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I am implementing a polyline drawing tool in a software for commenting over videos frames.

By polyline I am referring to connected segments forming an open shape.

The usual experience is that the user left click somewhere in the canvas to start the drawing, then each subsequent left click adds a segment. To provide immediate feedback, while the drawing is being created a segment is also drawn from the last point to the current mouse cursor. This is the standard way to do polyline drawings as far as I can tell.

Once the user is finished though, there is a problem on how to "end" the polyline. There is this dangling segment attached to the cursor that the user do not want, and each time you click away it actually adds another point to the polyline.

polyline ending

Some softwares use a right click to mean the end the polyline and the dangling segment is discarded. Google docs use the addition of a point very close to the last point. Other softwares use the Escape key. For the curve editor in Blender I always have to check the documentation.

There may or may not be a polygon tool in the future. For polygons I think the usual way is for the user to create a "closing" point, near the first point of the shape.

I have always found this particular piece of user experience unsatisfying as a user and I have refrained from implementing this drawing tool for this reason.

Right click usually means something else. Adding a nearby point… well what if you really wanted to add a small segment? I already use Escape to switch from the current tool to the pointer tool.

What is the best way to terminate a polyline? Is there a way that is widespread enough so that the user will expect it?

  • The dangling element isn't a necessity. I've used double click for an action that both places the last point and ends the polyline, and it worked well. – Vitaly Mijiritsky Sep 27 '14 at 21:11
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You don't need to provide only one solution!

Escape should do it - to complete the line at the last point added.

So should double click - to add a point and finish the line

And yes, right click can mean other things, which is why right click should bring up a menu to resolve any ambiguity as to what the click actually means: eg.

  • Finish
  • Add point and finish
  • Close shape and finish
  • [Any other relevant actions]
  • Close menu

But why include Close menu on the popup? It's unusual and seems unnecessary, but it's because the user is doing an action where every click is relevant to the action. Often popups are discarded by clicking outside the menu if none of the choices are wanted at the time. However, users can be unsure as to whether clicking to discard the menu will also add another unwanted point to the polyline. I've seen both cases of where the off-menu click does and doesn't add a point. The 'Close menu' option is a safe way for unsure users to discard the menu by effectively giving them an on-menu place to click to discard. Sure there are other ways to close the menu, some still potentially confusing and other less so, but this is an explicit 'get me out of here'.

  • Thank you very much! I'll definitely go with the multi solution approach. Thanks for the heads up on having an extra menu item to simply close the context menu without other impacts. – Joan Charmant Sep 28 '14 at 12:02
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Totally agree with RogerAttrill's answer, but I would suggest you to give to the user a more intuitive option.

I think the best option is what MS Word do when you select a text, a visual-friendly contextual menu (notice that the menu fades out when you take your mouse away from it):

Word Contextual Menu

I would add something like this (of course you can put other options on the menu or remove the ones I put):

Contextual menu example

If you can simulate the opacity effect like MS Word, is a great indicator too. And don't forget to add tooltips on the buttons in the context menu to indicate what is the button for.

And yes, I think you should provide more than one solution. Even implementing this menu, the escape key will be used if your users are a little more technical.

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    Thanks very interesting. I feel there might be complications if the user wanted to create a point that happens to be under the context menu though. – Joan Charmant Sep 28 '14 at 11:56
  • Yes, if the users can create points near each other, maybe a closing button on the context menu would be the best approach (I really miss it on MS Word sometimes). – Dinei Oct 2 '14 at 20:41

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