[Impeller] Incorporate difference clips in stencil coverage

impeller/entity/contents/clip_contents.cc

@@ -42,10 +42,9 @@ Contents::StencilCoverage ClipContents::GetStencilCoverage(
   }
   switch (clip_op_) {
     case Entity::ClipOperation::kDifference:
-      // This can be optimized further by considering cases when the bounds of
-      // the current stencil will shrink.
       return {.type = StencilCoverage::Type::kAppend,
-              .coverage = current_stencil_coverage};
+              .coverage = current_stencil_coverage->Cutout(
+                  geometry_->GetCoverage(entity.GetTransformation()).value())};
     case Entity::ClipOperation::kIntersect:
       return {
           .type = StencilCoverage::Type::kAppend,

impeller/geometry/geometry_unittests.cc

@@ -1369,6 +1369,65 @@ TEST(GeometryTest, RectIntersectsWithRect) {
   }
 }
 
+TEST(GeometryTest, RectCutout) {
+  // No cutout.
+  {
+    Rect a(0, 0, 100, 100);
+    Rect b(0, 0, 50, 50);
+    auto u = a.Cutout(b);
+    ASSERT_TRUE(u.has_value());
+    ASSERT_RECT_NEAR(u.value(), a);
+  }
+
+  // Full cutout.
+  {
+    Rect a(0, 0, 100, 100);
+    Rect b(-10, -10, 120, 120);
+    auto u = a.Cutout(b);
+    ASSERT_FALSE(u.has_value());
+  }
+
+  // Cutout from top.
+  {
+    auto a = Rect::MakeLTRB(0, 0, 100, 100);
+    auto b = Rect::MakeLTRB(-10, -10, 110, 90);
+    auto u = a.Cutout(b);
+    auto expected = Rect::MakeLTRB(0, 90, 100, 100);
+    ASSERT_TRUE(u.has_value());
+    ASSERT_RECT_NEAR(u.value(), expected);
+  }
+
+  // Cutout from bottom.
+  {
+    auto a = Rect::MakeLTRB(0, 0, 100, 100);
+    auto b = Rect::MakeLTRB(-10, 10, 110, 110);
+    auto u = a.Cutout(b);
+    auto expected = Rect::MakeLTRB(0, 0, 100, 10);
+    ASSERT_TRUE(u.has_value());
+    ASSERT_RECT_NEAR(u.value(), expected);
+  }
+
+  // Cutout from left.
+  {
+    auto a = Rect::MakeLTRB(0, 0, 100, 100);
+    auto b = Rect::MakeLTRB(-10, -10, 90, 110);
+    auto u = a.Cutout(b);
+    auto expected = Rect::MakeLTRB(90, 0, 100, 100);
+    ASSERT_TRUE(u.has_value());
+    ASSERT_RECT_NEAR(u.value(), expected);
+  }
+
+  // Cutout from right.
+  {
+    auto a = Rect::MakeLTRB(0, 0, 100, 100);
+    auto b = Rect::MakeLTRB(10, -10, 110, 110);
+    auto u = a.Cutout(b);
+    auto expected = Rect::MakeLTRB(0, 0, 10, 100);
+    ASSERT_TRUE(u.has_value());
+    ASSERT_RECT_NEAR(u.value(), expected);
+  }
+}
+
 TEST(GeometryTest, RectContainsPoint) {
   {
     // Origin is inclusive

impeller/geometry/rect.h

@@ -204,6 +204,39 @@ struct TRect {
   constexpr bool IntersectsWithRect(const TRect& o) const {
     return Intersection(o).has_value();
   }
+
+  /// @brief Returns the new boundary rectangle that would result from the
+  ///        rectangle being cutout by a second rectangle.
+  constexpr std::optional<TRect<T>> Cutout(const TRect& o) const {
+    const auto& [a_left, a_top, a_right, a_bottom] = GetLTRB();  // Source rect.
+    const auto& [b_left, b_top, b_right, b_bottom] = o.GetLTRB();  // Cutout.
+    if (b_left <= a_left && b_right >= a_right) {
+      if (b_top <= a_top && b_bottom >= a_bottom) {
+        // Full cutout.
+        return std::nullopt;
+      }
+      if (b_top <= a_top) {
+        // Cuts off the top.
+        return TRect::MakeLTRB(a_left, b_bottom, a_right, a_bottom);
+      }
+      if (b_bottom >= b_bottom) {
+        // Cuts out the bottom.
+        return TRect::MakeLTRB(a_left, a_top, a_right, b_top);
+      }
+    }
+    if (b_top <= a_top && b_bottom >= a_bottom) {
+      if (b_left <= a_left) {
+        // Cuts out the left.
+        return TRect::MakeLTRB(b_right, a_top, a_right, a_bottom);
+      }
+      if (b_right >= a_right) {
+        // Cuts out the right.
+        return TRect::MakeLTRB(a_left, a_top, b_left, a_bottom);
+      }
+    }
+
+    return *this;
+  }
 };
 
 using Rect = TRect<Scalar>;