lottie: Properly handle the TrimPath mode.

This correction addresses the drawing order of
objects when a group contains a TrimPath and
is merging shapes.

Additionally, the TrimPath method is can applied
to the engine to control the drawing behavior by
7f3dc541d6b7abcdc03facd884489f37c327fd98

issue: https://github.com/thorvg/thorvg/issues/2047

src/loaders/lottie/tvgLottieBuilder.cpp

@@ -198,7 +198,7 @@ static void _updateTransform(LottieGroup* parent, LottieObject** child, float fr
 
     uint8_t opacity;
 
-    if (parent->mergeable) {
+    if (parent->mergeable()) {
         if (!ctx->transform) ctx->transform = (Matrix*)malloc(sizeof(Matrix));
         _updateTransform(transform, frameNo, false, *ctx->transform, opacity);
         return;
@@ -232,10 +232,10 @@ static void _updateGroup(LottieGroup* parent, LottieObject** child, float frameN
     group->reqFragment |= ctx->reqFragment;
 
     //generate a merging shape to consolidate partial shapes into a single entity
-    if (group->mergeable) _draw(parent, ctx);
+    if (group->mergeable()) _draw(parent, ctx);
 
     Inlist<RenderContext> contexts;
-    contexts.back(new RenderContext(*ctx, group->mergeable));
+    contexts.back(new RenderContext(*ctx, group->mergeable()));
 
     _updateChildren(group, frameNo, contexts);
 
@@ -897,9 +897,7 @@ static void _updateTrimpath(TVG_UNUSED LottieGroup* parent, LottieObject** child
         end = (length * end) + pbegin;
     }
 
-    P(ctx->propagator)->strokeTrim(begin, end, false);
+    P(ctx->propagator)->strokeTrim(begin, end, trimpath->type == LottieTrimpath::Type::Individual ? true : false);
-
-    //TODO: individual or simultaenous mode
 }
 
 

src/loaders/lottie/tvgLottieModel.cpp

@@ -142,20 +142,20 @@ void LottieGroup::prepare(LottieObject::Type type)
     size_t fillCnt = 0;
 
     for (auto c = children.end() - 1; c >= children.begin(); --c) {
-        if (!mergeable && reqFragment) break;
-
         auto child = static_cast<LottieObject*>(*c);
 
+        if (child->type == LottieObject::Type::Trimpath) trimpath = true;
+
         /* Figure out if this group is a simple path drawing.
            In that case, the rendering context can be sharable with the parent's. */
-        if (mergeable && !child->mergeable()) mergeable = false;
+        if (allowMerge && (child->type == LottieObject::Group || !child->mergeable())) allowMerge = false;
 
         if (reqFragment) continue;
 
         /* Figure out if the rendering context should be fragmented.
            Multiple stroking or grouping with a stroking would occur this.
            This fragment resolves the overlapped stroke outlines. */
-        if (child->type == LottieObject::Group && !static_cast<LottieGroup*>(child)->mergeable) {
+        if (child->type == LottieObject::Group && !child->mergeable()) {
             if (strokeCnt > 0 || fillCnt > 0) reqFragment = true;
         } else if (child->type == LottieObject::SolidStroke || child->type == LottieObject::GradientStroke) {
             if (strokeCnt > 0) reqFragment = true;
@@ -165,6 +165,25 @@ void LottieGroup::prepare(LottieObject::Type type)
             else ++fillCnt;
         }
     }
+
+    //Reverse the drawing order if this group has a trimpath.
+    if (!trimpath) return;
+
+    for (uint32_t i = 0; i < children.count - 1; ) {
+        auto child2 = children[i + 1];
+        if (!child2->mergeable() || child2->type == LottieObject::Transform) {
+            i += 2;
+            continue;
+        }
+        auto child = children[i];
+        if (!child->mergeable() || child->type == LottieObject::Transform) {
+            i++;
+            continue;
+        }
+        children[i] = child2;
+        children[i + 1] = child;
+        i++;
+    }
 }
 
 

src/loaders/lottie/tvgLottieModel.h

@@ -572,13 +572,15 @@ struct LottieGroup : LottieObject
     }
 
     void prepare(LottieObject::Type type = LottieObject::Group);
+    bool mergeable() override { return allowMerge; }
 
     Scene* scene = nullptr;               //tvg render data
     Array<LottieObject*> children;
 
     bool reqFragment = false;   //requirment to fragment the render context
     bool buildDone = false;     //completed in building the composition.
-    bool mergeable = true;     //if this group is consisted of simple (transformed) shapes.
+    bool allowMerge = true;     //if this group is consisted of simple (transformed) shapes.
+    bool trimpath = false;      //this group has a trimpath.
 };
 
 
@@ -595,6 +597,8 @@ struct LottieLayer : LottieGroup
         return transform->opacity(frameNo);
     }
 
+    bool mergeable() override { return false; }
+
     void prepare();
     float remap(float frameNo);