DarkRoom/core/dr-gpu/src/shaders/mask.wgsl

// Rasterise one local-adjustment mask into a layer of the mask array.
//
// ARCH §5.4: every mask becomes pixels here and never in CPU memory. One draw
// per layer, each targeting its own array slice, run only when a mask's
// *shape* changes — moving a slider on a masked layer re-runs the adjust
// shader and not this one.
//
// # Why this is a render pass and not a compute one
//
// The natural shape for this is a compute shader writing a storage texture,
// and the format is what rules that out: **R8Unorm is not a core storage
// format**, so a compute path has to widen the mask to R32Float or RGBA8 —
// four bytes per pixel per layer. At eight layers over a 24 MP export that is
// 768 MB of masks, against 192 MB at one byte. A colour attachment takes
// R8Unorm happily, so the mask stays one byte and the pass becomes a
// full-screen triangle.
//
// The array slice is chosen by the *view* the caller attaches, so there is no
// slot uniform here — one less thing that can disagree with the shader.

struct MaskParams {
    // Output size, which is the render size rather than the segmentation's.
    width: u32,
    height: u32,
    // Label field size. Different from the above: the watershed runs at a
    // proxy resolution, and the mask is drawn at whatever the display or the
    // export asked for.
    label_width: u32,
    label_height: u32,

    // 0 = regions, 1 = linear, 2 = radial, 3 = subject, 4 = brush,
    // 5 = luminance range, 6 = colour range.
    //
    // A brush does not read this — it has its own entry points, because it is
    // the one mask that is not a function of the whole frame — but it is set
    // anyway so a captured frame says which kind of mask a pass was drawing.
    mode: u32,
    // How many regions the label field holds, so an out-of-range label is
    // caught rather than read past the end of `selected`.
    region_count: u32,
    // Softening applied to a region mask, in output pixels.
    feather: f32,
    // 0 hard, 1 linear, 2 smooth, 3 gaussian, 4 exponential. Kept in step with
    // `falloff_code` on the Rust side.
    falloff: u32,

    // Geometry. Meaning depends on `mode`. The centre is in normalised 0..1
    // coordinates; every distance below it is in the isotropic frame units
    // `frame_delta` establishes.
    centre: vec2<f32>,
    // Linear: (cos, sin) of the ramp direction. Radial: semi-axes.
    axis: vec2<f32>,
    // Linear: ramp width. Radial: edge falloff as a fraction of the radius.
    // A range: the fade at each edge of its band, in the band's own units.
    softness: f32,
    // Radial only: rotation of the ellipse.
    angle: f32,

    // TRACES: FR-DEV-10
    // How many source texels one mask texel spans, per axis.
    //
    // The mask array is rasterised at a proxy size and the photograph is not,
    // so one texel here covers several there. A range mask is a function of
    // pixel *values*, and point-sampling one source texel in four would make
    // its edge follow the sensor's noise wherever the picture has fine
    // texture — speckle that is then a mask, and therefore visible in the
    // adjustment. Averaging the footprint is what makes the band land on the
    // tone the area actually is.
    source_step: vec2<f32>,

    // Camera RGB → linear sRGB, one row each. Only a range reads these: it is
    // the one mask that looks at the photograph, and a hue is the body's own
    // primaries until this matrix has been applied — so the same stored arc
    // would select a different set of colours on every make of sensor.
    cam_to_srgb_0: vec4<f32>,
    cam_to_srgb_1: vec4<f32>,
    cam_to_srgb_2: vec4<f32>,
    // rgb: as-shot white balance. w: non-zero when the source arrived
    // gamma-encoded rather than linear.
    as_shot_wb: vec4<f32>,

    // Whether this part is turned over before it joins the mask.
    //
    // Read by `fs_combine` and by nothing else, deliberately. A brush deposits
    // dabs onto an empty field and has no idea what the rest of the frame is,
    // so a stroke shader cannot invert anything; doing it where the finished
    // part is read back is the one place that works for every kind of source.
    invert: u32,
    // Three scalars rather than a `vec3<u32>`: a three-component vector is
    // aligned to sixteen bytes in the uniform address space, so it would sit
    // at offset 144 and make this struct 160 bytes against the Rust side's
    // 144 — a mismatch wgpu reports as a binding too small for the shader,
    // several layers away from the padding that caused it.
    _pad0: u32,
    _pad1: u32,
    _pad2: u32,
}

