There's a trivial solution: Let your decorative pattern have discontinuities of the same type than a randomly placed seam would make. Then nobody notices the seams in the pattern. You can print your pattern quite freely onto the fabric.
Unfortunately the trivial solution exludes large recognizable shapes and long continuous curves.If you want your shapes to continue smoothly over the seams, the situation is complex.
Lets see an example. Here we have a couple of fabric cutting patterns which can belong to some shirt:
1) A half of the front side. AB=hole for a sleeve, BC and FA =seam with the back, DC=bottom edge, DE=edge that will get buttons, EF=seam with the collar
2) A sleeve. HI and JK will be one seam, KGH is a curved seam with the front and back parts, the dashed line at G is the symmetry axis
3) The magenta lines show the actual fabric edges. The space between black and magenta is for seam allowances and edge foldings. It's not visible, but can be printed to give some room for sewing inaccuracy.
The details of the decorative patterns cannot be drawn without these pieces. You can have the general idea beforehand, but you must carafully place the details.
Straight linear seams are quite easy. You can draw the pattern first to continue over the seam. Then you can add the rest of the shapes in the middle. In Illustrator you can for example align two copies of the sleeve side by side and draw over the seam. The seam and edge folding allowances are hided to reduce the clutter.
4) Two copies of the sleeve are aligned (drag with the direct selection tool one corner to snap, rotate with the rotation tool to make another corner to snap, smart quides must be ON, other snaps =OFF). The decorative shapes are drawed over the seam. Also the curved top seam KGH is covered. I gave a blue color to indicate that this is not exact at the other side of the seam.
5) All is grouped to keep it together. A copy is made and dragged & rotated to get both edges of the seam rady. All was ungrouped and the extras are deleted.
6) The decorative pattern is completed. The seam and folding allowances and are turned visible only to see the full piece.
Next we match seam AB-GK by continuing the pattern over the seam. Curves AB and GK are different in plane, but they fit when the fabric is bended. Fabric is not stretched, so the curve lengths must be same. We can measure the places, where the decorative shapes cross curve GK. If we express the places as distances from the endpoint along the curve, the same values should be ok for both GK and AB.
To make the measurement
- separate curve GK with the scissors tool
- let the decorative shapes on the curve stay
- arrange GK to front, insert an anchor point to every crossing
- take the scissors tool and split curve GK at every croossing.
In the following image the pieces of GK have different colors:
Illustrator shows curve lengths in the document info panel. Take subpanel Objects to see the length of the selected curve segment.
A bunch of rectangles is chained. The rectangles have widths = Curve segment lengths. This way we transfer the places of the crossings to a straight line. We do not use the rectangles as measuring tape, but draw a little simpler version with only line segments. At the bottom we have our measuring tape and a horizontally flipped copy of it. We drag them both to the brush collection and mark them to pattern brushes. The flipped version can be used, if our curve has reversed direction.
It's useful to test our measurind tape brush. It should fit perfectly to the segment GK. We test it to image 5 to reveal possible inaccuracy:
It looks out perfect. Next we separate segment AB in our fabric, duplicate it and change it's stroke to our measuring tape pattern brush:
7) Segment AB is separated with the scissors tool and got our measuring tape brush. A copy of the original segmen is moved a little aside. The fine placement relied on snaps ad was made with the direct selection tool. The red color was possible after expanding (Object > Expand Appearance).
8) The same blue circles were copied which were drawed over GK. Their placement is a little tricky. One point snaps easily. The circle häd to be rotated with the rotation tool to get another point to fit.
9) The circles are placed ok, the original AB is placed back and joined.
The drawing of the decorative pattern can be continued.