Seamless pattern guide
Why do white grid lines appear between pattern tiles, and are they real seams?
Thin white lines can be preview artifacts, but they can also reveal transparency, fractional positioning, or altered edge pixels. Test before hiding them.
A display line is not always in the file
Design applications sometimes draw hairlines between adjacent tiles while zooming or using GPU acceleration. Resampling places a tile boundary between screen pixels, and the preview blends neighboring colors with the canvas. The line may disappear at 100 percent, at another zoom level, after disabling GPU preview, or in a flattened export.
This artifact is especially common when copies sit at fractional coordinates. A tile that is 511.5 display pixels wide cannot align every edge with the device-pixel grid. Use whole-pixel placement for a diagnostic preview and compare several zoom levels before editing a source that may already be correct.
Real lines have identifiable causes
A line that survives export usually comes from transparent edge pixels, a matte color, a one-pixel gap, unequal opposite edges, or compression. Transforming each tile separately can also round positions differently. In 3D rendering, texture filtering and mipmaps may sample pixels outside the tile unless wrapping and padding are configured correctly.
Inspect the alpha channel against black, white, and a saturated color. Check the outermost pixel rows rather than only the visible motif. If the tile is opaque, compare exact left-right and top-bottom values. Do not overlap copies as a permanent fix because overlap changes motif spacing and can create dark double edges.
Use an export test to decide
Create a flattened three-by-three image at whole-pixel coordinates and export it losslessly. Open that export in a second viewer. If the line is absent, the original repeat is probably sound and the first application showed a rendering artifact. If the line remains in the exported pixels, repair the tile or placement.
The local checker adds seam guides independently of the pattern, so do not confuse its blue diagnostic overlay with image content. Toggle guides off, compare the heatmap, and inspect the PNG report. A high normalized edge score supports a real mismatch; a clean score plus a zoom-dependent hairline points toward preview rendering.
