fix: Apply text opacity to outline in ML

There's no mention of this in Maplibre docs but from my testing it seems
that Maplibre applies text opacity to the outline also. This commit
applies this logic when converting Maplibre style into Galileo style
rules. It also modifies `WithOpacity` expression to blend alpha value
insted of replacing it, as it seems to be closer to what Maplibre does.

Author: Maximkaaa

galileo-maplibre/src/layer/vector_tile.rs

@@ -96,15 +96,18 @@ fn get_background(layers: &[&MaplibreStyleLayer]) -> ColorExpr {
 
     get_color_value(
         &layer.paint.background_color,
-        &layer.paint.background_opacity,
+        Some(&layer.paint.background_opacity),
     )
     .map(Into::into)
     .unwrap_or(DEFAULT_TILE_BACKGROUND)
 }
 
-fn get_color_value(color: &MlStyleValue<MlColor>, opacity: &MlStyleValue<f64>) -> Option<Expr> {
+fn get_color_value(
+    color: &MlStyleValue<MlColor>,
+    opacity: Option<&MlStyleValue<f64>>,
+) -> Option<Expr> {
     let galileo_color = get_galileo_value(color)?;
-    let galileo_opacity = get_galileo_value(opacity);
+    let galileo_opacity = opacity.map(|v| get_galileo_value(v).unwrap_or(1.0.into()));
 
     Some(match galileo_opacity {
         Some(v) => Expr::WithOpacity(WithOpacityExpr {
@@ -245,9 +248,13 @@ fn symbol_rule(symbol: &SymbolLayer, tile_schema: &TileSchema) -> Option<StyleRu
         .and_then(|lod| tile_schema.lod_resolution(lod.round() as u32));
     let filter = symbol.filter.as_ref().and_then(|v| v.to_galileo_expr());
 
-    let font_color = get_color_value(&symbol.paint.text_color, &symbol.paint.text_opacity)?;
-    let outline_color =
-        get_color_value(&symbol.paint.text_halo_color, &MlStyleValue::Literal(1.0))?;
+    let font_color = get_color_value(&symbol.paint.text_color, Some(&symbol.paint.text_opacity))?;
+    // Even though Maplibre docs don't mention this, but it seems that text opacity is also
+    // applied to the outline color, so we use it here.
+    let outline_color = get_color_value(
+        &symbol.paint.text_halo_color,
+        Some(&symbol.paint.text_opacity),
+    )?;
     let font_size = get_galileo_value(
         &symbol
             .layout
@@ -264,6 +271,7 @@ fn symbol_rule(symbol: &SymbolLayer, tile_schema: &TileSchema) -> Option<StyleRu
     )?;
 
     let (font_family, weight, font_style) = parse_ml_fonts(&symbol.layout.text_font);
+    dbg!(&font_color);
 
     let style = VtTextStyle {
         font_family,
@@ -385,7 +393,7 @@ fn fill_rule(fill: &FillLayer, tile_schema: &TileSchema) -> Option<StyleRule> {
 
     let fill_color = &fill.paint.fill_color;
     let fill_opacity = &fill.paint.fill_opacity;
-    let color = get_color_value(fill_color, fill_opacity)?;
+    let color = get_color_value(fill_color, Some(fill_opacity))?;
 
     if !fill.paint.fill_antialias {
         log::debug!(
@@ -443,7 +451,7 @@ fn line_rule(line: &LineLayer, tile_schema: &TileSchema) -> Option<StyleRule> {
 
     let stroke_color = &line.paint.line_color;
     let stroke_opacity = &line.paint.line_opacity;
-    let color = get_color_value(stroke_color, stroke_opacity)
+    let color = get_color_value(stroke_color, Some(stroke_opacity))
         .unwrap_or(Color::TRANSPARENT.into())
         .into();
     let stroke_width = &line.paint.line_width;

galileo/src/expr/mod.rs

@@ -258,7 +258,7 @@ impl WithOpacityExpr {
             return ExprValue::Null;
         };
 
-        ExprValue::Color(color.with_alpha_float(opacity))
+        ExprValue::Color(color.with_alpha_float(color.a() as f64 / 255.0 * opacity))
     }
 }