True darts tools

I am looking for explanation about the true darts tools. I found some explanations in dismine / Valentina / issues / #301 - True darts — Bitbucket

I know it requires 5 points:

  • 1 and 2 on the base line
  • 3 at the left top of the V
  • 4 at the bottom edge of the V
  • 5 at the right top of the V

It generates 2 points D1 and D2 to make the dart.

Is the geometry defined by these conditions:

  • |P4 D1| = |P4 D2|
  • angle(P1 D1, D1 P4) + angle(P2 D2, D2 P4) = 180°
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I have never understood the purpose of this tool. What it is made for? This tool changes the place of the seam.

The truing of darts have different meaning for me. It is like in this video Truing the dart - YouTube

The truing should extend the line A5-A2 to be equal to A5-A4


Yes, you are correct. The True-Darts tool should be improved. Would you please add this to the issues list?

I created a tool in the prototype accepted the three points of the dart, plus the points along the curve or line on both sides of the dart, plus choice of the side points where the fold should be directed.

Example: If the dart in the gif is to be folded towards A1, then the parameters would be A, A4, A5, A3, A1. The tool would return A6, A7, A8 where A8 is the extension point.

The results are a little more complex when the dart lies along a curve (like a dart along the back neckline). If A to A1 were a curve with c1 and c2 control points, then the tool would return a new curve from A to A6 with c1a and c1b control points, curve from A6 to A8 with c1c and c1d, curve from A8 to A7 with c2a and c2b, and curve from A7 to A1 with c2c and c2d.

I’ll add it in to issue list.


Thanks for this comment, i also stumpled yesterday about this tool and had the same thoughs.

was this ever resolved? I am just learning how to do darts, mostly on curves no less and just now trying to figure out this tool


If you create a dart on a sheet of paper you can see that the edge of the paper is deformed into a ‘V’ shape.

If a dart is ‘trued’ it will create a straight seam.

This tool is working correctly.


Even after doing some search around the forum and references, the purpose of this tool is not entirely clear to me.

I hold the same meaning of truing a dart as @Stinde does, which is folding to determine the shape of the dart bulk and making sure the darts legs are the same size. Is this true darts tool doing the same thing, but it’s just a different drafting approach?

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Yes, If you create your dart on a straight line, and stitch the dart closed, the straight line gives a slight V effect and if the legs aren’t the same length, then the seamline will have a step in it. This tool takes the straight line and raises it a bit so that, when the dart is sewn closed, the seamline remains straight and also makes sure that the legs are the same length.


Purpose of the dart is to create 3D shape to the fabric. After closing the dart in the 2D plane the outline might not be straight but in 3D the outline is correct as it should be.

Truing the dart should not change the shape of the outline. If you cut the fabric without truing the dart and close and fold the dart, you will find out the in back of the folded edge is too short and does not reach to the fabric edge.

The gap is bigger with bigger darts. Specially with women garment the darts are so wide that without truing the folded edge pops out from the seam.


Yeah, that’s what’s throwing me off. The true darts tool is changing the shape of the outline into something I don’t expect. However, I have limited drafting experience, so I can’t tell if this is a traditional pattern drafting technique that I should know or if it’s an innovation in Seamly.

I also can’t tell if that’s this case with this dart tool. It’s hard wrapping my head around the resulting shape. Although, I guess I can go down the empirical route and try it myself. :slight_smile:


Yes this technique (true the dart) has been around for as long as there have been darts. :smiley:

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Maybe I should show of visual of what I mean. I’m familiar with the traditional way of truing darts, but when I use the tool in Seamly2D, I end up with something like this:

When I expect something like this: Screen Shot 2021-01-23 at 9.50.23 PM

So when I said:

I was referring to the results of the true darts tool in the above gif. There must be something silly I am missing here and I am overthinking this!


Hi @Adyst

In some places, I also use your method of adding the dart fabric without truing the seamline as well. I then use the Internal Path Tool to show the dart legs. You can use either method, depending on what your end result needs to look like, but with either way, you will need to manually add the fabric for the dart closure manually.

Screenshot 2021-01-24 121710 Screenshot 2021-01-24 121649 Screenshot 2021-01-24 121611 Ich habe einmal beide Abnäher zugedreht. Beim ersten beispiel entsteht eine gerader Nahtverlauf. Beim zweiten Beispiel bildet sich ein V. Normalerweise müsste hier jetzt noch ein Kurve nachgezeichnet werden. In den Büchern steht oft, das: “Der Nahtverlauf muss ausgeglichen werden.” Screenshot 2021-01-24 122410


Yes… but this not really truing the dart - it’s doing as Stinde said, simply adding fabric so when you sew the dart and then a seam, you don’t have the end of the dart sticking out and opened.

Truing a dart like the app is doing makes it so you end up with straight (180%) edge and not a Vee after the dart is stitched. It also adds length to the dart so it adds fullness to actually fit - such as in bust darts.

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THis tool isn’t complete. The prototype tool filled in the center of the dart and extended the center of the dart out so that it blends in with the seam line. Couldn’t get the programmer to do this :frowning: I would like to finish this tool, and add another tool from the prototype which can true a dart along a curve. It’s been done and done well in TMTP, it’s just not in Seamly yet. So these Dart Tools are future features :smiley: ! I’m starting to get comfortable with C++, maybe I’ll get to this sometime soon. I could do this quickly if this program were in python :snake:


Yes… currently the tool would add too much length to the dart.

C++ is not the problem… I picked up on it in a couple months - I was already familiar with C and Java so that helped. And it takes a bit to get the hang of the Qt way… Hatd part is the code itself. Aside from style and naming issues - it’s a big and complex set of applications. Adding a (new) tool is not an easy task, as one ties into so many classes with the tool itself, the dialogs, the ui, the pattern, the pref settings, the defs, possible pattern conversion, the XML schema, the icons, the geometry, the property explorer, the layout, the dxf export, etc etc. Even adding a simple feature like showing / hiding the point names is spread across a lot of files. After 3+ years I still haven’t gotten through all the code yet.


Yes, it’s a barrier. The architecture is awful.

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