diff --git a/ActionManager.cpp b/ActionManager.cpp
index 5277192..9595ea5 100644
--- a/ActionManager.cpp
+++ b/ActionManager.cpp
@@ -169,7 +169,7 @@ void Action::restore(uint target, uint64_t id)
         return;
 
     // sessionsources contains list of ids of all sources currently in the session
-    std::list<uint64_t> sessionsources = se->getIdList();
+    SourceIdList sessionsources = se->getIdList();
 //    for( auto it = sessionsources.begin(); it != sessionsources.end(); it++)
 //        Log::Info("sessionsources  id %s", std::to_string(*it).c_str());
 
@@ -180,7 +180,7 @@ void Action::restore(uint target, uint64_t id)
     loader.load( sessionNode );
 
     // loadersources contains list of ids of all sources generated by loader
-    std::list<uint64_t> loadersources = loader.getIdList();
+    SourceIdList loadersources = loader.getIdList();
 //    for( auto it = loadersources.begin(); it != loadersources.end(); it++)
 //        Log::Info("loadersources  id %s", std::to_string(*it).c_str());
 
diff --git a/CMakeLists.txt b/CMakeLists.txt
index a91b219..eaf8498 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -280,6 +280,7 @@ set(VMIX_SRCS
     TextureView.cpp
     TransitionView.cpp
     Source.cpp
+    SourceList.cpp
     Session.cpp
     Selection.cpp
     SessionSource.cpp
diff --git a/LayerView.cpp b/LayerView.cpp
index c073fe8..3a913dc 100644
--- a/LayerView.cpp
+++ b/LayerView.cpp
@@ -109,7 +109,7 @@ void LayerView::draw()
 
         // manipulation of sources in Mixing view
         if (ImGui::Selectable( ICON_FA_ALIGN_CENTER "   Distribute" )){
-            SourceList dsl = depthSorted(Mixer::selection().getCopy());
+            SourceList dsl = depth_sorted(Mixer::selection().getCopy());
             SourceList::iterator  it = dsl.begin();
             float depth = (*it)->depth();
             float depth_inc   = (dsl.back()->depth() - depth) / static_cast<float>(Mixer::selection().size()-1);
@@ -120,7 +120,7 @@ void LayerView::draw()
             View::need_deep_update_++;
         }
         if (ImGui::Selectable( ICON_FA_RULER_HORIZONTAL "  Space equally" )){
-            SourceList dsl = depthSorted(Mixer::selection().getCopy());
+            SourceList dsl = depth_sorted(Mixer::selection().getCopy());
             SourceList::iterator  it = dsl.begin();
             float depth = (*it)->depth();
             for (it++; it != dsl.end(); it++) {
@@ -130,7 +130,7 @@ void LayerView::draw()
             View::need_deep_update_++;
         }
         if (ImGui::Selectable( ICON_FA_EXCHANGE_ALT "  Reverse order" )){
-            SourceList dsl = depthSorted(Mixer::selection().getCopy());
+            SourceList dsl = depth_sorted(Mixer::selection().getCopy());
             SourceList::iterator  it = dsl.begin();
             SourceList::reverse_iterator  rit = dsl.rbegin();
             for (; it != dsl.end(); it++, rit++) {
diff --git a/Mixer.cpp b/Mixer.cpp
index da14cc7..f46430a 100644
--- a/Mixer.cpp
+++ b/Mixer.cpp
@@ -28,6 +28,7 @@ using namespace tinyxml2;
 #include "StreamSource.h"
 #include "NetworkSource.h"
 #include "ActionManager.h"
+#include "MixingGroup.h"
 #include "Streamer.h"
 
 #include "Mixer.h"
@@ -208,6 +209,22 @@ void Mixer::update()
     // update session and associated sources
     session_->update(dt_);
 
+    // update session's mixing groups
+    auto group_iter = session_->beginMixingGroup();
+    while ( group_iter != session_->endMixingGroup() ){
+        // update all valid groups
+        if ((*group_iter)->size() > 1) {
+            (*group_iter)->update(dt_);
+            group_iter++;
+        }
+        // delete invalid groups (singletons)
+        else {
+            mixing_.scene.fg()->detach( (*group_iter)->group() );
+            delete (*group_iter);
+            group_iter = session_->eraseMixingGroup(group_iter);
+        }
+    }
+
     // grab frames to recorders & streamers
     FrameGrabbing::manager().grabFrame(session_->frame(), dt_);
 
@@ -768,7 +785,7 @@ Source * Mixer::findSource (uint64_t id)
 SourceList Mixer::findSources (float depth_from, float depth_to)
 {
     SourceList found;
-    SourceList dsl = depthSorted( session_->getCopy() );
+    SourceList dsl = session_->depthSorted();
     SourceList::iterator  it = dsl.begin();
     for (; it != dsl.end(); it++) {
         if ( (*it)->depth() > depth_to )
@@ -1037,6 +1054,16 @@ void Mixer::merge(Session *session)
         attach(s);
     }
 
+    // import and attach session's mixing groups
+    auto group_iter = session->beginMixingGroup();
+    while ( group_iter != session->endMixingGroup() ){
+        if (session_->addMixingGroup( *group_iter ) )
+            mixing_.scene.fg()->attach( (*group_iter)->group() );
+        else
+            delete (*group_iter);
+        group_iter = session->eraseMixingGroup(group_iter);
+    }
+
     // needs to update !
     View::need_deep_update_++;
 
@@ -1064,7 +1091,7 @@ void Mixer::merge(SessionSource *source)
         float target_depth = source->depth();
 
