Fix for #34739 and #35060, avoid ambiguity in compositor viewer nodes.

The design changes coming with pynodes for the node editor allow editing multiple node groups or pinning. This is great for working on different node groups without switching between them all the time, but it causes a problem for viewer nodes: these nodes all write to the same Image data by design, causing access conflicts and in some cases memory corruption. This was not a problem before pynodes because the editor would only allow 1 edited node group at any time. With the new flexibility of node editors this restriction is gone.

In order to avoid concurrent write access to the viewer image buffer and resolve the ambiguity this patch adds an "active viewer key" to the scene->nodetree (added in bNodeTree instead of Scene due to otherwise circular DNA includes). This key identifies a specific node tree/group instance, which enables the compositor to selectively enable only 1 viewer node.

The active viewer key is switched when opening/closing node groups (push/pop on the snode->treepath stack) or when selecting a viewer node. This way only the "last edited" viewer will be active.

Eventually it would be nicer if each viewer had its own buffer per node space so one could actually compare viewers without switching. But that is a major redesign of viewer nodes and images, not a quick fix for bcon4 ...

15 files changed, 165 insertions, 136 deletions

diff --git a/source/blender/blenkernel/BKE_node.h b/source/blender/blenkernel/BKE_node.h
index adef4853002..d0b3b47c361 100644
--- a/source/blender/blenkernel/BKE_node.h
+++ b/source/blender/blenkernel/BKE_node.h
@@ -507,6 +507,7 @@ typedef struct bNodeInstanceHash
 typedef void (*bNodeInstanceValueFP)(void *value);
 
 extern const bNodeInstanceKey NODE_INSTANCE_KEY_BASE;
+extern const bNodeInstanceKey NODE_INSTANCE_KEY_NONE;
 
 bNodeInstanceKey       BKE_node_instance_key(bNodeInstanceKey parent_key, struct bNodeTree *ntree, struct bNode *node);
 
diff --git a/source/blender/blenkernel/intern/node.c b/source/blender/blenkernel/intern/node.c
index a62bfebed45..f41d6387e4e 100644
--- a/source/blender/blenkernel/intern/node.c
+++ b/source/blender/blenkernel/intern/node.c
@@ -2562,6 +2562,7 @@ int BKE_node_clipboard_get_type(void)
 
 /* magic number for initial hash key */
 const bNodeInstanceKey NODE_INSTANCE_KEY_BASE = {5381};
+const bNodeInstanceKey NODE_INSTANCE_KEY_NONE = {0};
 
 /* Generate a hash key from ntree and node names
  * Uses the djb2 algorithm with xor by Bernstein:
diff --git a/source/blender/compositor/intern/COM_CompositorContext.h b/source/blender/compositor/intern/COM_CompositorContext.h
index 3c7db703bf9..2120c12b04e 100644
--- a/source/blender/compositor/intern/COM_CompositorContext.h
+++ b/source/blender/compositor/intern/COM_CompositorContext.h
@@ -84,6 +84,7 @@ private:
 	/* @brief color management settings */
 	const ColorManagedViewSettings *m_viewSettings;
 	const ColorManagedDisplaySettings *m_displaySettings;
+
 public:
 	/**
 	 * @brief constructor initializes the context with default values.
diff --git a/source/blender/compositor/intern/COM_ExecutionSystem.cpp b/source/blender/compositor/intern/COM_ExecutionSystem.cpp
index 450d1b03917..ad29405d2e9 100644
--- a/source/blender/compositor/intern/COM_ExecutionSystem.cpp
+++ b/source/blender/compositor/intern/COM_ExecutionSystem.cpp
@@ -53,16 +53,6 @@ ExecutionSystem::ExecutionSystem(RenderData *rd, bNodeTree *editingtree, bool re
 	this->m_context.setbNodeTree(editingtree);
 	this->m_context.setPreviewHash(editingtree->previews);
 	this->m_context.setFastCalculation(fastcalculation);
-#if 0	/* XXX TODO find a better way to define visible output nodes from all editors */
-	bNode *gnode;
-	for (gnode = (bNode *)editingtree->nodes.first; gnode; gnode = gnode->next) {
-		if (gnode->type == NODE_GROUP && gnode->typeinfo->group_edit_get(gnode)) {
-			this->m_context.setActivegNode(gnode);
-			break;
-		}
-	}
-#endif
-
 	/* initialize the CompositorContext */
 	if (rendering) {
 		this->m_context.setQuality((CompositorQuality)editingtree->render_quality);
diff --git a/source/blender/compositor/intern/COM_ExecutionSystemHelper.cpp b/source/blender/compositor/intern/COM_ExecutionSystemHelper.cpp
index eb0c9cbdf11..9f0a943c8a2 100644
--- a/source/blender/compositor/intern/COM_ExecutionSystemHelper.cpp
+++ b/source/blender/compositor/intern/COM_ExecutionSystemHelper.cpp
@@ -47,12 +47,12 @@ void ExecutionSystemHelper::addbNodeTree(ExecutionSystem &system, int nodes_star
 	vector<Node *>& nodes = system.getNodes();
 	vector<SocketConnection *>& links = system.getConnections();
 	
