ClipBrook

Transparent PNGs for print on demand

A print-on-demand upload needs three things from a transparent PNG: enough pixels for the printed size, a bounding box that hugs the design, and edges whose color is stored honestly, with no background baked into the half-transparent pixels. ClipBrook cuts AI renders into that kind of PNG, in your browser, free.

The pixel arithmetic

Print is measured in dots per inch, and 300 DPI is the usual standard for a sharp result. Divide the pixels by 300 and you have the printed inches. The export panel offers four choices:

  • 4500 px on the long side is 15 inches at 300 DPI, which covers a large apparel print.
  • 3000 px is 10 inches, enough for mugs, stickers and most smaller goods.
  • 2048 px is 6.8 inches, which suits listing images and previews.
  • Keep the original size leaves the render at whatever it came out of the generator at.

Each preset resizes the long side and keeps the aspect ratio. Upscaling a small render adds no detail, so generate at 4K where you can and size down rather than up. The size is applied at download time, after the cut, so one batch can be exported at two sizes without cutting anything twice.

The bounding box

Print platforms place your design by its bounding box: the smallest rectangle that contains every pixel that is not transparent. One stray speck ten pixels wide in a corner, and the platform centers and scales your artwork as though the design included the speck, shrinking the part you actually drew. ClipBrook’s trim option crops the PNG to its content with a padding margin you set, and the keyer already deletes tiny stray islands as part of the cut, so a fragment has to be fairly large to survive into the bounding box at all.

Straight alpha, and why edges go wrong

A PNG stores straight alpha, which means each half-transparent edge pixel keeps its true subject color and records its transparency separately. That is what lets a printer or a compositor lay your design over a black shirt and get a clean blend: the edge pixel still knows it is “petal pink at 40% coverage” rather than “pink already mixed with something”.

Any pipeline that routes its output through a browser canvas multiplies the color by the alpha and rounds the result to whole numbers. At 10% alpha that rounding leaves about 26 usable levels per color channel instead of 256, and undoing the multiplication gives back the wrong color. At thumbnail size on a screen you would not see it. On a 15-inch print it shows as a dirty fringe along the edge of the design. ClipBrook never touches a canvas on the way out: the keyer solves each edge pixel’s true color and coverage, the resizer works on premultiplied values internally and converts back at the end, and the file is encoded straight from the raw pixel buffer.

There is a second source of fringes, and it is the render itself. A background that reflects onto the subject, a soft glow, or a vignette leaves tinted edges no exporter can fix, because the tint is genuinely part of the artwork. The prompt guide shows how to generate on a flat key color so the cut has clean material to work with, with prompts you can copy for each generator.

The workflow, in order

Take a prompt from the guide, render at 4K, drop the batch in, turn trim on, choose 4500 px, download the ZIP, upload it.