         // get how much space we need from there
-        SourceList dsl = depthSorted( session->getCopy() );
+        SourceList dsl = session->depthSorted();
         float  start_depth = dsl.front()->depth();
         float  end_depth = dsl.back()->depth();
         float  need_depth = MAX( end_depth - start_depth, LAYER_STEP);
@@ -1112,6 +1139,16 @@ void Mixer::merge(SessionSource *source)
             attach(s);
         }
 
+        // import and attach session's mixing groups
+        auto group_iter = session->beginMixingGroup();
+        while ( group_iter != session->endMixingGroup() ){
+            if (session_->addMixingGroup( *group_iter ) )
+                mixing_.scene.fg()->attach( (*group_iter)->group() );
+            else
+                delete (*group_iter);
+            group_iter = session->eraseMixingGroup(group_iter);
+        }
+
         // needs to update !
         View::need_deep_update_++;
 
@@ -1133,6 +1170,9 @@ void Mixer::swap()
         // detatch current session's nodes from views
         for (auto source_iter = session_->begin(); source_iter != session_->end(); source_iter++)
             detach(*source_iter);
+        // detatch current session's mixing groups
+        for (auto group_iter = session_->beginMixingGroup(); group_iter != session_->endMixingGroup(); group_iter++)
+            mixing_.scene.fg()->detach( (*group_iter)->group() );
     }
 
     // swap back and front
@@ -1150,6 +1190,10 @@ void Mixer::swap()
     layer_.scene.root()->copyTransform( session_->config(View::LAYER) );
     appearance_.scene.root()->copyTransform( session_->config(View::TEXTURE) );
 
+    // attach new session's mixing group to mixingview
+    for (auto group_iter = session_->beginMixingGroup(); group_iter != session_->endMixingGroup(); group_iter++)
+        mixing_.scene.fg()->attach( (*group_iter)->group() );
+
     // set resolution
     session_->setResolution( session_->config(View::RENDERING)->scale_ );
 
@@ -1258,4 +1302,6 @@ void Mixer::paste(const std::string& clipboard)
         }
     }
 
+    // TODO : Mixing groups
+
 }
diff --git a/MixingGroup.cpp b/MixingGroup.cpp
index 933ec9d..81300be 100644
--- a/MixingGroup.cpp
+++ b/MixingGroup.cpp
@@ -1,5 +1,4 @@
 #include <algorithm>
-#include <vector>
 
 #include <glm/gtx/vector_angle.hpp>
 #include <glm/gtx/rotate_vector.hpp>
@@ -7,41 +6,33 @@
 #include "defines.h"
 #include "Source.h"
 #include "Decorations.h"
+#include "Visitor.h"
 #include "Mixer.h"
 #include "Log.h"
 
 #include "MixingGroup.h"
 
-// utility to sort Sources in MixingView in a clockwise order
-// in reference to a center point
-struct clockwise_centered {
-    clockwise_centered(glm::vec2 c) : center(c) { }
-    bool operator() (Source * first, Source * second) {
-        glm::vec2 pos_first = glm::vec2(first->group(View::MIXING)->translation_)-center;
-        float angle_first = glm::orientedAngle( glm::normalize(pos_first), glm::vec2(1.f, 0.f) );
-        glm::vec2 pos_second = glm::vec2(second->group(View::MIXING)->translation_)-center;
-        float angle_second = glm::orientedAngle( glm::normalize(pos_second), glm::vec2(1.f, 0.f) );
-        return (angle_first < angle_second);
-    }
-    glm::vec2 center;
-};
 
 MixingGroup::MixingGroup (SourceList sources) : root_(nullptr), lines_(nullptr), center_(nullptr),
-    center_pos_(glm::vec2(0.f, 0.f)), active_(true)
+    center_pos_(glm::vec2(0.f, 0.f)), active_(true), update_action_(ACTION_NONE), updated_source_(nullptr)
 {
     // fill the vector of sources with the given list
     for (auto it = sources.begin(); it != sources.end(); it++){
-        (*it)->mixinggroup_ = this;
-        sources_.push_back(*it);
-        // compute barycenter (1)
-        center_pos_ += glm::vec2((*it)->group(View::MIXING)->translation_);
+        // add only if not linked already
+        if ((*it)->mixinggroup_ == nullptr) {
+            (*it)->mixinggroup_ = this;
+            sources_.push_back(*it);
+            // compute barycenter on the way (1)
+            center_pos_ += glm::vec2((*it)->group(View::MIXING)->translation_);
+        }
     }
     // compute barycenter (2)
     center_pos_ /= sources_.size();
 
     // sort the vector of sources in clockwise order around the center pos_
-    std::sort(sources_.begin(), sources_.end(), clockwise_centered(center_pos_));
+    sources_ = mixing_sorted( sources_, center_pos_);
 
+    // scene elements
     root_ = new Group;
     center_ = new Symbol(Symbol::CIRCLE_POINT);
     center_->visible_ = false;
@@ -54,10 +45,17 @@ MixingGroup::MixingGroup (SourceList sources) : root_(nullptr), lines_(nullptr),
 
 MixingGroup::~MixingGroup ()
 {
-    delete center_;
+    for (auto it = sources_.begin(); it != sources_.end(); it++) {
+        (*it)->mixinggroup_ = nullptr;
+        (*it)->overlay_mixinggroup_->visible_ = false;
+    }
+
     delete root_;
-    if (lines_)
-        delete lines_;
+}
+
+void MixingGroup::accept(Visitor& v)
+{
+    v.visit(*this);
 }
 
 void MixingGroup::update (float dt)
@@ -168,9 +166,11 @@ void MixingGroup::setActive (bool on)
 void MixingGroup::detach (Source *s)
 {
     // find the source
-    std::vector<Source *>::iterator its = std::find(sources_.begin(), sources_.end(), s);
+    SourceList::iterator its = std::find(sources_.begin(), sources_.end(), s);
     // ok, its in the list !
     if (its != sources_.end()) {
+        // tell the source
+        (*its)->mixinggroup_ = nullptr;
         // erase the source from the list
         sources_.erase(its);
         // clear index, delete lines_, and recreate path and index with remaining sources
@@ -178,32 +178,52 @@ void MixingGroup::detach (Source *s)
     }
 }
 
