Read the one-star reviews of any struggling coloring book and the same words appear: *blurry*, *pixelated*, *fuzzy lines*, *looks like a bad photocopy*. Colorists spend hours inside your line art at a distance of thirty centimetres — print quality is not a detail to them, it is the product. Here is what actually determines whether your printed lines come out crisp, and where publishers go wrong.

Vector and raster, in one minute

A vector file (SVG, some PDFs) stores shapes as mathematics — curves that can scale to any size without quality loss. A raster file (PNG, JPG) stores a grid of pixels, and its quality at print time depends entirely on how many pixels it has relative to the printed size. Vector sounds automatically superior, but that is not the full story: KDP's printers rasterize everything before ink hits paper. What reaches the page is always pixels — the only question is whether there are enough of them at your trim size.

The number that matters: pixels at trim size

Print needs roughly 300 dots per inch to look sharp at reading distance. For an 8.5×8.5in book that means about 2550×2550 pixels; for 6×9in, about 1800×2700. A 1000-pixel-wide image stretched across an 8.5-inch page delivers ~120 DPI — and that is exactly the soft, fuzzy line work those one-star reviews describe. The mistake is almost never "used a raster format"; it is printing a small raster large.

Where blur really comes from

  • Upscaling: enlarging a small image invents pixels and softens every edge. Always generate or scan at final size or larger, never inflate afterwards.
  • JPG compression on line art: JPG artifacts halo around high-contrast edges — precisely what line art is made of. Keep interiors in PNG or PDF end to end.
  • Grey anti-aliased edges: soft grey edge pixels print as dotty halftone speckle. Strong black-on-white contrast prints cleanest.
  • Re-exporting through Word or slide software: office tools silently downsample embedded images. Assemble your interior in a proper PDF pipeline.

What this means for AI-generated interiors

AI image models output rasters, so the two things to verify before publishing any AI-generated book are the output resolution and the aspect ratio versus your trim size. magicpaint.io handles both in the pipeline: pages are generated at high resolution (2K), matched to your book's KDP trim ratio so nothing is stretched or cropped at layout time, and compiled into the PDF at native resolution — no upscaling step anywhere. That, far more than the vector-versus-raster label, is what "print-ready" has to mean. If you want to compare how other generators approach this, directories like AI Toolz Dir catalog AI tools by category — check any tool's output resolution against the ~300 DPI bar before trusting it with an interior.

A five-minute pre-flight check

Before you upload any interior: zoom to 300% in your PDF viewer and inspect the thinnest lines for softness; confirm the page pixel dimensions divided by trim inches clear ~300 DPI; and after publishing, always order a proof copy. Five minutes of checking is cheaper than the permanent one-star review that "blurry lines" leaves on your listing.