+	bool is_active_group = (parent_key.value == system.getContext().getbNodeTree()->active_viewer_key.value);
+	
 	/* add all nodes of the tree to the node list */
 	bNode *node = (bNode *)tree->nodes.first;
 	while (node != NULL) {
-		/* XXX TODO replace isActiveGroup by a more accurate check, all visible editors should do this! */
-		bool isActiveGroup = true;
-		Node *nnode = addNode(nodes, node, isActiveGroup, system.getContext().isFastCalculation());
+		Node *nnode = addNode(nodes, node, is_active_group, system.getContext().isFastCalculation());
 		if (nnode) {
 			nnode->setbNodeTree(tree);
 			nnode->setInstanceKey(BKE_node_instance_key(parent_key, tree, node));
diff --git a/source/blender/compositor/intern/COM_compositor.cpp b/source/blender/compositor/intern/COM_compositor.cpp
index a0f660058f9..a0eb5206805 100644
--- a/source/blender/compositor/intern/COM_compositor.cpp
+++ b/source/blender/compositor/intern/COM_compositor.cpp
@@ -92,7 +92,8 @@ void COM_execute(RenderData *rd, bNodeTree *editingtree, int rendering,
 		}
 	}
 
-	ExecutionSystem *system = new ExecutionSystem(rd, editingtree, rendering, false, viewSettings, displaySettings);
+	ExecutionSystem *system = new ExecutionSystem(rd, editingtree, rendering, false,
+	                                              viewSettings, displaySettings);
 	system->execute();
 	delete system;
 
diff --git a/source/blender/editors/include/ED_node.h b/source/blender/editors/include/ED_node.h
index 448a15334c7..2f16d84aed0 100644
--- a/source/blender/editors/include/ED_node.h
+++ b/source/blender/editors/include/ED_node.h
@@ -65,6 +65,8 @@ void ED_node_tree_pop(struct SpaceNode *snode);
 int ED_node_tree_depth(struct SpaceNode *snode);
 struct bNodeTree *ED_node_tree_get(struct SpaceNode *snode, int level);
 
+void ED_node_set_active_viewer_key(struct SpaceNode *snode);
+
 /* drawnode.c */
 void ED_node_init_butfuncs(void);
 void ED_init_custom_node_type(struct bNodeType *ntype);
diff --git a/source/blender/editors/space_node/drawnode.c b/source/blender/editors/space_node/drawnode.c
index d326bd6eca5..5ab8a248d63 100644
--- a/source/blender/editors/space_node/drawnode.c
+++ b/source/blender/editors/space_node/drawnode.c
@@ -2835,135 +2835,143 @@ void ED_init_node_socket_type_virtual(bNodeSocketType *stype)
 