-void MixingGroup::move (Source *s)
+
+uint MixingGroup::size()
+{
+    return sources_.size();
+}
+
+SourceList::iterator MixingGroup::begin ()
+{
+    return sources_.begin();
+}
+
+SourceList::iterator MixingGroup::end ()
+{
+    return sources_.end();
+}
+
+bool MixingGroup::contains (Source *s)
 {
     // find the source
-    std::vector<Source *>::iterator its = std::find(sources_.begin(), sources_.end(), s);
-    // ok, its in the list !
-    if (its != sources_.end() && lines_) {
+    SourceList::iterator its = std::find(sources_.begin(), sources_.end(), s);
+    // in the list ?
+    return its != sources_.end();
+}
+
+void MixingGroup::move (Source *s)
+{
+    if (contains(s) && lines_) {
         // modify one point in the path
         lines_->editPath(index_points_[s], glm::vec2(s->group(View::MIXING)->translation_));
     }
-
 }
 
 void MixingGroup::createLineStrip()
 {
-    if (lines_) {
-        root_->detach(lines_);
-        delete lines_;
-    }
-
-    index_points_.clear();
-
     if (sources_.size() > 1) {
+
+        if (lines_) {
+            root_->detach(lines_);
+            delete lines_;
+        }
+
+        index_points_.clear();
+
+        // path linking all sources
         std::vector<glm::vec2> path;
-        auto it = sources_.begin();
-        // link sources
-        for (; it != sources_.end(); it++){
+        for (auto it = sources_.begin(); it != sources_.end(); it++){
             index_points_[*it] = path.size();
             path.push_back(glm::vec2((*it)->group(View::MIXING)->translation_));
         }
@@ -211,7 +231,6 @@ void MixingGroup::createLineStrip()
         // create
         lines_ = new LineLoop(path, 1.5f);
         lines_->shader()->color = glm::vec4(COLOR_MIXING_GROUP, 0.96f);
-
         root_->attach(lines_);
     }
 }
diff --git a/MixingGroup.h b/MixingGroup.h
index 7a2d999..5e448ab 100644
--- a/MixingGroup.h
+++ b/MixingGroup.h
@@ -4,6 +4,7 @@
 #include <map>
 
 #include "View.h"
+#include "SourceList.h"
 
 class LineLoop;
 class Symbol;
@@ -15,29 +16,34 @@ public:
     MixingGroup (SourceList sources);
     ~MixingGroup ();
 
+    // actions for update
     typedef enum {
         ACTION_NONE = 0,
         ACTION_GRAB_ONE = 1,
         ACTION_GRAB_ALL = 2,
         ACTION_ROTATE_ALL = 3
     } Action;
-
-    // actions for update
     inline void setAction (Action a) { update_action_ = a; }
     inline Action action () { return update_action_; }
     inline void follow (Source *s) { updated_source_ = s; }
 
-    // get node to draw
+    // node to draw
     inline Group *group () { return root_; }
 
+    // accept all kind of visitors
+    virtual void accept (Visitor& v);
+
     // to update in Mixing View
     void update (float dt);
     inline  bool active () const { return active_; }
     void setActive (bool on);
 
-    // individual change of a source in the group
+    // Source list manipulation
+    SourceList::iterator begin ();
+    SourceList::iterator end ();
+    uint size ();
+    bool contains (Source *s);
     void detach (Source *s);
-    void move   (Source *s);
 
 private:
 
@@ -46,10 +52,11 @@ private:
     LineLoop *lines_;
     Symbol *center_;
     void createLineStrip();
+    void move (Source *s);
 
     // properties linked to sources
-    glm::vec2 center_pos_;
-    std::vector<Source *> sources_;
+    glm::vec2  center_pos_;
+    SourceList sources_;
     std::map< Source *, uint> index_points_;
 
     // status and actions
diff --git a/MixingView.cpp b/MixingView.cpp
index cb3a1cc..ab4574f 100644
--- a/MixingView.cpp
+++ b/MixingView.cpp
@@ -129,8 +129,7 @@ void MixingView::draw()
         // special action of Mixing view
         if (ImGui::Selectable( ICON_FA_DRAW_POLYGON "  Link" )){
             // TODO create MixingGroup
-            MixingGroup *mg = new MixingGroup(Mixer::selection().getCopy());
-            groups_.push_back(mg);
+            MixingGroup *mg = Mixer::manager().session()->createMixingGroup(Mixer::selection().getCopy());
             scene.fg()->attach(mg->group());
             Source *cur = Mixer::selection().front();
             Mixer::manager().unsetCurrentSource();
@@ -196,9 +195,9 @@ void MixingView::update(float dt)
 {
     View::update(dt);
 
-    // always update the mixinggroups
-    for (auto g = groups_.begin(); g != groups_.end(); g++)
-        (*g)->update(dt);
+//    // always update the mixinggroups
+//    for (auto g = groups_.begin(); g != groups_.end(); g++)
+//        (*g)->update(dt);
 
     // a more complete update is requested
     // for mixing, this means restore position of the fading slider
@@ -398,8 +397,9 @@ void MixingView::terminate()
 {
     View::terminate();
 
-    // terminate all group actions
-    for (auto g = groups_.begin(); g != groups_.end(); g++)
+    // terminate all mixing group actions
+    for (auto g = Mixer::manager().session()->beginMixingGroup();
+         g != Mixer::manager().session()->endMixingGroup(); g++)
         (*g)->setAction( MixingGroup::ACTION_NONE );
 
