Duplicate, mirror and union leaves a curve along the join

[Paul Mc]

I'm trying to create a kettlebell icon which is symmetrical around a vertical axis. I'm pretty sure that in the past, with a similar requirement, I've created half the design, duplicated it, flipped the copy, repositioned it and then unioned it with the original to create the intended shape. I don't recall any issues. This one has me scratching my head. In order to ensure the smooth continuity of the curves between the two halves I need the vertical curves at the join to be removed. I was expecting that to happen when I clicked on the union button. These are pixel aligned nodes with snapping on.

Is there a clean way to do this without all the fiddle of breaking the curves and joining them back up again? Any thoughts on the "correct" way to do this?

kettlebell idea1.afdesign

[Alfred]

Select one half with the Node Tool and drag on the middle of each vertical so that it overlaps the corresponding vertical in the other half. The Boolean ‘Add’ operation should then give you the result you expect.

[Paul Mc]

@Alfred thanks for the suggestion. I did try this but it adds more nodes to the curve in unwanted places and also breaks the symmetry.

I can't help thinking that all the precision given through the pixel alignment, snapping options and transform panel are completely undermined by the eyeball and nudge method that seems to be required here.

Also, if I follow the same procedure as this with a rectangle it works as expected with no vertical curves dividing the result.

[Alfred]

4 hours ago, Paul Mc said:

I did try this but it adds more nodes to the curve in unwanted places and also breaks the symmetry.

It shouldn’t do either of those things. When you ‘Add’ the two curves you get coincident — or nearly coincident — nodes in four places (at each end of the straight line segments) but those are pre-existing nodes from the original shapes, and you can easily delete the top one of each pair. The symmetry should be unimpaired, if you’ll excuse the pun.

Try adding together the two shapes in the attached adapted version of your file.

kettlebell idea1 nudged.afdesign

[PixelPest]

Then you will get double points at the seam.

[Old Bruce]

Using the Move tool duplicate the curve, mirror it and then reverse the curve (need the Node tool active). Now the nodes should add properly.

[PixelPest]

Good luck with it.

[Alfred]

58 minutes ago, PixelPest said:

Then you will get double points at the seam.

Quite so, which is why I wrote:

1 hour ago, Alfred said:

those are pre-existing nodes from the original shapes, and you can easily delete the top one of each pair

 

[PixelPest]

Fine if you don´t care about symmetry. 

[Paul Mc]

OK, well I've solved it.

In spite of having Force pixel alignment and snapping enabled as well as the Transform panel showing that the x-coords of all the nodes on the join line are 100px this was clearly not the case. I set the user-interface pixel decimal places to 6 and then it showed as:

I then selected all the nodes on the join line and overtyped the 100px with "100" and pressed Enter. No apparent change but those x-coordinates were definitely 100px. Then repeating the steps above created the result that the curves on the join were no longer in the resultant shape. Bingo!

This is clearly a rounding error issue. Those 100px values were not exact and so never were co-incident after the move and the union.

Maybe what's needed is a way of forcing all (selected nodes) to whole value pixels coordinates before this type of operation to ensure alignment.

For me I now consider this solved. I don't think this is a bug as it is working as designed but any devs out there might consider the UX here.

Thanks for all the comments and suggestions everyone.