Underbase generator: choked white, and a highlight white that works

Why the underbase is choked

An underbase that exactly matches the artwork outline is a trap. The instant a screen shifts by half a millimetre — and it will — white peeks out from behind the color and rims the whole design. The fix is to choke it: shrink the underbase inward by a small, deliberate amount so the color plates over-print past its edge and hide it. Too little choke and you get the halo; too much and colors on dark garments go dull at the edges because there is nothing bright behind them. The right number depends on your registration accuracy and your press, so it is a control here, not a hidden constant.

How it is generated

The underbase is built from the combined coverage of every color plate in the separation, then eroded inward by your choke value in pixels at the output resolution. Because it is generated from the separation rather than drawn by hand, it updates when you merge inks, change the ink count, or delete a plate — you are never maintaining it separately. On a design with soft edges or halftoned areas, the underbase carries that tone too, so a 40% shadow does not get a solid slab of white beneath it.

Highlight white is a different screen

The white under the print and the white on top of the print are two different jobs. The underbase brightens the substrate; the highlight white is the specular hit — the glint in an eye, the edge of a chrome letter — and it should print last, over everything, at full opacity. Merging the two produces flat prints, which is why it is a separate plate here with its own threshold control. Turn it off for artwork that has no true whites, and turn it on for anything with metal, glass, water or a face in it.

Choke, spread and trapping, all in one pass

Choke and spread are the same operation in opposite directions: pull a plate in, or push it out. Both are per-ink here, so you can choke the underbase, spread a thin lettering plate that would otherwise starve, and leave everything else alone. Trapping handles the other registration failure — a hairline gap between two adjacent colors — by overlapping them slightly so a small misregistration shows overlap rather than garment. None of it is mysterious; all of it is measured in pixels at your output resolution, so the numbers mean something you can reason about.

Set the garment color and stop guessing

The separation takes the garment color as an input, and the proof renders your plates back over that color — underbase, colors, highlight white, in print order. That is the difference between checking a separation and hoping about one. Look at the proof on your actual shirt color before you burn anything, and if the underbase edge is visible in it, it will be visible on the press.

What it costs

Underbase generation is part of the print-ready render: 10 credits per image, with unlimited free re-renders of that image. That matters here more than anywhere, because finding your shop's correct choke value is an iterative exercise — you will render, look, adjust, and render again, and charging per render would only teach you to avoid the control that most needs using. Credits come with a plan from $8.50 a month for 600, or a $9 top-up for 500 that never expire. The separation itself, if all you want is flat spot plates, stays free and client-side.