 }
diff --git a/MixingView.h b/MixingView.h
index bee41da..24cdad8 100644
--- a/MixingView.h
+++ b/MixingView.h
@@ -3,7 +3,7 @@
 
 #include "View.h"
 
-class MixingGroup;
+//class MixingGroup;
 
 class MixingView : public View
 {
@@ -36,9 +36,6 @@ private:
     Disk *stashCircle_;
     Mesh *mixingCircle_;
     Mesh *circle_;
-
-    // linked sources
-    std::list< MixingGroup *> groups_;
 };
 
 
diff --git a/Selection.cpp b/Selection.cpp
index 37867ae..c58c8e8 100644
--- a/Selection.cpp
+++ b/Selection.cpp
@@ -173,16 +173,15 @@ std::string Selection::xml() const
         SourceList selection_clones_;
         SessionVisitor sv(&xmlDoc, selectionNode);
         for (auto iter = selection_.begin(); iter != selection_.end(); iter++, sv.setRoot(selectionNode) ){
-            // keep the clones for later
+            // start with clones
             CloneSource *clone = dynamic_cast<CloneSource *>(*iter);
             if (clone)
                 (*iter)->accept(sv);
             else
                 selection_clones_.push_back(*iter);
         }
-        // add the clones at the end
+        // add others in front
         for (auto iter = selection_clones_.begin(); iter != selection_clones_.end(); iter++, sv.setRoot(selectionNode) ){
-
             (*iter)->accept(sv);
         }
 
diff --git a/Session.cpp b/Session.cpp
index 10add59..2584914 100644
--- a/Session.cpp
+++ b/Session.cpp
@@ -7,6 +7,7 @@
 #include "FrameGrabber.h"
 #include "SessionCreator.h"
 #include "SessionSource.h"
+#include "MixingGroup.h"
 
 #include "Log.h"
 
@@ -37,6 +38,9 @@ Session::Session() : failedSource_(nullptr), active_(true), fading_target_(0.f)
 
 Session::~Session()
 {
+    // TODO delete all mixing groups?
+
+
     // delete all sources
     for(auto it = sources_.begin(); it != sources_.end(); ) {
         // erase this source from the list
@@ -87,6 +91,18 @@ void Session::update(float dt)
         }
     }
 
+//    // update the mixinggroups
+//    for (auto g = mixing_groups_.begin(); g != mixing_groups_.end(); ) {
+//        // update all valid groups
+//        if ((*g)->size() > 1) {
+//            (*g)->update(dt);
+//            g++;
+//        }
+//        // delete invalid groups (singletons)
+//        else
+//            g = deleteMixingGroup(g);
+//    }
+
     // apply fading (smooth dicotomic reaching)
     float f = render_.fading();
     if ( ABS_DIFF(f, fading_target_) > EPSILON) {
@@ -139,6 +155,9 @@ SourceList::iterator Session::deleteSource(Source *s)
     if (its != sources_.end()) {
         // remove Node from the rendering scene
         render_.scene.ws()->detach( s->group(View::RENDERING) );
+        // inform group
+        if (s->mixingGroup() != nullptr)
+            s->mixingGroup()->detach(s);
         // erase the source from the update list & get next element
         its = sources_.erase(its);
         // delete the source : safe now
@@ -241,11 +260,9 @@ SourceList::iterator Session::find(float depth_from, float depth_to)
     return std::find_if(sources_.begin(), sources_.end(), Source::hasDepth(depth_from, depth_to));
 }
 
-SourceList Session::getCopy() const
+SourceList Session::depthSorted() const
 {
-    SourceList list;
-    list = sources_;
-    return list;
+    return depth_sorted(sources_);
 }
 
 uint Session::numSource() const
@@ -253,17 +270,9 @@ uint Session::numSource() const
     return sources_.size();
 }
 
-std::list<uint64_t> Session::getIdList() const
+SourceIdList Session::getIdList() const
 {
-    std::list<uint64_t> idlist;
-
-    for( auto sit = sources_.begin(); sit != sources_.end(); sit++)
-        idlist.push_back( (*sit)->id() );
-
-    // make sure no duplicate
-    idlist.unique();
-
-    return idlist;
+    return ids(sources_);
 }
 
 bool Session::empty() const
@@ -315,6 +324,73 @@ void Session::move(int current_index, int target_index)
     sources_.insert(to, s);
 }
 
+MixingGroup *Session::createMixingGroup(SourceList sources)
+{
+    // will return the pointer to created MixingGroup
+    MixingGroup *g = nullptr;
+
+    // verify that all sources given are valid in the sesion
+    SourceList valid_sources;
+    for (auto _it = sources.begin(); _it != sources.end(); _it++) {
+        SourceList::iterator found = std::find(sources_.begin(), sources_.end(), *_it);
+        if ( found != sources_.end())
+            valid_sources.push_back(*found);
+    }
+
+    // create mixing group with valid sources (if it is a group of at least 2 sources)
+    if (valid_sources.size() > 1) {
+        g = new MixingGroup(valid_sources);
+        mixing_groups_.push_back(g);
+    }
+
+    return g;
+}
+
+
+bool Session::addMixingGroup (MixingGroup *mg)
+{
+    // verify that all sources given in MixingGroup are valid in the sesion
+    bool ok = true;
+    for (auto _it = mg->begin(); _it != mg->end(); _it++) {
+        SourceList::iterator found = std::find(sources_.begin(), sources_.end(), *_it);
+        if ( found == sources_.end()) {
+            ok = false;
+            break;
+        }
+    }
+    if (ok)
+        mixing_groups_.push_back(mg);
+    return ok;
+}
+
+void Session::createMixingGroups (std::list<SourceList> groups)
+{
+    // create mixing groups (if any)
+    for (auto group_it = groups.begin(); group_it != groups.end(); group_it++){
+         createMixingGroup( *group_it );
+    }
+}
+
+std::vector<MixingGroup *>::iterator Session::eraseMixingGroup (std::vector<MixingGroup *>::iterator g)
+{
+    return mixing_groups_.erase(g);
+}
+
+uint Session::numMixingGroup() const
+{
+    return mixing_groups_.size();
+}
+
+std::vector<MixingGroup *>::iterator Session::beginMixingGroup()
+{
+    return mixing_groups_.begin();
+}
+
+std::vector<MixingGroup *>::iterator Session::endMixingGroup()
+{
+    return mixing_groups_.end();
+}
+
 void Session::lock()
 {
     access_.lock();
@@ -328,9 +404,11 @@ void Session::unlock()
 
 Session *Session::load(const std::string& filename, uint recursion)
 {
+    // create session
     SessionCreator creator(recursion);
     creator.load(filename);
 
+    // return created session
     return creator.session();
 }
 
diff --git a/Session.h b/Session.h
index 38b31ea..80c985e 100644
--- a/Session.h
+++ b/Session.h
@@ -7,6 +7,7 @@
 #include "Source.h"
 
 class FrameGrabber;
+class MixingGroup;
 
 class Session
 {
@@ -39,10 +40,10 @@ public:
     SourceList::iterator find (std::string name);
     SourceList::iterator find (Node *node);
     SourceList::iterator find (float depth_from, float depth_to);
-    SourceList getCopy() const;
+    SourceList depthSorted() const;
 
     SourceList::iterator find (uint64_t id);
-    std::list<uint64_t> getIdList() const;
+    SourceIdList getIdList() const;
 
     SourceList::iterator at (int index);
     int index (SourceList::iterator it) const;    
@@ -75,6 +76,15 @@ public:
     void setFilename(const std::string &filename) { filename_ = filename; }
     std::string filename() const { return filename_; }
 
+    // mixing group
+    bool addMixingGroup(MixingGroup *mg);
+    MixingGroup *createMixingGroup (SourceList sources);
+    std::vector<MixingGroup *>::iterator eraseMixingGroup (std::vector<MixingGroup *>::iterator g);
+    void createMixingGroups (std::list<SourceList> groups);
+    uint numMixingGroup() const;
+    std::vector<MixingGroup *>::iterator beginMixingGroup();
+    std::vector<MixingGroup *>::iterator endMixingGroup();
+
     // lock and unlock access (e.g. while saving)
     void lock();
     void unlock();
@@ -84,6 +94,7 @@ protected:
     std::string filename_;
     Source *failedSource_;
     SourceList sources_;
+    std::vector<MixingGroup *> mixing_groups_;
     std::map<View::Mode, Group*> config_;
     bool active_;
     std::list<FrameGrabber *> grabbers_;
diff --git a/SessionCreator.cpp b/SessionCreator.cpp
index afe6644..5f9864c 100644
--- a/SessionCreator.cpp
+++ b/SessionCreator.cpp
@@ -91,8 +91,36 @@ void SessionCreator::load(const std::string& filename)
     // ready to read sources
     SessionLoader::load( xmlDoc_.FirstChildElement("Session") );
 
+    // create groups
+    session_->createMixingGroups( getMixingGroups() );
+
     // all good
     session_->setFilename(filename);
+
+//    for (std::map< uint64_t, Source* >::iterator qwe = sources_id_.begin(); qwe != sources_id_.end(); qwe++){
+//        Log::Info("%ld - %ld ", qwe->first, qwe->second->id());
+//    }
+
+//    int count = 0;
+//    std::list< SourceIdList >::iterator git;
+//    for (git = groups_sources_id_.begin(); git != groups_sources_id_.end(); git++)
+//    {
+//        Log::Info("XML Groups %d", count++);
+//        std::list<uint64_t>::iterator sit;
+//        for (sit = (*git).begin(); sit != (*git).end(); sit++ ) {
+//            Log::Info("- %ld", (*sit) );
+//        }
+//    }
+
+//    std::list< SourceList > ngrou = getMixingGroupsIdList();
+//    count = 0;
+//    for (auto git = ngrou.begin(); git != ngrou.end(); git++)
+//    {
+//        Log::Info("NEW Groups %d", count++);
+//        for (auto sit = (*git).begin(); sit != (*git).end(); sit++  ) {
+//            Log::Info("- %ld", (*sit)->id() );
+//        }
+//    }
 }
 
 
@@ -108,11 +136,48 @@ void SessionCreator::loadConfig(XMLElement *viewsNode)
     }
 }
 
-SessionLoader::SessionLoader(Session *session, int recursion): Visitor(), session_(session), recursion_(recursion)
+SessionLoader::SessionLoader(Session *session, int recursion): Visitor(),
+    session_(session), xmlCurrent_(nullptr), recursion_(recursion)
 {
 