 /* ************** Generic drawing ************** */
 
-void draw_nodespace_back_pix(const bContext *C, ARegion *ar, SpaceNode *snode)
-{
-	if ((snode->flag & SNODE_BACKDRAW) && ED_node_is_compositor(snode)) {
-		Image *ima = BKE_image_verify_viewer(IMA_TYPE_COMPOSITE, "Viewer Node");
-		void *lock;
-		ImBuf *ibuf = BKE_image_acquire_ibuf(ima, NULL, &lock);
-		if (ibuf) {
-			float x, y; 
-			
-			glMatrixMode(GL_PROJECTION);
-			glPushMatrix();
-			glMatrixMode(GL_MODELVIEW);
-			glPushMatrix();
-
-			/* keep this, saves us from a version patch */
-			if (snode->zoom == 0.0f) snode->zoom = 1.0f;
-			
-			/* somehow the offset has to be calculated inverse */
-			
-			glaDefine2DArea(&ar->winrct);
-			/* ortho at pixel level curarea */
-			wmOrtho2(-GLA_PIXEL_OFS, ar->winx - GLA_PIXEL_OFS, -GLA_PIXEL_OFS, ar->winy - GLA_PIXEL_OFS);
+void draw_nodespace_back_pix(const bContext *C, ARegion *ar, SpaceNode *snode, bNodeInstanceKey parent_key)
+{
+	bNodeInstanceKey active_viewer_key = (snode->nodetree ? snode->nodetree->active_viewer_key : NODE_INSTANCE_KEY_NONE);
+	Image *ima;
+	void *lock;
+	ImBuf *ibuf;
+	
+	if (!(snode->flag & SNODE_BACKDRAW) || !ED_node_is_compositor(snode))
+		return;
+	
+	if (parent_key.value != active_viewer_key.value)
+		return;
+	
+	ima = BKE_image_verify_viewer(IMA_TYPE_COMPOSITE, "Viewer Node");
+	ibuf = BKE_image_acquire_ibuf(ima, NULL, &lock);
+	if (ibuf) {
+		float x, y; 
+		
+		glMatrixMode(GL_PROJECTION);
+		glPushMatrix();
+		glMatrixMode(GL_MODELVIEW);
+		glPushMatrix();
+		
+		/* keep this, saves us from a version patch */
+		if (snode->zoom == 0.0f) snode->zoom = 1.0f;
+		
+		/* somehow the offset has to be calculated inverse */
+		
+		glaDefine2DArea(&ar->winrct);
+		/* ortho at pixel level curarea */
+		wmOrtho2(-GLA_PIXEL_OFS, ar->winx - GLA_PIXEL_OFS, -GLA_PIXEL_OFS, ar->winy - GLA_PIXEL_OFS);
+		
+		x = (ar->winx - snode->zoom * ibuf->x) / 2 + snode->xof;
+		y = (ar->winy - snode->zoom * ibuf->y) / 2 + snode->yof;
+		
+		if (ibuf->rect || ibuf->rect_float) {
+			unsigned char *display_buffer = NULL;
+			void *cache_handle = NULL;
 			
