# How it works

> One instanced quad per primitive, exact signed distance functions per pixel, analytic coverage from screen-space derivatives and dithered OKLab ramps.

Source: https://pixi-silk.schmooky.dev/docs/how-it-works/

## One instance per primitive

`SilkGraphics` is a Pixi `Mesh` with an instanced unit quad. Each primitive you draw is one
instance of 40 floats: its bounds, shape parameters, paints, gradient geometry and dash pattern.
The vertex shader expands the quad to the primitive's bounds plus an anti-aliasing margin. The
margin is measured in device pixels, using the smallest singular value of the local-to-pixel
Jacobian. That keeps it right at any zoom and for rotated or skewed objects.

## Exact distance, analytic coverage

For every pixel, the fragment shader computes the signed distance `d` from the pixel centre to the
shape (negative inside). It supports rounded boxes with per-corner radii and superellipse corners,
ellipses (trig-free Newton iterations), arcs, sectors, capsules, hearts, stars and triangles. It
turns the distance into coverage with a one-pixel box filter:

```glsl
float px = sqrt(abs(dFdx(p).x * dFdy(p).y - dFdx(p).y * dFdy(p).x)); // one device pixel, in local units
float coverage = clamp(0.5 - d / px, 0.0, 1.0);
```

The filter is exactly one device pixel wide, because `px` comes from screen-space derivatives. This
holds at 1000x zoom, when rotated, and inside filters and render textures. With `blur`, the box
filter becomes a gaussian: `0.5 - 0.5 * erf(d / (σ√2))`.

## Fill and stroke without seams

A primitive with both a fill and a stroke is shaded once. The shader computes the coverage of the
whole shape and of the stroke band. It splits each pixel exactly into "stroke" and "fill outside the
stroke", so no anti-aliased fill edge shows through an inner stroke.

## Polylines

Each polyline segment is an instance with its neighbours' geometry attached. A segment shades a
fragment only if it is the nearest one, so joints are round and translucent lines never
double-blend. A tiny antisymmetric bias breaks ties. Tapered strokes use the exact distance to an
uneven capsule.

## Areas

Filled chart areas are split into vertical columns that tile exactly. Shared column edges get no
anti-aliasing margin, only the top and bottom edges do. That keeps translucent fills uniform, with
no seams between columns.

## Hairlines

A stroke thinner than a device pixel is widened to one pixel, with its alpha scaled by the ratio.
The amount of "ink" stays the same, and thin lines never break apart.

## Gradients

Stops are interpolated (premultiplied, in OKLab by default) and baked into a shared 256x256 RGBA16F
atlas, one reference-counted row per ramp. Sampling adds ±0.5 LSB of interleaved gradient noise,
which removes 8-bit banding but leaves exact 8-bit colours untouched.

## Hit testing

The same distance functions run on the CPU, so `containsPoint` tests the real shape, including
holes and stroke widths.