Creating Smooth Curves: The Kolson Method

(I just called it the ‘Kolson Method’ here just to differentiate it from the ‘Sspencer Method’, etc.)

(Mod Edit: As of January 2019, Keith put a more complete tutorial on the wiki: Creating Smooth Curves: The Kolson Method - Seamly2D)

Based on my (somewhat limited) experience with V, here is what you need to watch out for:

  1. Always do curves last. They depend on control/reference points and lines that must be in place before you place your curves. It’s an issue with V itself, which needs to be dealt with at some point.
  2. Never do freehand curves. Use formulas for setting the position of control/reference points. If you don’t, they won’t be nearly as easy to make work, and won’t work at all with different measurements.
  3. Unless you are doing something fancy, all curve ends must be at ninety degrees to the lines that they attach to. This is to ensure that when you sew the other half of the garment together, where the two curves meet forms a smooth line. (i.e. 90° + 90° = 180° - a straight line.)

Setting up a curve using reference points like I do in the example isn’t hard. (I used X as a prefix for control/reference points to keep my main numbering simple. You’ll also notice that I drag the point labels around to make things neater.)

  1. Use the ‘Point from X and Y of two other points’ tool based on adjoining points on the curve (X2-X7).
  2. Create lines between the reference points and the curve points (light-blue, dashed lines).
  3. Use the ‘Curved path’ tool to lay your curve. (Don’t worry if it looks like crap, to begin with.)
  4. Right-click the curve and select ‘Options’.
  5. For each curve point, make sure that the control point angles are at right angles to whatever lines they are attached to. (If a line isn’t attached to a point, see if it makes sense to be oriented horizontal or vertical, like A16 in my example.)
  6. For the length of the first control point of each curve point, use the line length from the previous reference point (X3 in the example) to the previous curve point (A16 in the example) and multiply by .55. (The proper value is 0.551915, but that much precision isn’t needed; just remember `.55’. This value makes bezier curves come closest to approximating the curve of a circle, which makes for the smoothest curves.)
  7. For the length of the second control point, do the same thing, but with the next reference and curve points. (X4 and A32 in the example).

As you can see by my example, even odd armhole curves can turn out very smoothly, and changing any measurement won’t bother their result in the slightest.

Hope this helps!

[Update 1: I will be editing this first post as needed over the next few days. I just finished walking through the process in V. and took four dozen screenshots(!) to support the text. I’m taking a break now, but plan to get at least the images up and some of the text later today.]


@slspencer, would Keith’s tutorial fit somewhere in the curves section in tools in the user manual? UserManual:Tools - Seamly2D

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That is something I came to realize. …which is why I’ve spent a couple of hours already making sure that it will be as complete and as easy to understand and translate as possible.

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More importantly the two sides of a seam should add up to 180. There’s dozens of examples where seams are not at 90 and that’s doing nothing fancy… for EX: the armhole & 2 piece sleeve of any men’s suit.


Well, I’m just a graphics guy who is just getting started pattern making, so things like a two-piece sleeve sound pretty fancy to me! :wink:

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Here’s a draft of a typical 2 piece sleeve… as you can see most of the seams are not 90… the only place that is at a 90 is the back seam at the cuff - point N.

In any case… I think it’s an important point you brought up, in that most curves should vary with the measurements.



I bet if you have worn a suit “off the rack” it had a 2 piece sleeve. They are pretty common

Thank you for your input. I’ll change my tutorial to stress that what is important is that the angles of the two sides of a seam add up to 180 degrees in most cases.

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Tutorial is up…

If you have any questions, comments, or concerns, please post them here.


yay. I am glad to see that I am not the only one who believes in using the /wiki/talk: pages

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Hi! I’ve just upgraded S2D and I’ve noticed that the curved path tool it’s very slow (every time I make a change I need to wait few seconds before the curve starts to bend) while before it was immediate. Have you come across this problem?