Halftone and dither

Modern maps have millions of colours. Print did not, and the techniques it used to cope — halftone screens, limited inks, dithering — are a look. The palette ops give it to you directly.

Dither to a fixed palette

"riso": {
  "op": "dither",
  "input": "@composite",
  "palette": ["#0c1b33", "#f5f0e6", "#e04a3f", "#4a7bb7"],
  "method": "floyd-steinberg",
  "space": "lab"
}

• method: "floyd-steinberg" (default) diffuses quantization error to neighbouring pixels. Organic and non-repeating — closest to a photographic reproduction. It is inherently scan-ordered, so keep it for single images and bbox renders rather than pyramids where you will study the seams.
• method: "ordered" uses a 4×4 Bayer matrix, with amount (0–1) setting strength. Regular, screen-like, position-deterministic — the choice for tiles.
• space is lab (default, perceptual nearest-colour) or rgb (numeric). Lab is nearly always right; rgb is faster and sometimes harder-edged.

Alpha is preserved, so a dithered layer still composites.

Four inks, error-diffused. The stipple is what keeps the gradients readable.

Reduce without dithering

"flat": { "op": "posterize", "input": "@composite", "steps": 4 },
"inks": { "op": "quantize",  "input": "@composite",
          "palette": ["#0c1b33", "#f5f0e6", "#e04a3f"], "space": "lab" }

posterize snaps each channel to steps levels — banding, deliberately. quantize maps every pixel to its nearest palette entry with no error diffusion: flat, graphic, cheaper than dither. Same palette, very different feel — dither keeps gradients readable, quantize destroys them on purpose.

posterize (left) bands each channel; quantize (right) snaps to a palette. Neither diffuses error, so both go flat.

A newsprint stack

"grey":  { "op": "hsl", "input": "@composite", "saturation": -1 },
"boost": { "op": "levels", "input": "@grey", "in-black": 0.15, "in-white": 0.85 },
"dots":  { "op": "dither", "input": "@boost", "method": "ordered", "amount": 0.8,
           "palette": ["#111111", "#f7f5ef"] }

Desaturate, expand the tonal range so the two-colour screen has something to bite on, then screen. The levels step is the one people skip, and it is the difference between legible and mud. levels also takes gamma, out-black, and out-white if you want to lift the blacks the way ink on newsprint does.

Halftone blocks

mosaic averages the image into square blocks, which is a coarse screen:

"screen": { "op": "mosaic", "input": "@composite", "block": 4, "anchor": "world" }

An 8 px world-anchored block grid.

anchor: "world" keeps the block grid continuous across map tiles, at the cost of extra padding; anchor: "tile" restarts the grid at each tile’s corner and needs no padding — fine for a single image, visible as a grid-of-grids in a pyramid. mode: "nearest" samples each block’s centre instead of averaging, for hard blocks with no colour blending.

Plate wobble

Print looks convincing partly because the plates are never perfectly registered. displace pushes pixels around according to a displacement raster, so low-frequency noise gives a gently misregistered plate:

"wob_src": { "op": "noise", "scale-px": 64, "octaves": 2, "anchor": "world" },
"wobbled": { "op": "displace", "input": "@composite",
             "displacement": "@wob_src", "amp-px": 1.5, "boundary": "mirror" }

Keep amp-px under about 2. More reads as a bug; less is invisible. anchor: "world" on the noise is what keeps the wobble continuous across tiles — see tiles and determinism.

Where in the graph

Palette reduction belongs at the end, over the finished composite, for the same reason a print shop screens assembled artwork rather than each element: dithering per layer and then blending gives you dithered dithering, and the palette stops meaning anything.

The exception is when one layer should look printed on an otherwise smooth map — then dither that layer alone and blend it over.

Padding is computed across the whole chain, so you never add it up by hand; but if edges look wrong after adding displace or a world-anchored mosaic, pad is the first thing to check.

A worked example

risograph.json puts the whole chapter together: a masked point-grid screen over the area fills, solid linework, drums knocked out of register with transform, and one closing quantize onto the ink palette. See it rendered in the gallery.