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I'm playing with a celtic knot work shape, which means that I want to split a path which crosses itself at that intersection. How do I tell it what segment I want to split?

It's one path but different segments of that path. For instance, there is the horizontal segment, and then a segment which is tilted at 120 degrees. It doesn't matter if I select the horizontal segment or the tilted segment, it always cuts the horizontal segment.

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3 Answers

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If the path involves curves (which a Celtic Knot will), snipping away from the intersection then stretching the path will pull the curves, distorting them slightly. For precision, you will need to know how to control the 'stacking' order within a path.


Short answer: Use the pen tool to select the path endpoint on the segment of the path you don't want to be snipped. Then snip the exact intersection point.


Huh? What? Why would that help? Every path in Illustrator has a start point and a finish point (this is normal in vector paths, not just in Illy). When you use the scissors tool or similar at a precise intersection where a line crosses itself, it always cuts the section of the path closest to the start of the path (tested in CS4 and CS6).

Think of illustrator paths as always going under themselves, coiling downwards. It's kinda counterintuitive... I remember it by imagining illustrator sewing the paths into the canvas, carefully sowing the thread underneath earlier stitches (that works for me but probably no-one else...).

If it's a complex case (which any good Celtic knot will be...) and you need to see which direction paths are pointing (i.e. which end point is the start and which is the finish) add an "add arrowheads" effect, then remove the effect when you're done.

So why the pen tool? You can switch which way round an open line is pointing by using the pen tool to select the point that is currently the start point, as if you were going to continue drawing from there. This instantly turns it from start point into end point, reversing the path. If you've added arrowheads, you'll see this - the direction of the arrow will reverse.

enter image description here

In the above image, a curved path with an arrowhead on the finish point and its copy with the direction reversed were snipped at the exact point they intersect. You can see from the arrowhead on the newly created 'cut off bit' which segment of the path was snipped: it was the one closest to the arrow's start point.

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That's a nuance I didn't know. Thanks! –  Hack Saw Jul 16 '12 at 19:59
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The key when doing this kind of work in Illustrator is to avoid manually moving anchor points unless you have a very specific guide or intersection you can snap to. A specific use case would be curved segments.

An alternative to changing the winding order (or "direction") of the path is to locate an existing anchor point near to the intersection at which you wish to cut. Cut on this existing point. Deselect and then reselect the segment you're trying to cut - bring to front. Now cut on the intersection (make sure Smart Guides are turned on so you cut exactly at the intersection). Now, with the Direct Select (white arrow) tool, draw a tiny marquee around the point you cut earlier (it's actually two points in the exact same space) and re-join (CTRL-J / CMD-J). Illustrator will respect the previous bezier handles and the curve will be reassembled exactly as it was before.

Pathfinder

Pathfinder can really be your friend here, if you're doing a lot of complicated cuts - the Celtic Knot is a prime example. Pathfinder Divide will cut on all intersections, allowing you to then re-assemble or delete as necessary with geometric perfection. Just remember that when using Pathfinder Divide, you must always un-group afterward to access the split pieces (or double-click to enter Isolation Mode).

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Often it's a matter of stacking. Illustrator always wants to cut or select the top most path or anchor.

The easiest solution is to not be as precise in where you are cutting. For example, cut the tilted path a bit above the horizontal. Then just use the direct selection tool to move the anchor into the proper position.

cutting

The other option is to draw a path so that the areas you intend to cut are on top. For example, if you draw the horizontal first, then up and then the angle, the angled path is on top and will be cut. But if you draw the angle, then down then the horizontal, the horizontal gets cut.

A third option is to draw independent paths so you can control the stacking order. For example draw the horizontal, then as a separate path draw the angle. Cut the angle where you want, then connect it to the horizontal path.

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Well precision is of high import to me. I think your advice of independent paths is the best way. Thanks. –  Hack Saw Jul 12 '12 at 4:32
    
Actually, provided you have smart guides turned on, you CAN use the first option, because when you drag the point back into place, the intersections is still there, so it will snap precisely into position. –  Tom Auger Jul 12 '12 at 19:05
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@TomAuger - If it's a straight line, then yes that's fine. If it's a curve, then while it's true that you can drag the point to the exact intersection, doing so will change the curve slightly (it will stretch it out, making it slightly flatter). Celtic knots are crazy complicated collections of very precise, repeating, overlapping curves, so I'm not surprised that the asker is reluctant to do this. –  user568458 Jul 13 '12 at 8:53
    
@user568458 Excellent point. See my answer –  Tom Auger Jul 13 '12 at 15:40
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