What a print check catches before you waste film
Every print shop keeps a shelf of sheets that came out wrong and were paid for anyway. Most of them failed for one of a small number of reasons, and most of those reasons were visible in the file before it was printed. This is the list, in the order we see them.
A background that should not be there
The most common one. A logo saved as a JPEG has a white rectangle behind it; a screenshot has the page behind it. On a transfer, that rectangle prints. It can be removed by colour (everything close to white goes) or by a model that separates the subject from the background; the second handles photos and gradients that the first cannot. Either way the cut edge has to be smoothed, or the result has a jagged white fringe.
Empty space around the artwork
A transparent border is not free: it takes sheet area the customer pays for and makes the artwork print smaller than they expected. Cropping to the visible pixels fixes the size, the price and the layout at once.
Stray specks
Dust on a scan, stray pixels from a sloppy magic wand, a single dot left behind by a removed background. Each one prints as a tiny transfer that has to be weeded or ends up on the garment. They are easy to find by size and easy to remove.
Half transparent edges
Anti-aliased edges on a transparent PNG are partly see-through. DTF ink is not: a 40% pixel prints as a pale, fuzzy halo around the design. The fix hardens the edge so every pixel is either ink or nothing. This one surprises customers most, because the file looks fine on a screen.
Glow and JPEG ghosts
A screenshot of a logo on a dark site carries a faint glow; a JPEG carries compression noise that becomes a grey smudge once the background is gone. Both print. Removing them is a matter of distance from the solid artwork.
Low resolution
A 400 pixel logo printed at 10 inches is 40 dots per inch, and it looks like it. The right first answer is to tell the customer the largest size that image can print sharply and offer it in one click. The second answer, where the shop offers it, is upscaling: a model redraws the image at up to four times its size, which rescues flat artwork and logos well and photographs less well.
Fine detail that will not survive
Hairlines, tiny text and thin outlines fall off the transfer at pressing. A check can flag lines thinner than the film and powder can hold, which is a conversation worth having before the print, not after.
When to run it
Before the order. A check that runs in your queue after payment turns every finding into an email. A check that runs at upload shows the customer the problem and the fix while they are still deciding, and most apply the fix themselves. SheetGangster's print check runs on every upload, on every plan, with background removal and cropping free; the rest of the fixes above come with Basic and Premium, and upscaling with Premium. Each finding carries a one click fix, and the shop can turn any check off.