Confused on curves, need help

Hi Everyone,

As I’m new to Seamly2D and the community I didn’t feel I should post something until I did my homework :slight_smile: However, I just can’t seem to wrap my head around curves (no pun intended).

I’m working on a basic block for trousers from Muller & Sohn. But after reading through every curve documentation and video there is, I still can’t really seem to make it work. I have a few questions I hope you can help me with:

  1. How do curves work? For example, I see nearly everyone divide the part of a curve by 3 or mulplying it by 0,3. Why is this? I can only make my curve on the inseam/outseam sort of work when I use 0,05… This is but one mystery of the curve…

  2. What am I doing wrong? After hours of fiddling I’m getting some result, and it’s starting to look like that picture in the book, but it’s not making any sense as to how it actually works, and why the knee (at the outseam) isn’t curving right.

I’ve added some files (pattern photo, measurement file, attempt at trouser pattern) to this post, hopefully someone can help me out. I feel like once I understand the curve I can make it work. But for now I’m still grasping in the dark.

Thanks for thinking along!


ps. Since I’m a new user I can only add one file at a time. So I’ll upload the rest after.

Pattern photo|265x799

1 Like

Here’s the file for the pattern.

basic block trouser without dart.val (11,3 KB)

1 Like

Here’s the file with the measurements.

Maten 13-11-2021.vit (1,7 KB)

1 Like

That’s a loaded question… but I’ll try to break it down to just the basic idea with 1 curve using interactive control points. Note that Seamly2D uses bezier curves… which we’ve defined as:

  • Curve - interactive
  • Curve - fixed
  • Spline - interactiive
  • Spline - Fixed

where a “spline” is nothing more than consecutive curves, with the control points between curves automatically set 180 degs apart so as to provide a smooth transition from 1 curve into another.

The other factor that determines how smooth a curve flows is the length of the control points. Not only is this important in just drafting a single pattern, it’s essential to change the length of the control points with the measurements. That’s where you will see users defining the length of the control point by some formula of length * a factor. It’s easier to look at curves within a triangle… with the control points along each leg and the curve along the hypotenuse… like:


Where the length of the the control point at A = the length of Line_A_A1 * factor and the length of the control point at A2 = Line_A1_A2 * factor.

So just what is the value of the factor? Well without getting into the math it’s an approx for bezier curves and can be defined as 0.551915. So in other words the length of control point A = Line_A_A1 * 0.551915… so basically 55% or a little over 1/2 the length. So again, what this accomplishes is a smooth transition for whatever is before and after this curve, and regardless of the “size” it will always be proportional.

Something that I do is put the bezier factor into a variable in the “variables table” so it makes it clearer WHAT the factor is for.


So now the formulas become:


You may then be wondering well how would all this apply if you need to find those triangle leg lengths. if they not given? Well then you have to add points to allow you to have access to the line lengths as illustrated here:


Here you can see I needed a lengths for the curve between F3 and F1… so I added the point X3. Now I have access to Line_F3_X3 and Line_F1_X3 and the control point lengths become Line_F3_X3 * #bez_approx and Line_F1_X3 * #bez_approx. BTW… I could have just added lines F3 - F2 and F1 - F2… but I find it gets messy with over lapping lines.

Just another note… as you will run into this issue at some point - you need to define any lines you need BEFORE creating the curve as you can only access tools that have created BEFORE the current tool.

Hope this helps.


Thank you Douglas for your answer! Couldn’t resist to look in the middle of a meeting. I’m gonna work through this tonight and see if I can make it work. :slight_smile:

I added another curve to illustrate how one curve transitions into another:



curve.val (1.9 KB)

So what you should get is a smooth transition from A5 to A4. `

And Here’s an example of using the angle of Line_A_A6 to set the angle of the control point to smoothly transition from a new (pink) curve into Line_A_A6:



Note the subtraction of 180 degrees to the angle… this is flip the control point direction as if the line was actually Line_A6_A… if that makes sense.

3curves.val (2.3 KB)


Hi & welcome to our forum, @sjoerdg

These 2 posts will explain some of the ways people do curves:

But they both involve the very curvy armhole curve and can be applied to the crotch curve at the hip line on the trousers.

I only answer you with: There are no hard & fast rules to creating curves, except that, if you wish to have your pattern to resize smoothly between different sizes/measurement files, you need to use formulas that refer to the corresponding parts of the pattern.

I normally place a straight line from the ankle to the knee on both the inner & outer seam and then do a curve from the knee to the crotch or waist.

Oftern, the pattern making instructions tell you to make the curve 1cm inwards, and what I have found is that they actually want you to make a line from the knee to the hip/crotch. on the inner seam, add a node 2/3 of the line and from this node, add another node 1cm away at right angles to the line from knee to hip. The create your curve touching this point and use the angles and lenghts of these lines in the formulas.

I’m going to add the nodes and do a new curve on your pattern, so that you can see what I do.

z pattern.val (14.0 KB)

Maten 13-11-2021.vit (1.7 KB)

Line that I’ve added are in dark red & new curves are in blue.

Basically, the angle of the curve handle needs to either follow on a line or be at a 90° of another line or curve handle. If the curve is very pronounced, like the crotch, you need to use /2 and if it is a gentle curve, then /3 or more.

I hope that this helps you to understand curves better.


