Attached is a screenshot from my seamly sleeve pattern (apologies for the black background it is not in my control). I am surely not the only one who has ever had to adjust a sleeve cap curve to match the length of an armscye and I would like to do it in the most professional way possible.
My point A9 determines sleeve cap width and the length of the back notch curve (Spl_A1_A9) is dependant on it. Its origin point is A8 which is at the same Y coordinate, which is by design so that I full control over the position of the point.
Is there a way to set the length of Line_A8_A9 (value X from the title) to be such (or to increase until) Spl_A1_A9 (Y) is a given value (the length of the scye of the back bodice pattern).
I remember once trying to read the seamly manual (emphasis on trying) and seeing something similar but it seemed too complicated for me to even think about it.
Theoretically the length of Line_A8_A9 should be proportional to the back half of the arm scye, and by trrail an error you could figure out what that ratio is by setting a value for Line_A8_A9 and then comparing the sleeve to bodice lengths… repeat until the length match. Once you’ve figured out that ratio it could be applied to other sizes… BUT I don’t think there’s a direct way to have Line_A8_A9 adjust to the length of back half of the armscye so it matches the back half of the sleeve cap. i.e. There is no way to draw a spline of a given length.
I agree with @Douglas and believe me, I’ve tried. There are too many factors to take into consideration that make a spline/curve length quite unstable in this regard. For instance, we’re taught that the curve must touch a point at a distance from a point between A7 & A9, going downwards towards the centre & another distance at a point between A1 & A7 going upwards and outwards. I find that these standards don’t necessarily create a nice smooth curve over a range of sizes, nor do they work well if the cap height is shortened. And, over a range of sizes, the ratio of the cap curve to armscye curve also changes when these methods are used. Hence my ongoing studies of sleeve caps…
While not inifinite, there’s an astronomical number of possible combinations of control point lengths and angles that will yield a spline of a given length. The only spline we can draw between 2 points with a known length (= the length between the 2 points) is a straight line. Just to put it in perspective… each control point length and an angle is represented by a qreal number with a max value of 1.79769e+308 with a LOT of values between 0 and 1.79769e+308 without even talking about decimal places. Again the only way to draw a spline with a known length is by each control point length = 0, or a straight line.
Plus in the asked senario it includes the complexity of a 3rd parameter - the length of LIne_A8_A9 to move the point A9 to adjust the length the spline from A1 to A9.
1st sleeve being more your typical sleeve, 2nd being more a t-shirt with little cap. Eventually you would hit the limit where A25 to A74 = Armscye length with no cap… i.e. a rectangular or trapazoidal sleeve - like a kimono.
I don’t mind sharing, but mostly it comes from trial, error & practice. Lots of studying how people do things in all sorts of books & reading everything I can on the internet, and trying those methods in Seamly using the tools available, variables and formulas.
I did a short tutorial on armhole curves in this topic. I use pretty much the same methods with the sleeve cap and any other curves that I need. Have a look at it & let me know what you think and then we can discuss another tutorial on the sleeve cap.
