[Impeller] Fix division by zero for transparent shadows

impeller/display_list/dl_dispatcher.cc

@@ -1103,7 +1103,7 @@ void DlDispatcher::drawShadow(const SkPath& path,
 
     Scalar color_scale = color_alpha * (1 - greyscale_alpha);
     Scalar tonal_alpha = color_scale + greyscale_alpha;
-    Scalar unpremul_scale = color_scale / tonal_alpha;
+    Scalar unpremul_scale = tonal_alpha != 0 ? color_scale / tonal_alpha : 0;
     spot_color = Color(unpremul_scale * spot_color.red,
                        unpremul_scale * spot_color.green,
                        unpremul_scale * spot_color.blue, tonal_alpha);

impeller/display_list/dl_unittests.cc

@@ -17,10 +17,14 @@
 #include "flutter/display_list/effects/dl_image_filter.h"
 #include "flutter/display_list/effects/dl_mask_filter.h"
 #include "flutter/testing/testing.h"
+#include "gtest/gtest.h"
+#include "impeller/display_list/dl_dispatcher.h"
 #include "impeller/display_list/dl_image_impeller.h"
 #include "impeller/display_list/dl_playground.h"
+#include "impeller/entity/contents/rrect_shadow_contents.h"
 #include "impeller/geometry/constants.h"
 #include "impeller/geometry/point.h"
+#include "impeller/geometry/scalar.h"
 #include "impeller/playground/widgets.h"
 #include "impeller/scene/node.h"
 #include "third_party/imgui/imgui.h"
@@ -825,6 +829,38 @@ TEST_P(DisplayListTest, CanDrawShadow) {
   ASSERT_TRUE(OpenPlaygroundHere(builder.Build()));
 }
 
+TEST_P(DisplayListTest, TransparentShadowProducesCorrectColor) {
+  flutter::DisplayListBuilder builder;
+  {
+    builder.Save();
+    builder.Scale(1.618, 1.618);
+    builder.DrawShadow(SkPath{}.addRect(SkRect::MakeXYWH(0, 0, 200, 100)),
+                       SK_ColorTRANSPARENT, 15, false, 1);
+    builder.Restore();
+  }
+  auto dl = builder.Build();
+
+  DlDispatcher dispatcher;
+  dispatcher.drawDisplayList(dl, 1);
+  auto picture = dispatcher.EndRecordingAsPicture();
+
+  std::shared_ptr<RRectShadowContents> rrect_shadow;
+  picture.pass->IterateAllEntities([&rrect_shadow](Entity& entity) {
+    if (ScalarNearlyEqual(entity.GetTransformation().GetScale().x, 1.618f)) {
+      rrect_shadow =
+          std::static_pointer_cast<RRectShadowContents>(entity.GetContents());
+      return false;
+    }
+    return true;
+  });
+
+  ASSERT_NE(rrect_shadow, nullptr);
+  ASSERT_EQ(rrect_shadow->GetColor().red, 0);
+  ASSERT_EQ(rrect_shadow->GetColor().green, 0);
+  ASSERT_EQ(rrect_shadow->GetColor().blue, 0);
+  ASSERT_EQ(rrect_shadow->GetColor().alpha, 0);
+}
+
 // Draw a hexagon using triangle fan
 TEST_P(DisplayListTest, CanConvertTriangleFanToTriangles) {
   constexpr Scalar hexagon_radius = 125;

impeller/entity/contents/rrect_shadow_contents.cc

@@ -7,6 +7,7 @@
 
 #include "impeller/entity/contents/content_context.h"
 #include "impeller/entity/entity.h"
+#include "impeller/geometry/color.h"
 #include "impeller/geometry/path.h"
 #include "impeller/geometry/path_builder.h"
 #include "impeller/renderer/render_pass.h"
@@ -33,6 +34,10 @@ void RRectShadowContents::SetColor(Color color) {
   color_ = color.Premultiply();
 }
 
+Color RRectShadowContents::GetColor() const {
+  return color_;
+}
+
 std::optional<Rect> RRectShadowContents::GetCoverage(
     const Entity& entity) const {
   if (!rect_.has_value()) {

impeller/entity/contents/rrect_shadow_contents.h

@@ -30,6 +30,8 @@ class RRectShadowContents final : public Contents {
 
   void SetColor(Color color);
 
+  Color GetColor() const;
+
   // |Contents|
   std::optional<Rect> GetCoverage(const Entity& entity) const override;