-			x = (ar->winx - snode->zoom * ibuf->x) / 2 + snode->xof;
-			y = (ar->winy - snode->zoom * ibuf->y) / 2 + snode->yof;
-
-			if (ibuf->rect || ibuf->rect_float) {
-				unsigned char *display_buffer = NULL;
-				void *cache_handle = NULL;
-
-				if (snode->flag & (SNODE_SHOW_R | SNODE_SHOW_G | SNODE_SHOW_B)) {
-					int ofs;
-
-					display_buffer = IMB_display_buffer_acquire_ctx(C, ibuf, &cache_handle);
-
+			if (snode->flag & (SNODE_SHOW_R | SNODE_SHOW_G | SNODE_SHOW_B)) {
+				int ofs;
+				
+				display_buffer = IMB_display_buffer_acquire_ctx(C, ibuf, &cache_handle);
+				
 #ifdef __BIG_ENDIAN__
-					if      (snode->flag & SNODE_SHOW_R) ofs = 2;
-					else if (snode->flag & SNODE_SHOW_G) ofs = 1;
-					else                                 ofs = 0;
+				if      (snode->flag & SNODE_SHOW_R) ofs = 2;
+				else if (snode->flag & SNODE_SHOW_G) ofs = 1;
+				else                                 ofs = 0;
 #else
-					if      (snode->flag & SNODE_SHOW_R) ofs = 1;
-					else if (snode->flag & SNODE_SHOW_G) ofs = 2;
-					else                                 ofs = 3;
+				if      (snode->flag & SNODE_SHOW_R) ofs = 1;
+				else if (snode->flag & SNODE_SHOW_G) ofs = 2;
+				else                                 ofs = 3;
 #endif
-
-					glPixelZoom(snode->zoom, snode->zoom);
-					/* swap bytes, so alpha is most significant one, then just draw it as luminance int */
-
-					glaDrawPixelsSafe(x, y, ibuf->x, ibuf->y, ibuf->x, GL_LUMINANCE, GL_UNSIGNED_INT,
-					                  display_buffer + ofs);
-
-					glPixelZoom(1.0f, 1.0f);
-				}
-				else if (snode->flag & SNODE_SHOW_ALPHA) {
-					display_buffer = IMB_display_buffer_acquire_ctx(C, ibuf, &cache_handle);
-
-					glPixelZoom(snode->zoom, snode->zoom);
-					/* swap bytes, so alpha is most significant one, then just draw it as luminance int */
+				
+				glPixelZoom(snode->zoom, snode->zoom);
+				/* swap bytes, so alpha is most significant one, then just draw it as luminance int */
+				
+				glaDrawPixelsSafe(x, y, ibuf->x, ibuf->y, ibuf->x, GL_LUMINANCE, GL_UNSIGNED_INT,
+				                  display_buffer + ofs);
+				
+				glPixelZoom(1.0f, 1.0f);
+			}
+			else if (snode->flag & SNODE_SHOW_ALPHA) {
+				display_buffer = IMB_display_buffer_acquire_ctx(C, ibuf, &cache_handle);
+				
+				glPixelZoom(snode->zoom, snode->zoom);
+				/* swap bytes, so alpha is most significant one, then just draw it as luminance int */
 #ifdef __BIG_ENDIAN__
-					glPixelStorei(GL_UNPACK_SWAP_BYTES, 1);
+				glPixelStorei(GL_UNPACK_SWAP_BYTES, 1);
 #endif
-					glaDrawPixelsSafe(x, y, ibuf->x, ibuf->y, ibuf->x, GL_LUMINANCE, GL_UNSIGNED_INT, display_buffer);
-
+				glaDrawPixelsSafe(x, y, ibuf->x, ibuf->y, ibuf->x, GL_LUMINANCE, GL_UNSIGNED_INT, display_buffer);
+				
 #ifdef __BIG_ENDIAN__
-					glPixelStorei(GL_UNPACK_SWAP_BYTES, 0);
+				glPixelStorei(GL_UNPACK_SWAP_BYTES, 0);
 #endif
-					glPixelZoom(1.0f, 1.0f);
-				}
-				else if (snode->flag & SNODE_USE_ALPHA) {
-					glEnable(GL_BLEND);
-					glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
-					glPixelZoom(snode->zoom, snode->zoom);
-					
-					glaDrawImBuf_glsl_ctx(C, ibuf, x, y, GL_NEAREST);
-					
-					glPixelZoom(1.0f, 1.0f);
-					glDisable(GL_BLEND);
-				}
-				else {
-					glPixelZoom(snode->zoom, snode->zoom);
-
-					glaDrawImBuf_glsl_ctx(C, ibuf, x, y, GL_NEAREST);
-					
-					glPixelZoom(1.0f, 1.0f);
-				}
-
-				if (cache_handle)
-					IMB_display_buffer_release(cache_handle);
+				glPixelZoom(1.0f, 1.0f);
 			}
-
-			/** @note draw selected info on backdrop */
-			if (snode->edittree) {
-				bNode *node = snode->edittree->nodes.first;
-				rctf *viewer_border = &snode->nodetree->viewer_border;
-				while (node) {
-					if (node->flag & NODE_SELECT) {
-						if (node->typeinfo->uibackdropfunc) {
-							node->typeinfo->uibackdropfunc(snode, ibuf, node, x, y);
-						}
+			else if (snode->flag & SNODE_USE_ALPHA) {
+				glEnable(GL_BLEND);
+				glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
+				glPixelZoom(snode->zoom, snode->zoom);
+				
+				glaDrawImBuf_glsl_ctx(C, ibuf, x, y, GL_NEAREST);
+				
+				glPixelZoom(1.0f, 1.0f);
+				glDisable(GL_BLEND);
+			}
+			else {
+				glPixelZoom(snode->zoom, snode->zoom);
+				
+				glaDrawImBuf_glsl_ctx(C, ibuf, x, y, GL_NEAREST);
+				
+				glPixelZoom(1.0f, 1.0f);
+			}
+			
+			if (cache_handle)
+				IMB_display_buffer_release(cache_handle);
+		}
+		
+		/** @note draw selected info on backdrop */
+		if (snode->edittree) {
+			bNode *node = snode->edittree->nodes.first;
+			rctf *viewer_border = &snode->nodetree->viewer_border;
+			while (node) {
+				if (node->flag & NODE_SELECT) {
+					if (node->typeinfo->uibackdropfunc) {
+						node->typeinfo->uibackdropfunc(snode, ibuf, node, x, y);
 					}
-					node = node->next;
-				}
-
-				if ((snode->nodetree->flag & NTREE_VIEWER_BORDER) &&
-				    viewer_border->xmin < viewer_border->xmax &&
-				    viewer_border->ymin < viewer_border->ymax)
-				{
-					glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
-					setlinestyle(3);
-					cpack(0x4040FF);
-
-					glRectf(x + snode->zoom * viewer_border->xmin * ibuf->x,
-					        y + snode->zoom * viewer_border->ymin * ibuf->y,
-					        x + snode->zoom * viewer_border->xmax * ibuf->x,
-					        y + snode->zoom * viewer_border->ymax * ibuf->y);
-
-					setlinestyle(0);
-					glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
 				}
+				node = node->next;
 			}
 			
