The Plaid Studio / Seamless pattern generator / Seamless pattern checker
Seamless pattern checker
Drop a pattern tile and see it repeated 3 × 3, offset by half so its edges meet in the middle, with each join measured against the texture of the tile itself. Find the seam before a print-on-demand check or a printer's proof does. Runs in your browser — the image never leaves your computer.
How the check works
A tile is seamless when whatever leaves its right edge comes back on its left, and whatever leaves the bottom comes back at the top. So the checker compares the last column of pixels with the first, and the last row with the first, colour by colour. A busy, textured pattern differs a little from one column to the next anyway, so each join is measured against the tile's own column-to-column and row-to-row difference: a join that is no rougher than the texture around it will not show; one several times rougher will.
The offset view is the classic test from image editors: shift the tile by half its width and height and the four edges meet in a cross in the middle, where the eye finds a break at once. Read what makes a pattern seamless for the common causes, and use the DPI calculator to check the tile is large enough for its print size.
Questions
How do I check if a pattern is seamless?
Repeat it 2 × 2 or 3 × 3 at its real print size and look at the joins, then offset it by half a tile so the edges meet in the middle. The checker above does both and also measures how much each join differs from the texture around it.
Why does my pattern show a line where the tiles meet?
Usually a motif stops short of the edge or is cut off on one side only, the tile is lighter on one side, the file was resized by an odd amount and left a half-pixel row, or JPEG compression softened the edges differently from the middle.
Is my image uploaded anywhere?
No. The image is read and measured in your browser with a canvas; it never leaves your computer.
What do the half-drop and brick options do?
They lay the tile the way those repeats do — every other column dropped by half, or every other row shifted by half — so you can see whether the design reads well in that layout. The edge measurement is the same for all three; a half-drop also needs the top and bottom to match across the drop.