rlottie: Fix precomp layer rendering issue when it has alpha value

When the precomp layer has alpha transparency and has more than 1 child layer and they overlap each other
if we just propagate the alpha to child layer it will be applied twice in overlapped area.
in this case we treat the precomp layer as complex content and don't propagate the alpha transparency.
instead we create a buffer and draw all the child layers and then we blend with the main buffer
with the alpha transparency value.

src/lottie/lottieitem.cpp

@@ -77,6 +77,7 @@ LOTCompItem::LOTCompItem(LOTModel *model)
 {
     mCompData = model->mRoot.get();
     mRootLayer = createLayerItem(mCompData->mRootLayer.get());
+    mRootLayer->setComplexContent(false);
     mViewSize = mCompData->size();
 }
 
@@ -552,6 +553,8 @@ LOTCompLayerItem::LOTCompLayerItem(LOTLayerData *layerModel)
     if (!layerModel->layerSize().empty()) {
         mClipper = std::make_unique<LOTClipperItem>(layerModel->layerSize());
     }
+
+    if (mLayers.size() > 1) setComplexContent(true);
 }
 
 void LOTCompLayerItem::buildLayerNode()
@@ -582,6 +585,27 @@ void LOTCompLayerItem::buildLayerNode()
 }
 
 void LOTCompLayerItem::render(VPainter *painter, const VRle &inheritMask, const VRle &matteRle)
+{
+    if (vIsZero(combinedAlpha())) return;
+
+    if (vCompare(combinedAlpha(), 1.0)) {
+        renderHelper(painter, inheritMask, matteRle);
+    } else {
+        if (complexContent()) {
+            VSize size = painter->clipBoundingRect().size();
+            VPainter srcPainter;
+            VBitmap srcBitmap(size.width(), size.height(), VBitmap::Format::ARGB32_Premultiplied);
+            srcPainter.begin(&srcBitmap);
+            renderHelper(&srcPainter, inheritMask, matteRle);
+            srcPainter.end();
+            painter->drawBitmap(VPoint(), srcBitmap, combinedAlpha() * 255);
+        } else {
+            renderHelper(painter, inheritMask, matteRle);
+        }
+    }
+}
+
+void LOTCompLayerItem::renderHelper(VPainter *painter, const VRle &inheritMask, const VRle &matteRle)
 {
     VRle mask;
     if (mLayerMask) {
@@ -702,8 +726,10 @@ void LOTCompLayerItem::updateContent()
         mClipper->update(combinedMatrix());
     }
     int mappedFrame = mLayerData->timeRemap(frameNo());
+    float alpha = combinedAlpha();
+    if (complexContent()) alpha = 1;
     for (const auto &layer : mLayers) {
-        layer->update( mappedFrame, combinedMatrix(), combinedAlpha());
+        layer->update( mappedFrame, combinedMatrix(), alpha);
     }
 }
 

src/lottie/lottieitem.h

@@ -95,6 +95,8 @@ public:
    int id() const {return mLayerData->id();}
    int parentId() const {return mLayerData->parentId();}
    void setParentLayer(LOTLayerItem *parent){mParentLayer = parent;}
+   void setComplexContent(bool value) { mComplexContent = value;}
+   bool complexContent() const {return mComplexContent;}
    virtual void update(int frameNo, const VMatrix &parentMatrix, float parentAlpha);
    VMatrix matrix(int frameNo) const;
    virtual void renderList(std::vector<VDrawable *> &){}
@@ -125,6 +127,7 @@ protected:
    float                                       mCombinedAlpha{0.0};
    int                                         mFrameNo{-1};
    DirtyFlag                                   mDirtyFlag{DirtyFlagBit::All};
+   bool                                        mComplexContent{false};
 };
 
 class LOTCompLayerItem: public LOTLayerItem
@@ -138,6 +141,7 @@ public:
 protected:
    void updateContent() final;
 private:
+    void renderHelper(VPainter *painter, const VRle &mask, const VRle &matteRle);
     void renderMatteLayer(VPainter *painter, const VRle &inheritMask, const VRle &matteRle,
                           LOTLayerItem *layer, LOTLayerItem *src);
 private: