[SVGDom] Opacity optimization

experimental/svg/model/SkSVGRenderContext.cpp

Index: experimental/svg/model/SkSVGRenderContext.cpp
diff --git a/experimental/svg/model/SkSVGRenderContext.cpp b/experimental/svg/model/SkSVGRenderContext.cpp
index 7e2e90e22c4ddaa5ca743fbce51ea1d2c9083456..147d64edb3998bf60ed82686527ac4b3aae34505 100644
--- a/experimental/svg/model/SkSVGRenderContext.cpp
+++ b/experimental/svg/model/SkSVGRenderContext.cpp
@@ -239,7 +239,8 @@ const SkSVGNode* SkSVGRenderContext::findNodeById(const SkString& id) const {
     return v ? v->get() : nullptr;
 }
 
-void SkSVGRenderContext::applyPresentationAttributes(const SkSVGPresentationAttributes& attrs) {
+void SkSVGRenderContext::applyPresentationAttributes(const SkSVGPresentationAttributes& attrs,
+                                                     uint32_t flags) {
 
 #define ApplyLazyInheritedAttribute(ATTR)                                               \
     do {                                                                                \
@@ -267,10 +268,36 @@ void SkSVGRenderContext::applyPresentationAttributes(const SkSVGPresentationAttr
 
     // Uninherited attributes.  Only apply to the current context.
 
-    auto* opacity = attrs.fOpacity.getMaybeNull();
-    if (opacity && opacity->value() < 1) {
+    if (auto* opacity = attrs.fOpacity.getMaybeNull()) {
+        this->applyOpacity(opacity->value(), flags);
+    }
+}
+
+void SkSVGRenderContext::applyOpacity(SkScalar opacity, uint32_t flags) {
+    if (opacity >= 1) {
+        return;
+    }
+
+    const bool hasFill   = SkToBool(this->fillPaint());
+    const bool hasStroke = SkToBool(this->strokePaint());
+
+    // We can apply the opacity as paint alpha iif it only affects one atomic draw.
+    // For now, this means a) the target node doesn't have any descendants, and
+    // b) it only has a stroke or a fill (but not both).  Going forward, we may need
+    // to refine this heuristic (e.g. to accommodate markers).
+    if ((flags & kLeaf) && (hasFill ^ hasStroke)) {
+        auto* pctx = fPresentationContext.writable();
+        if (hasFill) {
+            pctx->fFillPaint.setAlpha(
+                SkScalarRoundToInt(opacity * pctx->fFillPaint.getAlpha()));
+        } else {
+            pctx->fStrokePaint.setAlpha(
+                SkScalarRoundToInt(opacity * pctx->fStrokePaint.getAlpha()));
+        }
+    } else {
+        // Expensive, layer-based fall back.
         SkPaint opacityPaint;
-        opacityPaint.setAlpha(opacity_to_alpha(opacity->value()));
+        opacityPaint.setAlpha(opacity_to_alpha(opacity));
         // Balanced in the destructor, via restoreToCount().
         fCanvas->saveLayer(nullptr, &opacityPaint);
     }