is not None and self.config.bg_color is not None:
+ if self.config.bg_color is not None:
  _apply_bg_color(doc.get_body(), self.config.bg_color)
 
  # apply text color
  if doc.get_body() is not None and self.config.color is not None:
  doc.get_body().set_style(StyleProperties.Color, self.config.color)
 
+ # apply text align
  if doc.get_body() is not None and not self.config.preserve_text_align:
  doc.get_body().set_style(StyleProperties.TextAlign, TextAlignType.center)