-			glMatrixMode(GL_PROJECTION);
-			glPopMatrix();
-			glMatrixMode(GL_MODELVIEW);
-			glPopMatrix();
+			if ((snode->nodetree->flag & NTREE_VIEWER_BORDER) &&
+			        viewer_border->xmin < viewer_border->xmax &&
+			        viewer_border->ymin < viewer_border->ymax)
+			{
+				glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
+				setlinestyle(3);
+				cpack(0x4040FF);
+				
+				glRectf(x + snode->zoom * viewer_border->xmin * ibuf->x,
+				        y + snode->zoom * viewer_border->ymin * ibuf->y,
+				        x + snode->zoom * viewer_border->xmax * ibuf->x,
+				        y + snode->zoom * viewer_border->ymax * ibuf->y);
+				
+				setlinestyle(0);
+				glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);
+			}
 		}
-
-		BKE_image_release_ibuf(ima, ibuf, lock);
+		
+		glMatrixMode(GL_PROJECTION);
+		glPopMatrix();
+		glMatrixMode(GL_MODELVIEW);
+		glPopMatrix();
 	}
+	
+	BKE_image_release_ibuf(ima, ibuf, lock);
 }
 
 
diff --git a/source/blender/editors/space_node/node_draw.c b/source/blender/editors/space_node/node_draw.c
index 871d3751a4d..992caf5285f 100644
--- a/source/blender/editors/space_node/node_draw.c
+++ b/source/blender/editors/space_node/node_draw.c
@@ -1314,7 +1314,7 @@ void drawnodespace(const bContext *C, ARegion *ar)
 			UI_view2d_multi_grid_draw(v2d, (depth > 0 ? TH_NODE_GROUP : TH_BACK), U.widget_unit, 5, 2);
 			
 			/* backdrop */
-			draw_nodespace_back_pix(C, ar, snode);
+			draw_nodespace_back_pix(C, ar, snode, path->parent_key);
 			
 			draw_nodetree(C, ar, ntree, path->parent_key);
 		}
@@ -1339,7 +1339,7 @@ void drawnodespace(const bContext *C, ARegion *ar)
 		UI_view2d_multi_grid_draw(v2d, TH_BACK, U.widget_unit, 5, 2);
 		
