[Impeller] libImpeller: Expose APIs to draw shadows. (#165517)

Fixes https://github.com/flutter/flutter/issues/165146

engine/src/flutter/impeller/toolkit/interop/dl_builder.cc

@@ -181,4 +181,17 @@ void DisplayListBuilder::DrawParagraph(const Paragraph& paragraph,
   handle->Paint(&builder_, point.x, point.y);
 }
 
+void DisplayListBuilder::DrawShadow(const Path& path,
+                                    const flutter::DlColor& color,
+                                    float elevation,
+                                    bool occluder_is_transparent,
+                                    float device_pixel_ratio) {
+  builder_.DrawShadow(flutter::DlPath(path.GetPath()),  // path
+                      color,                            // shadow color
+                      elevation,                        // elevation
+                      occluder_is_transparent,          // occluder transparency
+                      device_pixel_ratio                // dpr
+  );
+}
+
 }  // namespace impeller::interop

engine/src/flutter/impeller/toolkit/interop/dl_builder.h

@@ -110,6 +110,12 @@ class DisplayListBuilder final
 
   void DrawParagraph(const Paragraph& paragraph, Point point);
 
+  void DrawShadow(const Path& path,
+                  const flutter::DlColor& color,
+                  float elevation,
+                  bool occluder_is_transparent,
+                  float device_pixel_ratio);
+
   ScopedObject<DisplayList> Build();
 
  private:

engine/src/flutter/impeller/toolkit/interop/impeller.cc

@@ -1150,6 +1150,21 @@ void ImpellerDisplayListBuilderDrawParagraph(ImpellerDisplayListBuilder builder,
   GetPeer(builder)->DrawParagraph(*GetPeer(paragraph), ToImpellerType(*point));
 }
 
+IMPELLER_EXTERN_C
+void ImpellerDisplayListBuilderDrawShadow(ImpellerDisplayListBuilder builder,
+                                          ImpellerPath path,
+                                          const ImpellerColor* color,
+                                          float elevation,
+                                          bool occluder_is_transparent,
+                                          float device_pixel_ratio) {
+  GetPeer(builder)->DrawShadow(*GetPeer(path),             //
+                               ToDisplayListType(*color),  //
+                               elevation,                  //
+                               occluder_is_transparent,    //
+                               device_pixel_ratio          //
+  );
+}
+
 IMPELLER_EXTERN_C
 ImpellerParagraphBuilder ImpellerParagraphBuilderNew(
     ImpellerTypographyContext context) {

engine/src/flutter/impeller/toolkit/interop/impeller.h

@@ -2098,6 +2098,30 @@ void ImpellerDisplayListBuilderDrawParagraph(
     ImpellerParagraph IMPELLER_NONNULL paragraph,
     const ImpellerPoint* IMPELLER_NONNULL point);
 
+//------------------------------------------------------------------------------
+/// @brief      Draw a shadow for a Path given a material elevation. If the
+///             occluding object is not opaque, additional hints (via the
+///             `occluder_is_transparent` argument) must be provided to render
+///             the shadow correctly.
+///
+/// @param[in]  builder    The builder.
+/// @param[in]  path       The shadow path.
+/// @param[in]  color      The shadow color.
+/// @param[in]  elevation  The material elevation.
+/// @param[in]  occluder_is_transparent
+///                        If the object casting the shadow is transparent.
+/// @param[in]  device_pixel_ratio
+///                        The device pixel ratio.
+///
+IMPELLER_EXPORT
+void ImpellerDisplayListBuilderDrawShadow(
+    ImpellerDisplayListBuilder IMPELLER_NONNULL builder,
+    ImpellerPath IMPELLER_NONNULL path,
+    const ImpellerColor* IMPELLER_NONNULL color,
+    float elevation,
+    bool occluder_is_transparent,
+    float device_pixel_ratio);
+
 //------------------------------------------------------------------------------
 // Display List Builder: Drawing Textures
 //------------------------------------------------------------------------------

engine/src/flutter/impeller/toolkit/interop/impeller.hpp

@@ -73,6 +73,7 @@ struct Proc {
   PROC(ImpellerDisplayListBuilderDrawOval)                  \
   PROC(ImpellerDisplayListBuilderDrawPaint)                 \
   PROC(ImpellerDisplayListBuilderDrawParagraph)             \
+  PROC(ImpellerDisplayListBuilderDrawShadow)                \
   PROC(ImpellerDisplayListBuilderDrawPath)                  \
   PROC(ImpellerDisplayListBuilderDrawRect)                  \
   PROC(ImpellerDisplayListBuilderDrawRoundedRect)           \
@@ -1345,6 +1346,20 @@ class DisplayListBuilder final
     return *this;
   }
 
+  //----------------------------------------------------------------------------
+  /// @see      ImpellerDisplayListBuilderDrawShadow
+  ///
+  DisplayListBuilder& DrawShadow(const Path& path,
+                                 const ImpellerColor& shadow_color,
+                                 float elevation,
+                                 bool occluder_is_transparent,
+                                 float device_pixel_ratio) {
+    gGlobalProcTable.ImpellerDisplayListBuilderDrawShadow(
+        Get(), path.Get(), &shadow_color, elevation, occluder_is_transparent,
+        device_pixel_ratio);
+    return *this;
+  }
+
   //----------------------------------------------------------------------------
   /// @see      ImpellerDisplayListBuilderDrawPath
   ///

engine/src/flutter/impeller/toolkit/interop/impeller_unittests.cc

@@ -474,4 +474,33 @@ TEST_P(InteropPlaygroundTest, CanRenderTextAlignments) {
       }));
 }
 
+TEST_P(InteropPlaygroundTest, CanRenderShadows) {
+  hpp::DisplayListBuilder builder;
+  {
+    builder.DrawRect(ImpellerRect{0, 0, 400, 400},
+                     hpp::Paint{}.SetColor(ImpellerColor{
+                         0.0, 1.0, 0.0, 1.0, kImpellerColorSpaceSRGB}));
+  }
+  ImpellerRect box = {100, 100, 100, 100};
+  {
+    hpp::PathBuilder path_builder;
+    path_builder.AddRect(box);
+    ImpellerColor shadow_color = {0.0, 0.0, 0.0, 1.0, kImpellerColorSpaceSRGB};
+    builder.DrawShadow(path_builder.Build(), shadow_color, 4.0f, false, 1.0f);
+  }
+  {
+    hpp::Paint red_paint;
+    red_paint.SetColor(
+        ImpellerColor{1.0, 0.0, 0.0, 1.0, kImpellerColorSpaceSRGB});
+    builder.DrawRect(box, red_paint);
+  }
+  auto dl = builder.Build();
+  ASSERT_TRUE(
+      OpenPlaygroundHere([&](const auto& context, const auto& surface) -> bool {
+        hpp::Surface window(surface.GetC());
+        window.Draw(dl);
+        return true;
+      }));
+}
+
 }  // namespace impeller::interop::testing