The principle of seamless is pretty simple: if an object within a ‘tile’ of a pattern extends beyond the edge of the pattern, the portion of the object that falls outside the edge of the tile must exist within the tile on the opposite side.
Translation:
First things first. ‘tile’ is one repeat of your design.
Say you are going an anatomical heart as your image. If the heart hits the edge of the roll it has to appear on the opposite edge of the roll too. IE: It has to match to be seamless. Al Wrath’s amazing Sketchy Trees is a great example of this.
More Official tech speak:
If the pattern is a straight match, the part of the object falling outside the edge must come on at the same level at the other side of the edge. If you are creating repeats that work as offset drops (eg. half drop), the part falling outside the tile must enter the tile on the other side of the tile precisely in the distance of the drop.
Here’s more about your different repeat options:
Straight match: the left and right edges of the tile match in a straight horizontal line with the left and right edge of the next tile. If your design has standalone motifs or dominant colour splashes it might be better to try other repeat options to create a better flow and rhythm for the design.
Half drop / offset match: the match is obtained by half dropping the repeat.
1/3 drop: the adjacent tile drops down 1/3 length of the design
1/4 drop: the adjacent tile drops down 1/4 length of the design
Mirror: the match is obtained by turning every second strip of wallpaper upside down.
Free match: the repeat has no match. The tiles can be hung next to each other without having to match any parts of the pattern. Super easy for hanging the wallpaper.
How to create brilliant seamless repeats:
Create work that comes to life on a whole wall. Don’t just design for the roll. Try and think how it’s going to feel surrounding someone.
Set up the canvas for your tile by choosing the size and shape you want. The width of the tile is 53cm. The length of the tile is totally down to your design and when printed will be repeated to fill the 10meter long roll. However, bear in mind that very long repeats can be difficult to hang and often come with wasted wallpaper. Try to stay under a meter for your vertical repeat.
Understand your idea as it relates to wallpaper. Is it half drop, quarter drop, 1/3 drop, straight match, reverse match that give your artwork the most depth and the right rhythm and flow?
Photoshop Offset feature comes in handy when you need to get the edges of your tile to match perfectly. Have a look at Sycra Yasin’s superb simple tutorial for creating a seamless straight match repeat
SewHeidi has a great tutorial for creating a seamless repeats including half drop, quarter drop and 1/3 drop matches (Illustrator CS6 and newer, for CS5 and earlier versions check out this video)
And here’s another great piece of advise from SewHeidi how to extract one tile from a repeating pattern in Ilustrator
We hope these are helpful and feel free to get in touch if you have any questions!