 		/* backdrop */
-		draw_nodespace_back_pix(C, ar, snode);
+		draw_nodespace_back_pix(C, ar, snode, NODE_INSTANCE_KEY_NONE);
 	}
 	
 	ED_region_draw_cb_draw(C, ar, REGION_DRAW_POST_VIEW);
diff --git a/source/blender/editors/space_node/node_edit.c b/source/blender/editors/space_node/node_edit.c
index bf4a41e35c3..80504dda80b 100644
--- a/source/blender/editors/space_node/node_edit.c
+++ b/source/blender/editors/space_node/node_edit.c
@@ -253,7 +253,8 @@ static void compo_startjob(void *cjv, short *stop, short *do_update, float *prog
 
 	// XXX BIF_store_spare();
 	
-	ntreeCompositExecTree(ntree, &cj->scene->r, 0, 1, &scene->view_settings, &scene->display_settings);  /* 1 is do_previews */
+	/* 1 is do_previews */
+	ntreeCompositExecTree(ntree, &cj->scene->r, FALSE, TRUE, &scene->view_settings, &scene->display_settings);
 
 	ntree->test_break = NULL;
 	ntree->stats_draw = NULL;
diff --git a/source/blender/editors/space_node/node_intern.h b/source/blender/editors/space_node/node_intern.h
index 4b31f22b172..c31bb5ba832 100644
--- a/source/blender/editors/space_node/node_intern.h
+++ b/source/blender/editors/space_node/node_intern.h
@@ -136,7 +136,7 @@ void node_draw_link(struct View2D *v2d, struct SpaceNode *snode, struct bNodeLin
 void node_draw_link_bezier(struct View2D *v2d, struct SpaceNode *snode, struct bNodeLink *link, int th_col1, int do_shaded, int th_col2, int do_triple, int th_col3);
 int  node_link_bezier_points(struct View2D *v2d, struct SpaceNode *snode, struct bNodeLink *link, float coord_array[][2], int resol);
 // void node_draw_link_straight(View2D *v2d, SpaceNode *snode, bNodeLink *link, int th_col1, int do_shaded, int th_col2, int do_triple, int th_col3 );
-void draw_nodespace_back_pix(const struct bContext *C, struct ARegion *ar, struct SpaceNode *snode);
+void draw_nodespace_back_pix(const struct bContext *C, struct ARegion *ar, struct SpaceNode *snode, bNodeInstanceKey parent_key);
 
 
 /* node_add.c */
diff --git a/source/blender/editors/space_node/node_select.c b/source/blender/editors/space_node/node_select.c
index 366fcce3dd5..ea745c47690 100644
--- a/source/blender/editors/space_node/node_select.c
+++ b/source/blender/editors/space_node/node_select.c
@@ -293,6 +293,7 @@ void node_select_single(bContext *C, bNode *node)
 	nodeSetSelected(node, TRUE);
 	
 	ED_node_set_active(bmain, snode->edittree, node);
+	ED_node_set_active_viewer_key(snode);
 	
 	ED_node_sort(snode->edittree);
 	
@@ -374,8 +375,10 @@ static int node_mouse_select(Main *bmain, SpaceNode *snode, ARegion *ar, const i
 	}
 	
 	/* update node order */
-	if (selected)
+	if (selected) {
+		ED_node_set_active_viewer_key(snode);
 		ED_node_sort(snode->edittree);
+	}
 	
 	return selected;
 }
diff --git a/source/blender/editors/space_node/space_node.c b/source/blender/editors/space_node/space_node.c
index e234b4255ed..ec5c4657b9c 100644
--- a/source/blender/editors/space_node/space_node.c
+++ b/source/blender/editors/space_node/space_node.c
@@ -90,6 +90,8 @@ void ED_node_tree_start(SpaceNode *snode, bNodeTree *ntree, ID *id, ID *from)
 	snode->id = id;
 	snode->from = from;
 	
+	ED_node_set_active_viewer_key(snode);
+	
 	WM_main_add_notifier(NC_SCENE | ND_NODES, NULL);
 }
 
@@ -117,6 +119,8 @@ void ED_node_tree_push(SpaceNode *snode, bNodeTree *ntree, bNode *gnode)
 	/* update current tree */
 	snode->edittree = ntree;
 	
