Spot color separation, free and in your browser

What a spot separation has to get right

A spot separation is one plate per ink, and the rule that matters is that a plate is defined by color, not by shape. A single cream ink might appear in eleven letters of a wordmark, two snowcaps and a scattering of highlights — that is one screen, printed once, so it has to be one plate. Tools that separate by connected region hand you eleven plates and a headache, and tools that filter small regions by area quietly drop the lettering entirely. The separator here clusters in LAB space using perceptual color distance, so inks are grouped the way a printer groups them and small shapes of a known ink stay with their ink.

Why grouping by colour and not by shape is the whole game

This is the failure that separates working tools from demos, so it is worth being concrete about — and the plate below is the concrete version, generated by the committed proof script through the shipped client engine. The source spells three words in cream: 19 disconnected glyphs and exactly one screen — one ink, mixed once, burned once, printed once. A separator that walks connected regions hands you 19 plates, and now you are merging them by hand and hoping you did not miss a small one. Worse are the tools that filter regions by pixel area to keep the plate count down: those quietly delete lettering, because a letterform at print resolution is a small region and the algorithm cannot tell the difference between a serif and a speck of JPEG noise. You find out at the light table. Grouping in colour space instead means the question asked of every pixel is "which ink is this?" rather than "which blob is this?" Clustering here happens in LAB space using perceptual distance, so two colours that look the same distance apart to your eye are the same distance apart to the maths — which is not true in RGB, where a fixed numeric step is a huge shift in some parts of the space and invisible in others.

What tolerance actually decides

Ink count sets a maximum palette size from 2 to 12, not a guaranteed number of screens: empty clusters or tolerance can return fewer, and the free engine keeps the largest colour field as the base. Most spot jobs still land at 2 to 6 printable inks. Tolerance is the more interesting control: it sets how far a pixel may drift from an ink centre before it is treated as evidence of a different ink. Set it too tight on a file that has been through JPEG compression and every flat area fragments, because compression does not store flat colour as flat — an 8-bit fill that was one value on the designer's screen arrives as a cloud of values a few steps wide, and a tight tolerance reads that cloud as three inks that all look identical in the preview. Set it too loose and two colours you meant to keep apart collapse: a navy and a black that are genuinely different inks in your drawer merge into one plate, and the design loses its depth. The practical method is to separate high and merge down. Run at eight inks, look at the plates, and merge the ones that are the same ink in the real world — merging is instant and free, and unmerging is just a re-separate, whereas discovering at the press that two plates should have been one costs you a screen. The ink list is fully editable while you do it: merge plates, rename the readable default to the ink you will actually mix, reorder the local plate output, or delete a plate knowing that deletion drops its coverage rather than redistributing it. The exact hex remains beside every generated name and follows into the manifest. There is no ink library and no PMS lookup here: the separator starts with names such as Navy, Cream or Red, and you supply the real shop ink name that then lands in the PSD, the ORA and the file names.

The anti-aliased edge between two inks

Every edge in a raster design is a lie told in a few pixels. Where cream meets navy, the renderer wrote a ramp of blended values so the edge looks smooth on screen, and none of those intermediate values is an ink you own. A naive separator treats that ramp as evidence of new colours and gives you a phantom plate that is nothing but a one-pixel outline tracing every shape in the design — which looks almost plausible in a layer panel and is unprintable. Here a blend that lies between two known inks is split as coverage between those two plates instead of becoming a third ink. The raw PNG and PSD masks therefore keep grayscale edge coverage; index output and the paid 1-bit-film option resolve coverage into solid dots. Two things to know about the result. First, adjacent plates often need intentional overlap for press misregistration; the paid render exposes per-ink spread for that, but no separate automatic-trapping control. Second, on a badly compressed file the ramp is wider and noisier than the designer intended, and tolerance is the control for that — raise it so the noise collapses into its parent inks instead of spawning phantom plates. If you brush the generated plate layers, use the editor's document export to keep those pixel edits; the Colours-panel plate and spot-channel writers still read the raw masks. What you should not do is chase a perfect edge in the separator when the source JPEG never had one.

Free, client-side, and not uploaded

Spot separation runs entirely in your browser. There is no artwork upload, no account and no queue during separation — your customer's pixels are never transmitted. A small anonymous outcome beacon records that the tool ran, without dimensions, file names, ink data or image content. Downloads need a free account. Once signed in, per-ink transparent PNGs, a numbered ZIP in stacking order and an OpenRaster file for GIMP or Krita are unlimited and still written locally.

Two PSD formats sharing one daily allowance

The layered PSD is the everyday hand-off: one named layer per ink, every plate written at the full working-canvas size so they stack back up in register. The multichannel PSD targets a separator's spot-channel hand-off. It carries one real spot channel per ink, each with its ink colour and the name you gave it attached to the channel itself; its resolution record uses the physical DPI implied by the print width you set. The garment base is deliberately left out, because the substrate is not a screen and including it would hand your shop an extra one to burn. Both exports are written in your browser and both count against the same allowance: one PSD export a day on a free account, uncapped on a monthly plan, and 3 credits each beyond that for non-subscribers. Neither writer applies an extra export-tier downscale or watermark; the editor can still fit an unusually large source to the current device's memory ceiling before separation. Worth saying plainly because it is unusual — the desktop packages charge $229 to $399 for spot-channel output, and here it is the free half. And the writer has been checked by something other than itself: the test suite reads a four-channel fixture back through ag-psd and psd-tools, checking channel names, colours, spot records, dimensions, DPI and ink polarity. That is a separate writer test, not the plate artwork pictured on this page. Photoshop has not opened the sample yet, so we are not going to say it has.

When you want the print math on top

Separation is one half of a press-ready file; the other half is print math, and that is the paid render: per-ink choke and spread (including deliberate overlap where you set it), a choked underbase on dark garments, a separate highlight white printed last, dot-gain compensation, registration marks, halftone screening if a plate needs tone, optional SVG tracing for suitable plates, and 1-bit film targeting your chosen DPI within the renderer's disclosed output caps. That render is 10 credits per image, and for 24 hours afterwards you can re-render the same image as often as you like at no extra cost — long enough to adjust a choke value five times, print a test, and come back to it after lunch.

Where spot separation is the wrong tool

Spot is for flat and mostly-flat art. Point it at a full-tonal photograph and you will get a posterized approximation, because a photograph does not have discrete inks in it. For photographs you want index separation (posterize plus dither, no halftone angles to worry about) or simulated process (tonal channels, halftoned, underbase, highlight white). Both are one tab away, and both start from the same clustering pass you already ran.