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Select Linked behavior inconsistent. Sometimes doesn't work. Sometimes overflows.
Closed, InvalidPublic

Description

System Information
OS X 10.6.8
NVIDIA GeForce 320M

Blender Version
Broken: 2.71 9337574
Worked: Never, as far as I know.

Short description of error
In Edit Mode sometimes pressing the L key will Select Linked vertices. Sometimes it does not.
Because the behavior is inconsistent I have made a habit of clicking a vertex first and then Selecting Linked.
But it really shouldn't be this way. While preparing some class curriculum for beginners of Blender I realized that I would not be able to explain to a student why Select Linked behaves as it does. This got me thinking it could actually be a bug.

There is also a problem with the selection overflowing into vertices which are NOT linked. I'm not sure if these two issues are related but it's worth mentioning them together since they might be. The overflow bug is hard to replicate consistently, however I was able to replicate it in this minute-long unedited screencast. See the .mov attached. I am also including a .txt file of the console output that goes along with that screencast. The selection overflow occurs 38 seconds into the video. You can see I start with the default scene and try to replicate the bug four times, then finally succeed on the fifth. Finally I select just one piece to demonstrate it is in fact a separate piece.

For some reason Blender is not always succeeding at selecting linked vertices. It doesn't seem to always know which are connected and which are not, which is strange because geometrically speaking it's pretty black and white. Either a piece is connected or it's not. Maybe Select Linked uses some viewport-based approximation to calculate and it's lacking in accuracy.

Exact steps for others to reproduce the error
For the bug where Select Linked fails:

  1. Starting from the default scene, Tab into Edit Mode, then Box Select or Shift-RClick to select four vertices that form a face of the cube (do not use face select or it will succeed)
  2. Perform an operation on that face, such as Grab, Rotate, Scale, or Extrude.
  3. Press the L key. Select Linked fails.

For the bug where Select Linked overflows:
This is a tough one because it doesn't happen every time, but you can see from my video that it does happen frequently enough to observe if you try several times.

  1. Starting from the default scene, Tab into Edit Mode and Shift D to Duplicate the cube mesh. Move it 2 Blender Units on any axis (I used Y).
  2. Rotate and/or zoom the viewport arbitrarily.
  3. Press the L key. Sometimes it will select the unconnected cube mesh as well as the connected one.

Screencast:

Console Output:

Details

Type
Bug

Event Timeline

Seth L. (quantumanomaly) set Type to Bug.
Seth L. (quantumanomaly) created this task.
Seth L. (quantumanomaly) raised the priority of this task from to Needs Triage by Developer.
Campbell Barton (campbellbarton) triaged this task as Needs Information from User priority.

Watched the video and not sure where the bug is.

Maybe you're aware of this, but to be clear.

  • Pick linked (LKey) uses the mouse position.
  • Select linked based on existing selection (Ctrl+LKey).

It seems like your expecting selection to be based on existing selection, instead of mouse location?

Thank you for clarifying that! OK, my mistake.
I was not clear on this for a very long time. I think the reason for it is that selecting a vertex and immediately pressing L will effectively select all the geometry linked to it in most cases... (have the same effect as Ctrl+L) but this is only because that's where your cursor is in most cases. So I got the impression that L selects all the linked vertices of the current selection, but actually that is what Ctrl+L does. It was my misunderstanding. I didn't realize there were two separate operations. Sorry about that! In this case there is no bug, there is only user error.

One thought I had was that it might be helpful if there was some visual feedback given to the user that an event was taking place at the cursor location when the L key is pressed. It would help distinguish a difference between L and Ctrl+L and serve as an indicator that "hey, something is happening near your cursor". So I made a small 48x48px animated .svg that could theoretically appear for a moment at the cursor location when Pick Linked is used. I don't know if .svg is a format that could be incorporated into Blender, but I could just as easily make it an APNG or series of PNGs etc. Since it's an .svg the stroke color and other parameters can be changed by opening it in any text editor. You can see the animation by opening it in a web browser.

Bastien Montagne (mont29) claimed this task.

So, no bug here in the end!

Thanks for the suggestions, but our tracker is no place for feature requests (please use IRC, forums or bf-funboard mailing list for such topic).