Duplicates: T34848

--- Operating System, Graphics card ---

OS: Mac OSX 10.6.8
Graphics Card: NVIDIA GeForce 9400

--- Blender version with error, and version that worked ---

Version with error: Blender trunk revision 55515. I'm gonna guess that this is not the specific revision which caused the error, rather it is the first revision I've used that I noticed the error in. More likely this is from the recent changes to group nodes, or perhaps the change in which output nodes will recalculate while editing.

Version that worked: Blender Official Release 2.66a

--- Short description of error ---

When you have a viewer inside a group node, it will override any and all viewers outside the group for display in the backdrop (or image editor, if it is set to "viewer node"). So if you are editing the main node tree, the only thing that will really display in the backdrop is the last selected viewer node which is inside a group. When you edit the tree, you will see the tiles updating the image to the currently selected viewer (outside of a group), but as soon as the update finishes, the backdrop goes back to the grouped viewer. This can all be seen in the included .blend.

--- Steps for others to reproduce the error (preferably based on attached .blend file) ---

Step 1. Open the blend file I've attached, and hit F12 to render.
Step 2. Notice that the final composited render is a pinkish-colored, heavily-blurred Suzanne.
Step 3. Exit the image editor window, and notice that while both the compositor backdrop and the image editor in the bottom left (set to viewer node) show a pinkified Suzanne, she does not appear blurred.
Step 4. So, now we can see there is a problem. Click the viewer node to make it active, and notice that while the node itself displays the correct blurred image, the backdrop has not changed. Obviously, then, this viewer must not be what is displayed in the backdrop, though it should be since it is the active viewer.
Step 5. Change the blur size in the blur node. You should be able to see the tiles in the backdrop updating the blur, but when it finishes the backdrop returns to the non-blurred image. If your computer is too fast to see this effect, feel free to create a more complex scene to slow down the compositing, and re-render.
Step 6. Select the group node and hit tab to edit it. Inside the group node you will find an RGB curves node, as well as a viewer.
Step 7. Change something in the RGB curves, and notice that the backdrop displays the results correctly (through the viewer in the group).
Step 8. Tab back out, and notice that the backdrop is the same. So we can see that it is indeed the viewer in the group which is being used.
Step 9. Tab back into the group, delete the viewer, tab back out, and notice that the backdrop is finally displaying the blurred result correctly.

And that's it! Hopefully this is plenty of information to make the bug easier to find, and thank you for taking the time to fix this!
- LswaN