+	ED_node_set_active_viewer_key(snode);
+	
 	WM_main_add_notifier(NC_SCENE | ND_NODES, NULL);
 }
 
@@ -135,6 +139,8 @@ void ED_node_tree_pop(SpaceNode *snode)
 	path = snode->treepath.last;
 	snode->edittree = path->nodetree;
 	
+	ED_node_set_active_viewer_key(snode);
+	
 	/* listener updates the View2D center from edittree */
 	WM_main_add_notifier(NC_SCENE | ND_NODES, NULL);
 }
@@ -208,6 +214,14 @@ void ED_node_tree_path_get_fixedbuf(SpaceNode *snode, char *value, int max_lengt
 	}
 }
 
+void ED_node_set_active_viewer_key(SpaceNode *snode)
+{
+	bNodeTreePath *path = snode->treepath.last;
+	if (snode->nodetree && path) {
+		snode->nodetree->active_viewer_key = path->parent_key;
+	}
+}
+
 void snode_group_offset(SpaceNode *snode, float *x, float *y)
 {
 	bNodeTreePath *path = snode->treepath.last;
diff --git a/source/blender/makesdna/DNA_node_types.h b/source/blender/makesdna/DNA_node_types.h
index b6a5c758dc0..3ed97c2480f 100644
--- a/source/blender/makesdna/DNA_node_types.h
+++ b/source/blender/makesdna/DNA_node_types.h
@@ -370,6 +370,13 @@ typedef struct bNodeTree {
 	 * Only available in base node trees (e.g. scene->node_tree)
 	 */
 	struct bNodeInstanceHash *previews;
+	/* XXX workaround for ambiguous viewer output:
+	 * Viewer nodes all write to the same image buffer.
+	 * This determines the tree instance containing the "active" output.
+	 * Only used in local scene->nodetree.
+	 */
+	bNodeInstanceKey active_viewer_key;
+	int pad;
 	
 	/* execution data */
 	/* XXX It would be preferable to completely move this data out of the underlying node tree,
@@ -403,7 +410,7 @@ typedef struct bNodeTree {
 #define NTREE_COM_OPENCL			2	/* use opencl */
 #define NTREE_TWO_PASS				4	/* two pass */
 #define NTREE_COM_GROUPNODE_BUFFER	8	/* use groupnode buffers */
-#define NTREE_VIEWER_BORDER		16	/* use a border for viewer nodes */
+#define NTREE_VIEWER_BORDER			16	/* use a border for viewer nodes */
 
 /* XXX not nice, but needed as a temporary flags
  * for group updates after library linking.
diff --git a/source/blender/render/intern/source/pipeline.c b/source/blender/render/intern/source/pipeline.c
index 5510c07e19c..0a92e09e664 100644
--- a/source/blender/render/intern/source/pipeline.c
+++ b/source/blender/render/intern/source/pipeline.c
@@ -1732,7 +1732,7 @@ static void do_merge_fullsample(Render *re, bNodeTree *ntree)
 			ntreeCompositTagRender(re->scene);
 			ntreeCompositTagAnimated(ntree);
 			
-			ntreeCompositExecTree(ntree, &re->r, 1, G.background == 0, &re->scene->view_settings, &re->scene->display_settings);
+			ntreeCompositExecTree(ntree, &re->r, TRUE, G.background == 0, &re->scene->view_settings, &re->scene->display_settings);
 		}
 		
 		/* ensure we get either composited result or the active layer */
@@ -1909,7 +1909,7 @@ static void do_render_composite_fields_blur_3d(Render *re)
 				if (re->r.scemode & R_FULL_SAMPLE)
 					do_merge_fullsample(re, ntree);
 				else {
-					ntreeCompositExecTree(ntree, &re->r, 1, G.background == 0, &re->scene->view_settings, &re->scene->display_settings);
+					ntreeCompositExecTree(ntree, &re->r, TRUE, G.background == 0, &re->scene->view_settings, &re->scene->display_settings);
 				}
 				
 				ntree->stats_draw = NULL;