Index color separation, free in your browser

Why index is the quiet favourite

Conventional tonal printing builds a gradient from dots of varying size, which forces you to pick screen angles that do not interfere, compensate for how much each dot will grow on press, and hope the fabric agrees. Index separation sidesteps all 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 are no screen angles, so there is no moiré, and because every dot is solid, dot gain moves the image slightly darker instead of destroying a tonal curve. It is forgiving on textured garments, forgiving on old equipment, and forgiving of a printer who is still learning tonal work.

How it works here

You choose the ink count. The image is clustered in LAB space to find the palette that best represents it — the same perceptual clustering the spot separator uses, so the inks it picks are inks a person would have picked. Then the transitions are dithered with blue noise rather than an ordered pattern, because ordered dithers leave a visible grid that fights the weave of a shirt. Each ink becomes its own plate of solid dots. Bump the ink count and the image gets smoother and the screen count goes up; drop it and you get a graphic, posterized look that a lot of merch actively wants.

What you can control

Ink count from 2 to 12. Tolerance, which decides how eagerly near-identical colors collapse into one ink. Garment color, so the substrate is accounted for instead of pretending you print on white. And the full ink list — merge two inks that are practically the same, rename a plate to the ink you will actually mix, reorder the print sequence, or delete a plate and let the rest absorb it. Per-plate preview shows each screen on its own before you burn anything.

Free, in the browser, with no upload

Index separation is client-side, like spot. Nothing uploads, no account is needed, and it works with the internet down. Export per-ink PNGs, a numbered ZIP, an ORA file, or a layered PSD (three a day free, full quality, no watermark ever). Because index plates are already solid dots rather than continuous tone, the free export is genuinely usable film artwork for a lot of jobs — which is not something we can say about a tonal separation.

When you still want the paid render

Two reasons. First, resolution: dithering is resolution-dependent, and a plate rendered at 1200 dpi for a 14-inch print is a 16,800-pixel image — comfortable on a server, painful in a browser tab on a shop PC. Second, print math: a choked underbase under an index separation on a dark garment, per-ink choke and spread, registration marks, and vector or 1-bit film output. That render is 10 credits per image, and re-renders after you adjust something are free.

Index versus simulated process

Both print photographs. Simulated process gives a smoother, more photographic result and generally handles skin tones and soft gradients better, but it needs halftones, angle choices, dot-gain compensation, and a human reviewing channels. Index is coarser and more graphic, but it is dramatically more predictable on press and much easier to get right the first time. Plenty of shops run index for exactly that reason. Try both on the same file — the tab keeps your document, so switching separation type does not mean starting over.