 }
 
+// get the list of new source id (value in sources_id_ map)
+SourceIdList SessionLoader::getIdList() const
+{
+    SourceIdList id_current_value;
+    for (auto it = sources_id_.begin(); it != sources_id_.end(); it++)
+        id_current_value.push_back( it->second->id() );
+
+    return id_current_value;
+}
+
+// groups_sources_id_ is parsed in XML and contains list of groups of ids
+// Here we return the list of groups of newly created sources
+// based on correspondance map sources_id_
+// NB: importantly the list is cleared from duplicates
+std::list< SourceList > SessionLoader::getMixingGroups() const
+{
+    std::list< SourceList > groups_new_sources_id;
+
+    // perform conversion from xml id to new id
+    for (auto git = groups_sources_id_.begin(); git != groups_sources_id_.end(); git++)
+    {
+        SourceList new_sources;
+        for (auto sit = (*git).begin(); sit != (*git).end(); sit++  ) {
+            if (sources_id_.count(*sit) > 0)
+                new_sources.push_back( sources_id_.at(*sit) );
+        }
+        new_sources.sort();
+        groups_new_sources_id.push_back( new_sources );
+    }
+
+    // remove duplicates
+    groups_new_sources_id.unique();
+
+    return groups_new_sources_id;
+}
+
 void SessionLoader::load(XMLElement *sessionNode)
 {
     sources_id_.clear();
@@ -133,9 +198,9 @@ void SessionLoader::load(XMLElement *sessionNode)
             Source *load_source = nullptr;
 
             // check if a source with the given id exists in the session
-            uint64_t id__ = 0;
-            xmlCurrent_->QueryUnsigned64Attribute("id", &id__);
-            SourceList::iterator sit = session_->find(id__);
+            uint64_t id_xml_ = 0;
+            xmlCurrent_->QueryUnsigned64Attribute("id", &id_xml_);
+            SourceList::iterator sit = session_->find(id_xml_);
 
             // no source with this id exists
             if ( sit == session_->end() ) {
@@ -179,8 +244,10 @@ void SessionLoader::load(XMLElement *sessionNode)
             // apply config to source
             load_source->accept(*this);
             load_source->touch();
+
             // remember
-            sources_id_.push_back( load_source->id() );
+            sources_id_[id_xml_] = load_source;
+//            sources_id_[id_xml_] = load_source->id();
         }
 
         // create clones after all sources, to be able to clone a source created above
@@ -194,9 +261,9 @@ void SessionLoader::load(XMLElement *sessionNode)
             if ( pType && std::string(pType) == "CloneSource") {
 
                 // check if a source with same id exists
-                uint64_t id__ = 0;
-                xmlCurrent_->QueryUnsigned64Attribute("id", &id__);
-                SourceList::iterator sit = session_->find(id__);
+                uint64_t id_xml_ = 0;
+                xmlCurrent_->QueryUnsigned64Attribute("id", &id_xml_);
+                SourceList::iterator sit = session_->find(id_xml_);
 
                 // no source clone with this id exists
                 if ( sit == session_->end() ) {
@@ -210,20 +277,23 @@ void SessionLoader::load(XMLElement *sessionNode)
                         if (origin != session_->end()) {
                             // create a new source of type Clone
                             Source *clone_source = (*origin)->clone();
+
                             // add source to session
                             session_->addSource(clone_source);
+
                             // apply config to source
                             clone_source->accept(*this);
                             clone_source->touch();
+
                             // remember
-                            sources_id_.push_back( clone_source->id() );
+                            sources_id_[id_xml_] = clone_source;
+//                            sources_id_[id_xml_] = clone_source->id();
                         }
                     }
                 }
             }
         }
-        // make sure no duplicate
-        sources_id_.unique();
+
     }
 
 }
@@ -514,6 +584,18 @@ void SessionLoader::visit (Source& s)
         s.setImageProcessingEnabled(on);
     }
 
+    xmlCurrent_ = sourceNode->FirstChildElement("MixingGroup");
+    if (xmlCurrent_) {
+        SourceIdList idlist;
+        XMLElement* sourceNode = xmlCurrent_->FirstChildElement("source");
+        for ( ; sourceNode ; sourceNode = sourceNode->NextSiblingElement()) {
+            uint64_t id__ = 0;
+            sourceNode->QueryUnsigned64Attribute("id", &id__);
+            idlist.push_back(id__);
+        }
+        groups_sources_id_.push_back(idlist);
+    }
+
     // restore current
     xmlCurrent_ = sourceNode;
 }
diff --git a/SessionCreator.h b/SessionCreator.h
index 3996632..0c637ee 100644
--- a/SessionCreator.h
+++ b/SessionCreator.h
@@ -1,10 +1,11 @@
 #ifndef SESSIONCREATOR_H
 #define SESSIONCREATOR_H
 
-#include <list>
+#include <map>
+#include <tinyxml2.h>
 
 #include "Visitor.h"
-#include <tinyxml2.h>
+#include "SourceList.h"
 
 class Session;
 
@@ -17,7 +18,8 @@ public:
     inline Session *session() const { return session_; }
 
     void load(tinyxml2::XMLElement *sessionNode);
-    inline std::list<uint64_t> getIdList() const { return sources_id_; }
+    SourceIdList getIdList() const;
+    std::list< SourceList > getMixingGroups() const;
 
     Source *createSource(tinyxml2::XMLElement *sourceNode, bool clone_duplicates = true);
 
@@ -46,10 +48,16 @@ public:
     void visit (NetworkSource& s) override;
 
 protected:
-    tinyxml2::XMLElement *xmlCurrent_;
+    // result created session
     Session *session_;
-    std::list<uint64_t> sources_id_;
+    // parsing current xml
+    tinyxml2::XMLElement *xmlCurrent_;
+    // level of loading recursion
     int recursion_;
+    // map of correspondance from xml source id (key) to new source pointer (value)
+    std::map< uint64_t, Source* > sources_id_;
+    // list of groups (lists of xml source id)
+    std::list< SourceIdList > groups_sources_id_;
 
     static void XMLToNode(tinyxml2::XMLElement *xml, Node &n);
 };
diff --git a/SessionVisitor.cpp b/SessionVisitor.cpp
index 2f8139b..068b7d3 100644
--- a/SessionVisitor.cpp
+++ b/SessionVisitor.cpp
@@ -13,6 +13,7 @@
 #include "ImageShader.h"
 #include "ImageProcessingShader.h"
 #include "MediaPlayer.h"
+#include "MixingGroup.h"
 
