Index color separation, and when to choose it over simulated process

Why index is the quiet favourite

Conventional tonal printing builds a gradient from dots of varying size, which forces you to space screen angles carefully, compensate for how much each dot will grow on press, and test the result against the fabric. Index separation sidesteps most of it. Every dot is the same size and 100% ink; tone comes from how densely those dots are scattered. Because there are no varying dot sizes there is no halftone angle set or rotated process grid, so inter-screen process moiré stops being the main failure mode. Dot growth can still make the image darker, and the dither texture or its repeated tile can still show at some scales. It is a simpler tonal hand-off, not a promise that mesh, emulsion and press variables disappear.

How it works here

You choose a maximum ink count. The image is clustered in LAB space to build a perceptually grouped candidate palette — the same colour-space approach the spot separator uses — and tolerance or empty clusters can return fewer masks. Then the transitions are dithered with blue noise rather than an ordered pattern. Bump the ink count and the image can get smoother while the screen count rises; drop it and the result becomes more graphic and posterized. Inspect the palette rather than treating the automatic choices as an ink recipe.

One thing to know first: the free engine keeps the biggest ink as the base

This is the sharp edge on the free path and it is easier to hear now than to discover at the light table. The free in-browser engine always treats the largest-area cluster as the base and leaves it in place rather than lifting it to a plate, on the theory that the biggest flat field in a design is the garment or the field the art sits on, and you do not burn a screen for the shirt. That is often right on flat artwork where the largest field really is the substrate. On a photograph it is a guess, and it is the kind of guess a photograph punishes: if the largest area of your image is a sky, a wall or a subject's jacket, that colour becomes the base instead of a plate — and it is a colour you almost certainly do need to print. So the free index separation of a photograph hands you one fewer printable screen than the returned palette contains, and the omitted screen may not be the one you would have chosen. Look at what landed in the base before you trust the plate list. The paid render decides this differently: it drops an ink to base only when that ink is close to the garment colour you set and records that decision in its warnings. That is why free and paid index separations of the same photograph can legitimately return different plate counts.

What you can control

A maximum palette count from 2 to 12. Tolerance, which decides how eagerly near-identical colors collapse into one ink. And the full local ink list — merge two inks that are practically the same, rename a plate to the ink you will actually mix, reorder the local plate output, or delete a plate and deliberately drop its coverage. Per-plate preview shows each screen on its own before you burn anything. Garment colour is a control too, and it is worth being exact about what it does at each tier: on the free client-side path it tints the on-screen proof so you are judging the composite against the shirt you will actually print rather than against white — it does not change the plates themselves. On the paid render it genuinely feeds the separation: the underbase is built for that substrate, and an ink close enough to the garment colour is recognised as the substrate rather than cut as a screen. So set it either way — on the free path for an honest proof, on the paid path because it changes the output.

Free, in the browser, with no upload

Index separation is client-side, like spot. No account is needed to separate, and your artwork is never uploaded; an anonymous outcome beacon sends only the run type, duration and resulting printable-screen count. Downloads require a free account. Once signed in, you can export per-ink PNGs, a numbered ZIP, an ORA file, a layered PSD, or a multichannel PSD with one real spot channel per printable ink (the two PSDs share one free daily export; a monthly plan removes that cap). The local masks already contain solid dithered dots and do not need a second halftone screen, but they remain working-canvas separation artwork: verify effective resolution, the chosen base, registration needs and output polarity in your own film path. The paid render is the route that adds output-DPI film and registration marks.

When you still want the paid render

Three reasons. First, resolution: dithering is resolution-dependent, and a 14-inch plate targeting 1200 dpi is 16,800 pixels along that dimension — server work, with the renderer's 16,800px/200MP caps and any clamp recorded in the manifest. Second, print math: a choked underbase under an index separation on a dark garment, per-ink choke and spread, registration marks, optional SVG tracing for suitable plates, and 1-bit film output. Third, the base decision above — the paid render only demotes an ink to the substrate when it actually matches your garment colour. That render is 10 credits per image, and for 24 hours afterwards you can re-render that image as often as you like at no extra cost.

Index vs simulated process: what actually differs

Both print a full-colour image with spot inks, and they differ in one structural way from which everything else follows. Index makes every dot the same size at 100% ink and builds tone out of dot *density*; simulated process varies dot size and builds tone out of dot *area*. Because index dots never change size there is no dot grid to rotate, so index needs no screen angles at all and there is no rotated grid for the plate next to it to interfere with — moiré is not the failure mode you spend the job managing. Simulated process has to halftone every channel, which means an angle set, and moiré is a live risk you manage. Against that, varying dot sizes carry more information: simulated process resolves more tonal detail and reads smoother, particularly in skin tones and long soft gradients, where index shows its dither pattern and its palette steps. Index also degrades gracefully — dot gain moves an index print slightly darker overall, because a solid dot growing is still a solid dot, whereas gain in a halftone distorts the tonal curve and is the classic cause of a muddy process print.

So which one should you run

Choose index when the image will tolerate a graphic look, when the garment is textured or heathered, when your equipment or your registration is not razor-sharp, when you are still learning tonal work, or when you want the job to behave the same way on the fiftieth shirt as on the first. Choose simulated process when the artwork is photographic and needs to look photographic — faces, chrome, soft product shots, anything where a visible dither pattern would read as a mistake rather than a style — and when you have the press control and the patience to correct channels. There is also an honest cost difference: index separation here is free and runs in your browser, and simulated process needs a server render and a human on the channels, because fully automatic simulated process is unreliable in every tool that sells it, ours included. Plenty of good shops run index for most of their tonal work and reserve simulated process for the jobs that genuinely demand it. Try both on the same file — switching separation type keeps your document, so it costs you a click rather than a restart.