common shape: fixing stroking composition issue.

When combining Shape, Stroke, and AlphaMasking, there is a missing
composition step which results in an incorrect output as expected by the user.

This problem is resolved by introducing shape fill and stroking composition.

@Issue: https://github.com/thorvg/thorvg/issues/209

inc/thorvg.h

@@ -70,6 +70,7 @@ protected: \
 #define _TVG_DECLARE_ACCESSOR() \
     friend Canvas; \
     friend Scene; \
+    friend Shape; \
     friend Picture; \
     friend Accessor; \
     friend IteratorAccessor
@@ -81,6 +82,7 @@ namespace tvg
 class RenderMethod;
 class IteratorAccessor;
 class Scene;
+class Shape;
 class Picture;
 class Canvas;
 class Accessor;

src/lib/tvgPictureImpl.h

@@ -142,8 +142,9 @@ struct Picture::Impl
         const Paint* target;
         auto method = picture->composite(&target);
         if (!target || method == tvg::CompositeMethod::ClipPath) return false;
-        if (target->pImpl->opacity < 255 && target->pImpl->opacity > 0) return true;
-        return false;
+        if (target->pImpl->opacity == 255 || target->pImpl->opacity == 0) return false;
+
+        return true;
     }
 
     RenderData update(RenderMethod &renderer, const RenderTransform* pTransform, uint32_t opacity, Array<RenderData>& clips, RenderUpdateFlag pFlag, bool clipper)

src/lib/tvgShape.cpp

@@ -32,7 +32,7 @@ constexpr auto PATH_KAPPA = 0.552284f;
 /* External Class Implementation                                        */
 /************************************************************************/
 
-Shape :: Shape() : pImpl(new Impl())
+Shape :: Shape() : pImpl(new Impl(this))
 {
     Paint::pImpl->id = TVG_CLASS_ID_SHAPE;
     Paint::pImpl->method(new PaintMethod<Shape::Impl>(pImpl));

src/lib/tvgShapeImpl.h

@@ -35,6 +35,12 @@ struct Shape::Impl
     RenderShape rs;                     //shape data
     RenderData rd = nullptr;            //engine data
     uint32_t flag = RenderUpdateFlag::None;
+    Shape* shape;
+    bool needComp;                      //for composition
+
+    Impl(Shape* s) : shape(s)
+    {
+    }
 
     bool dispose(RenderMethod& renderer)
     {
@@ -45,11 +51,40 @@ struct Shape::Impl
 
     bool render(RenderMethod& renderer)
     {
-        return renderer.renderShape(rd);
+        Compositor* cmp = nullptr;
+        bool ret;
+
+        if (needComp) {
+            cmp = renderer.target(bounds(renderer), renderer.colorSpace());
+            renderer.beginComposite(cmp, CompositeMethod::None, 255);
+        }
+        ret = renderer.renderShape(rd);
+        if (cmp) renderer.endComposite(cmp);
+        return ret;
+    }
+
+    bool needComposition(uint32_t opacity)
+    {
+        //In this case, paint(shape) would try composition by backends. (See: SwShapeTask::run())
+        //TODO: that composition should be moved here?
+        if (opacity < 255) return false;
+
+        //Shape composition is only necessary when stroking is valid.
+        if (!rs.stroke || rs.stroke->width < FLT_EPSILON || rs.stroke->color[3] == 0) return false;
+
+        //Composition test
+        const Paint* target;
+        auto method = shape->composite(&target);
+        if (!target || method == tvg::CompositeMethod::ClipPath) return false;
+        if (target->pImpl->opacity == 255 || target->pImpl->opacity == 0) return false;
+
+        return true;
     }
 
     RenderData update(RenderMethod& renderer, const RenderTransform* transform, uint32_t opacity, Array<RenderData>& clips, RenderUpdateFlag pFlag, bool clipper)
     {
+        needComp = needComposition(opacity);
+
         rd = renderer.prepare(rs, rd, transform, opacity, clips, static_cast<RenderUpdateFlag>(pFlag | flag), clipper);
         flag = RenderUpdateFlag::None;
         return rd;