 #include <iostream>
 
@@ -274,16 +275,6 @@ void SessionVisitor::visit(LineStrip &n)
         points_node->InsertEndChild(p);
     }
     xmlCurrent_->InsertEndChild(points_node);
-
-//    XMLElement *colors_node = xmlDoc_->NewElement("colors");
-//    std::vector<glm::vec4> colors = n.getColors();
-//    for(size_t i = 0; i < colors.size(); ++i)
-//    {
-//        XMLElement *p = XMLElementFromGLM(xmlDoc_, colors[i]);
-//        p->SetAttribute("index", (int) i);
-//        colors_node->InsertEndChild(p);
-//    }
-//    xmlCurrent_->InsertEndChild(colors_node);
 }
 
 void SessionVisitor::visit(LineSquare &)
@@ -392,6 +383,12 @@ void SessionVisitor::visit (Source& s)
     sourceNode->InsertEndChild(xmlCurrent_);
     s.processingShader()->accept(*this);
 
+    if (s.mixingGroup()) {
+        xmlCurrent_ = xmlDoc_->NewElement( "MixingGroup" );
+        sourceNode->InsertEndChild(xmlCurrent_);
+        s.mixingGroup()->accept(*this);
+    }
+
     xmlCurrent_ = sourceNode;  // parent for next visits (other subtypes of Source)
 }
 
@@ -423,11 +420,11 @@ void SessionVisitor::visit (SessionGroupSource& s)
 
     Session *se = s.session();
 
-    XMLElement *rootgroup = xmlDoc_->NewElement("Session");
-    xmlCurrent_->InsertEndChild(rootgroup);
+    XMLElement *sessionNode = xmlDoc_->NewElement("Session");
+    xmlCurrent_->InsertEndChild(sessionNode);
 
     for (auto iter = se->begin(); iter != se->end(); iter++){
-        setRoot(rootgroup);
+        setRoot(sessionNode);
         (*iter)->accept(*this);
     }
 
@@ -469,3 +466,14 @@ void SessionVisitor::visit (NetworkSource& s)
     xmlCurrent_->SetAttribute("type", "NetworkSource");
     xmlCurrent_->SetAttribute("connection", s.connection().c_str() );
 }
+
+void SessionVisitor::visit (MixingGroup& g)
+{
+    xmlCurrent_->SetAttribute("size", g.size());
+
+    for (auto it = g.begin(); it != g.end(); it++) {
+        XMLElement *sour = xmlDoc_->NewElement("source");
+        sour->SetAttribute("id", (*it)->id());
+        xmlCurrent_->InsertEndChild(sour);
+    }
+}
diff --git a/SessionVisitor.h b/SessionVisitor.h
index a16e113..564bbc5 100644
--- a/SessionVisitor.h
+++ b/SessionVisitor.h
@@ -50,6 +50,7 @@ public:
     void visit (PatternSource& s) override;
     void visit (DeviceSource& s) override;
     void visit (NetworkSource& s) override;
+    void visit (MixingGroup& s) override;
 
     static tinyxml2::XMLElement *NodeToXML(Node &n, tinyxml2::XMLDocument *doc);
 };
diff --git a/Source.cpp b/Source.cpp
index 7cc950f..9fe7014 100644
--- a/Source.cpp
+++ b/Source.cpp
@@ -238,10 +238,6 @@ Source::~Source()
         (*it)->detach();
     clones_.clear();
 
-    // inform group
-    if (mixinggroup_)
-        mixinggroup_->detach(this);
-
     // delete objects
     delete stored_status_;
     if (renderbuffer_)
diff --git a/Source.h b/Source.h
index 305ee0d..e2bf764 100644
--- a/Source.h
+++ b/Source.h
@@ -131,12 +131,17 @@ public:
     void setMask (FrameBufferImage *img);
     void storeMask (FrameBufferImage *img = nullptr);
 
+    // operations on depth
     float depth () const;
     void  setDepth (float d);
 
+    // operations on alpha
     float alpha () const;
     void  setAlpha (float a);
 
+    // groups for mixing
+    MixingGroup *mixingGroup() const { return mixinggroup_; }
+
     struct hasNode: public std::unary_function<Source*, bool>
     {
         bool operator()(const Source* elem) const;
@@ -179,6 +184,7 @@ public:
         float _to;
     };
 
+    // class-dependent icon
     virtual glm::ivec2 icon () const { return glm::ivec2(12, 11); }
 