@ sjoerdg

Exactly. Specifically the control points. If you don’t formulize the control points, curves will get wonky as the size changes AND sometimes the angle of what a curve is transitioning into… as illustrated in my last post. OR sometimes you want that beginning or ending control point to be perpendicular to a “seam” line to keep that 180 degs across the seam so as not to get V’s when stitched.


Hi @Grace & @Douglas

Finished work and drove right into it. I do think I’m starting to understand.

Since my pattern shows a different crotch curve than the one Grace made I tried to adapt it to the instructions. Does this look like it works (like you explained) and should scale? (Grace = blue, me = red)

Is there a difference as to why I would use the line going up + 180 or the line going down as it is? (see picture 1 and 2) Since both give the same end result?

Thanks again for responding so quickly and thoroughly. I have literally wasted hours on this so far -.- And I’m now getting back my excitement through seeing how it might actually work out.

ps. I couldn’t post this until after my ‘enthusiasm block’ was over :slight_smile:

Crotch curve 180

1 Like

Because they’re not the same line (segment)… even if they’re along the same axis. The start and end points matter, as it determines whether you’re going clockwise or counterclockwise. Does it matter… no as like you said the result is the same. For me though I tend to think in terms of the line and curve going in the same direction… thus I’d go with the angle + 180.



I used the line above as the angle of the 1st curve handle to create a smooth transition from the line into the curve. & then I used the line between the start node (of the curve) divided by 2 (or you can use *.55 for accuracy) to form the start of the curve.

The 2nd handle, I used the line A17 to A2 for the angle of the handle. The reason is that this curve will connect to the back of the trouser curve, so one needs to create a continuous transition from front to back, so by using this line, I would also use the corresponding line on the back panel. and then used the length of that same line divided by 2.

So in theory, this method should be the equivalent of using a French curve ruler to make that curve when making your patterns by hand, and thereby creating a smooth curve that will resize smoothly when you charge the measurements file.

However, I have no idea about Muller & Sohns methods, so they may be different & nor do I know what picture you have in your mind that you making a pattern for. So while I can tell you how I do things, you will develop your own methods that work better for you. Each person modifies these methods to suite themselves. There is no real right or wrong way.



Exactly… You can make it look good for 1 size, and it will work ok, but as soon as you start resizing with different measurements the transition between lines and curves can get wonky.

Yup… one can draft using whatever system using a French curve or tailors ruler to transition a curve… but that’s just a single pattern. If you draft another size by hand the process is repeated manually ie “fudged”, but it doesn’t translate digitally unless one uses a process ( dividing by 1/2 ) like you’ve described. Unless of course if we implement a “French curve” type tool.

1 Like

If you wish, you can make the upper handle longer and the lower handle shorter by changing the ‘line/2’ as I have done in this image, to get it closer to the appearance of your curve:


And you can also add or subtract a number of degrees in the direction formula (see below) but, once again, it may need to be adjusted when new measurements are added unless you create a rule in the Custom Variables:


1 Like

Hi Both,

Thanks for the added advice and suggestions, I just keep tinkering away :slight_smile: It look me a while to figure out what you meant with a smooth transition between front and back. But than I thought you might mean the ‘meeting of angles’.

But the pattern shows this kind of transition. So the angles seem to be doing something different than what you’re proposing. I’ll try and sew it together somewhere this week to see how things compare. I’ll be back! haha

1 Like

As someone who has more experience with origami than with pattern drafting, I get the impression that the crotch-point in this pattern is supposed to have a slightly angular join, like a paper bag.

Though I am far from discounting the possibility of fudge-factor either. Maybe it’s because my meager experience has mostly been with their costume patterns from the nineties, but it is my experience that even the “Big Four” here in USA tend to expect one to work with fudge.

1 Like

Ha … big four… it’s all Butterick. Even Vougue - which use to be decent patterns. Just recently used a Vouge jacket pattern - thinking it would save some time. Boy was I wrong. Wanna talk fudge factor… the customer measured a 45" chest… so one would think a size 46 woukd be the ticket. Nope… just measuring the pieces I said we better use the 44. Turns out that was even big. Had to take in the jacket. The 44 measured about a 50 put together. 6 inches of ease? I don’t think so. Sigh.


Hi again, @sjoerdg

Just looking at the front & back crotch, those in the image can work once the seam allowance is added when you do the Details.

I moved the blue line to the Waist 2 line & made slight adjustments to the curves:


I think it compares better with your image now.

:rofl: :rofl: :rofl: My patterns mostly turn out like Vogue’s then.

1 Like


Once I had the seam allowance added it did look a little smoother. I’m definitely curious to see how this turns out. But guess what, as a millennial I don’t have a printer, so this’ll have to be put on pauze for a few days. I hope to share an update next weekend :slight_smile:

1 Like

Just out of curiosity… Using one of the formats PDF, SVG, or DXF maybe you could see if a printing service in your area that has plotters that could plot the pattern(s)? Even though I have a few plotters at my shop I haven’t used one yet with a Seamly pattern, but we have a service a couple doors down from us that we’ve used with some pattern others have sent us.

1 Like

What do you mean by plotters?

I currently live in a city that has no copy shop whatsoever. Luckily I’m still physically at work 1 day a week, so a pdf print and than cutting and pasting it together should do.

1 Like