background-opacity support

include/mbgl/style/property_key.hpp

@@ -60,6 +60,7 @@ enum class PropertyKey {
     RasterContrast,
     RasterFade,
 
+    BackgroundOpacity,
     BackgroundColor
 };
 

include/mbgl/style/style_properties.hpp

@@ -92,6 +92,7 @@ struct RasterProperties {
 
 struct BackgroundProperties {
     inline BackgroundProperties() {}
+    float opacity = 1.0f;
     Color color = {{ 0, 0, 0, 1 }};
 };
 

src/map/map.cpp

@@ -626,7 +626,6 @@ void Map::renderLayers(std::shared_ptr<StyleLayerGroup> group) {
 void Map::renderLayer(std::shared_ptr<StyleLayer> layer_desc, RenderPass pass, const Tile::ID* id, const mat4* matrix) {
     if (layer_desc->type == StyleLayerType::Background) {
         // This layer defines a background color/image.
-        if (pass == RenderPass::Translucent) return;
 
         if (debug::renderTree) {
             std::cout << std::string(indent * 4, ' ') << "- " << layer_desc->id << " ("

src/renderer/painter.cpp

@@ -186,15 +186,23 @@ void Painter::renderTileLayer(const Tile& tile, std::shared_ptr<StyleLayer> laye
 void Painter::renderBackground(std::shared_ptr<StyleLayer> layer_desc) {
     const BackgroundProperties& properties = layer_desc->getProperties<BackgroundProperties>();
 
-    useProgram(plainShader->program);
-    plainShader->setMatrix(identityMatrix);
-    plainShader->setColor(properties.color);
-    backgroundArray.bind(*plainShader, backgroundBuffer, BUFFER_OFFSET(0));
+    Color color = properties.color;
+    color[0] *= properties.opacity;
+    color[1] *= properties.opacity;
+    color[2] *= properties.opacity;
+    color[3] *= properties.opacity;
 
-    glDisable(GL_STENCIL_TEST);
-    depthRange(strata + strata_epsilon, 1.0f);
-    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
-    glEnable(GL_STENCIL_TEST);
+    if ((color[3] >= 1.0f) == (pass == RenderPass::Opaque)) {
+        useProgram(plainShader->program);
+        plainShader->setMatrix(identityMatrix);
+        plainShader->setColor(color);
+        backgroundArray.bind(*plainShader, backgroundBuffer, BUFFER_OFFSET(0));
+
+        glDisable(GL_STENCIL_TEST);
+        depthRange(strata + strata_epsilon, 1.0f);
+        glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
+        glEnable(GL_STENCIL_TEST);
+    }
 }
 
 const mat4 &Painter::translatedMatrix(const mat4& matrix, const std::array<float, 2> &translation, const Tile::ID &id, TranslateAnchorType anchor) {

src/style/property_fallback.cpp

@@ -52,6 +52,7 @@ const std::map<PropertyKey, PropertyValue> PropertyFallbackValue::properties = {
     { PropertyKey::RasterContrast, defaultStyleProperties<RasterProperties>().contrast },
     { PropertyKey::RasterFade, defaultStyleProperties<RasterProperties>().fade },
 
+    { PropertyKey::BackgroundOpacity, defaultStyleProperties<BackgroundProperties>().opacity },
     { PropertyKey::BackgroundColor, defaultStyleProperties<BackgroundProperties>().color },
 };
 

src/style/style_layer.cpp

@@ -233,6 +233,7 @@ template <>
 void StyleLayer::applyStyleProperties<BackgroundProperties>(const float z, const timestamp now) {
     properties.set<BackgroundProperties>();
     BackgroundProperties &background = properties.get<BackgroundProperties>();
+    applyTransitionedStyleProperty(PropertyKey::BackgroundOpacity, background.opacity, z, now);
     applyTransitionedStyleProperty(PropertyKey::BackgroundColor, background.color, z, now);
 }
 

src/style/style_parser.cpp

@@ -636,6 +636,7 @@ void StyleParser::parseStyle(JSVal value, ClassProperties &klass) {
     parseOptionalProperty<Function<float>>("raster-fade-duration", Key::RasterFade, klass, value);
     parseOptionalProperty<PropertyTransition>("transition-raster-fade-duration", Key::RasterFade, klass, value);
 
+    parseOptionalProperty<Function<float>>("background-opacity", Key::BackgroundOpacity, klass, value);
     parseOptionalProperty<Function<Color>>("background-color", Key::BackgroundColor, klass, value);
 }