     // get the xml description text of a source
diff --git a/SourceList.cpp b/SourceList.cpp
new file mode 100644
index 0000000..b3a3b4f
--- /dev/null
+++ b/SourceList.cpp
@@ -0,0 +1,118 @@
+#include <algorithm>
+
+#include <glm/gtx/vector_angle.hpp>
+#include <glm/gtx/rotate_vector.hpp>
+
+#include "Source.h"
+#include "SourceList.h"
+
+// utility to sort Sources by depth
+bool compare_depth (Source * first, Source * second)
+{
+  return ( first->depth() < second->depth() );
+}
+
+SourceList depth_sorted(SourceList list)
+{
+    SourceList sl = list;
+    sl.sort(compare_depth);
+
+    return sl;
+}
+
+// utility to sort Sources in MixingView in a clockwise order
+// in reference to a center point
+struct clockwise_centered {
+    clockwise_centered(glm::vec2 c) : center(c) { }
+    bool operator() (Source * first, Source * second) {
+        glm::vec2 pos_first = glm::vec2(first->group(View::MIXING)->translation_)-center;
+        float angle_first = glm::orientedAngle( glm::normalize(pos_first), glm::vec2(1.f, 0.f) );
+        glm::vec2 pos_second = glm::vec2(second->group(View::MIXING)->translation_)-center;
+        float angle_second = glm::orientedAngle( glm::normalize(pos_second), glm::vec2(1.f, 0.f) );
+        return (angle_first < angle_second);
+    }
+    glm::vec2 center;
+};
+
+SourceList mixing_sorted(SourceList list, glm::vec2 center)
+{
+    SourceList sl = list;
+    sl.sort(clockwise_centered(center));
+
+    return sl;
+}
+
+
+SourceIdList ids (SourceList list)
+{
+    SourceIdList idlist;
+
+    for( auto sit = list.begin(); sit != list.end(); sit++)
+        idlist.push_back( (*sit)->id() );
+
+    // make sure no duplicate
+    idlist.unique();
+
+    return idlist;
+}
+
+
+SourceListCompare compare (SourceList first, SourceList second)
+{
+    SourceListCompare ret = SOURCELIST_DISTINCT;
+    if (first.empty() || first.empty())
+        return ret;
+
+    // a new test list: start with the second list and remove all commons with first list
+    SourceList test = second;
+    for (auto it = first.begin(); it != first.end(); it++){
+        test.remove(*it);
+    }
+
+    // all sources of the second list were in the first list
+    if (test.empty()) {
+        // same size, therefore they are the same!
+        if (first.size() == second.size())
+            ret = SOURCELIST_EQUAL;
+        // otherwise, first list contains all sources of the second list.
+        else
+            ret = SOURCELIST_SECOND_IN_FIRST;
+    }
+    // some sources of the second list were in the first
+    else if ( second.size() != test.size() ){
+        // if the number of sources removed from second is the number of sources in the first
+        if (second.size() - test.size() == first.size())
+            ret = SOURCELIST_FIRST_IN_SECOND;
+        // else, there is a patrial intersection
+        else
+            ret = SOURCELIST_INTERSECT;
+    }
+    // else no intersection, lists are distinct (return detault)
+
+    return ret;
+}
+
+
+SourceList intersect (SourceList first, SourceList second)
+{
+    // take second list and remove all elements also in first list
+    // -> builds the list of what remains in second list
+    SourceList l1 = second;
+    for (auto it = first.begin(); it != first.end(); it++)
+        l1.remove(*it);
+    // take second list and remove all elements in the remainer list
+    // -> builds the list of what is in second list and was part of the first list
+    SourceList l2 = second;
+    for (auto it = l1.begin(); it != l1.end(); it++)
+        l2.remove(*it);
+    return l2;
+}
+
+
+SourceList join (SourceList first, SourceList second)
+{
+    SourceList l = second;
+    for (auto it = first.begin(); it != first.end(); it++)
+        l.push_back(*it);
+    return l;
+}
diff --git a/SourceList.h b/SourceList.h
new file mode 100644
index 0000000..52ea72a
--- /dev/null
+++ b/SourceList.h
@@ -0,0 +1,28 @@
+#ifndef SOURCELIST_H
+#define SOURCELIST_H
+
+#include <list>
+#include <glm/glm.hpp>
+
+class Source;
+
+typedef std::list<Source *> SourceList;
+
+SourceList depth_sorted  (SourceList list);
+SourceList mixing_sorted (SourceList list, glm::vec2 center = glm::vec2(0.f, 0.f));
+SourceList intersect     (SourceList first, SourceList second);
+SourceList join          (SourceList first, SourceList second);
+
+typedef enum {
+    SOURCELIST_DISTINCT = 0,
+    SOURCELIST_INTERSECT = 1,
+    SOURCELIST_EQUAL = 2,
+    SOURCELIST_FIRST_IN_SECOND = 3,
+    SOURCELIST_SECOND_IN_FIRST = 4
+} SourceListCompare;
+SourceListCompare compare (SourceList first, SourceList second);
+
+typedef std::list<uint64_t> SourceIdList;
+SourceIdList ids (SourceList list);
+
+#endif // SOURCELIST_H
diff --git a/View.cpp b/View.cpp
index a3ce5df..d42a87f 100644
--- a/View.cpp
+++ b/View.cpp
@@ -275,16 +275,3 @@ void View::updateSelectionOverlay()
         overlay_selection_->scale_ = glm::vec3(0.f, 0.f, 1.f);
 }
 
-
-bool compare_depth (Source * first, Source * second)
-{
-  return ( first->depth() < second->depth() );
-}
-
-SourceList depthSorted(SourceList list)
-{
-    SourceList dsl = list;
-    dsl.sort(compare_depth);
-
-    return dsl;
-}
diff --git a/View.h b/View.h
index 82cfed4..c21db46 100644
--- a/View.h
+++ b/View.h
@@ -5,6 +5,7 @@
 
 #include "Scene.h"
 #include "FrameBuffer.h"
+#include "SourceList.h"
 
 class Session;
 class SessionFileSource;
@@ -16,8 +17,6 @@ class Disk;
 class Handles;
 class Source;
 
-typedef std::list<Source *> SourceList;
-SourceList depthSorted(SourceList);
 
 class View
 {
diff --git a/Visitor.h b/Visitor.h
index da0bdff..2842cdd 100644
--- a/Visitor.h
+++ b/Visitor.h
@@ -37,6 +37,7 @@ class SessionGroupSource;
 class RenderSource;
 class CloneSource;
 class NetworkSource;
+class MixingGroup;
 
 // Declares the interface for the visitors
 class Visitor {
@@ -69,6 +70,7 @@ public:
     virtual void visit (ImageProcessingShader&) {}
 
     // utility
+    virtual void visit (MixingGroup&) {}
     virtual void visit (Source&) {}
     virtual void visit (MediaSource&) {}
     virtual void visit (NetworkSource&) {}