diff --git a/include/xo/alloc/GC.hpp b/include/xo/alloc/GC.hpp
index 8ddb3e23..1e55c497 100644
--- a/include/xo/alloc/GC.hpp
+++ b/include/xo/alloc/GC.hpp
@@ -147,6 +147,8 @@ namespace xo {
             static up<GC> make(const Config & config);
 
             const Config & config() const { return config_; }
+            std::uint8_t nursery_polarity() const { return nursery_polarity_; }
+            std::uint8_t tenured_polarity() const { return tenured_polarity_; }
             const GCRunstate & runstate() const { return runstate_; }
             const GcStatistics & native_gc_statistics() const { return gc_statistics_; }
             GcStatisticsExt get_gc_statistics() const;
@@ -345,6 +347,13 @@ namespace xo {
             /** garbage collector configuration **/
             Config config_;
 
+            /** keep track of the identity of from-space and to-space.
+             *  assist for animation (see xo-imgui/example/ex2).
+             *  polarity alternates between 0 and 1 on each GC
+             **/
+            std::uint8_t nursery_polarity_ = 0;
+            std::uint8_t tenured_polarity_ = 0;
+
             /** contains allocated objects, along with unreachable garbage to be collected.
              *  roles reverse after each incremental, or full, collection.
              **/
diff --git a/src/alloc/GC.cpp b/src/alloc/GC.cpp
index 1b5481cb..5f75c2a0 100644
--- a/src/alloc/GC.cpp
+++ b/src/alloc/GC.cpp
@@ -501,6 +501,7 @@ namespace xo {
             up<ArenaAlloc> tmp = std::move(nursery_[role2int(role::to_space)]);
             nursery_[role2int(role::to_space)] = std::move(nursery_[role2int(role::from_space)]);
             nursery_[role2int(role::from_space)] = std::move(tmp);
+            nursery_polarity_ = 1 - nursery_polarity_;
         }
 
         void
@@ -509,6 +510,7 @@ namespace xo {
             up<ArenaAlloc> tmp = std::move(tenured_[role2int(role::to_space)]);
             tenured_[role2int(role::to_space)] = std::move(tenured_[role2int(role::from_space)]);
             tenured_[role2int(role::from_space)] = std::move(tmp);
+            tenured_polarity_ = 1 - tenured_polarity_;
         }
 
         void