@group(0) @binding(0) var<uniform> p: MaskParams;
// Compacted region id per pixel of the label field. Compacted rather than the
// watershed's raw basin roots: the roots are sparse indices into pixel space,
// so indexing a per-region array by one would need a table as large as the
// image. The compaction happens once, when the segmentation is built.
@group(0) @binding(1) var<storage, read> labels: array<u32>;
// One entry per region: non-zero if the region is in this mask. Small — a few
// thousand bytes — which is what makes changing a selection cheap.
@group(0) @binding(2) var<storage, read> selected: array<u32>;
// The **signed distance** from one subject's boundary, in proxy pixels:
// positive inside, negative outside. A 1x1 placeholder when the layer is not a
// subject — the binding is fixed, and a second pipeline differing only in what
// it ignores would be worse than a wasted texel.
//
// A distance field rather than a finished alpha is what makes growing,
// shrinking and feathering free: each is arithmetic on this, so a slider moves
// a uniform instead of rebuilding a mask.
@group(0) @binding(3) var subject: texture_2d<f32>;
// TRACES: FR-DEV-10
// The photograph itself, as the demosaicer left it: camera RGB, unbalanced,
// with no edit applied. A 1x1 placeholder for every mask that is a shape,
// because the bindings are fixed and a second pipeline differing only in what
// it ignores would cost more than one texel.
//
// **The unedited image, and that is the design rather than an accident of
// pass order.** A band over the *edited* result would move as the edit was
// made: raising the highlights would change which pixels counted as
// highlights, so the slider would chase its own mask. Measuring what the
// camera recorded means the selection stays where the photographer put it
// while they work on it.
@group(0) @binding(6) var image: texture_2d<f32>;

// A full-screen triangle rather than a quad: three vertices instead of six,
// no shared edge for the rasteriser to crack along, and no vertex buffer.
@vertex
fn vs(@builtin(vertex_index) i: u32) -> @builtin(position) vec4<f32> {
    let x = f32(i32(i) / 2) * 4.0 - 1.0;
    let y = f32(i32(i) & 1) * 4.0 - 1.0;
    return vec4<f32>(x, y, 0.0, 1.0);
}

fn region_at(px: vec2<i32>) -> u32 {
    // Nearest-neighbour from output space into the label field. Deliberately
    // not bilinear: region ids are *names*, and the average of region 4 and
    // region 9 is not region 6.
    let fx = (f32(px.x) + 0.5) / f32(p.width);
    let fy = (f32(px.y) + 0.5) / f32(p.height);
    let lx = clamp(i32(fx * f32(p.label_width)), 0, i32(p.label_width) - 1);
    let ly = clamp(i32(fy * f32(p.label_height)), 0, i32(p.label_height) - 1);
    return labels[u32(ly) * p.label_width + u32(lx)];
}

fn in_selection(px: vec2<i32>) -> f32 {
    let r = region_at(px);
    if (r >= p.region_count) {
        return 0.0;
    }
    return select(0.0, 1.0, selected[r] != 0u);
}

fn region_mask(px: vec2<i32>) -> f32 {
    let hard = in_selection(px);
    if (p.feather <= 0.0) {
        return hard;
    }

    // Box-average the binary selection over the feather radius. Cheap, and it
    // is the whole reason a region mask does not look cut out with scissors:
    // the watershed boundary is pixel-exact, which is correct and also harsher
    // than any edit wants at a subject's edge.
    let r = i32(ceil(p.feather));
    var total = 0.0;
    var n = 0.0;
    for (var dy = -r; dy <= r; dy = dy + 1) {
        for (var dx = -r; dx <= r; dx = dx + 1) {
            let q = clamp(
                px + vec2<i32>(dx, dy),
                vec2<i32>(0, 0),
                vec2<i32>(i32(p.width) - 1, i32(p.height) - 1),
            );
            total = total + in_selection(q);
            n = n + 1.0;
        }
    }
    return total / n;
}

// Offset from a gradient's centre, in the frame's own **isotropic** units:
// y spans 0..1 and x spans 0..aspect, so a step of the same length means the
// same distance whichever way it points.
//
// Without this the geometry lives in raw 0..1, where one axis is compressed
// against the other by the aspect ratio — so a 45° ramp is not at 45° on
// anything but a square frame, and a radial with equal radii draws an ellipse.
// Both faults are invisible in the stored numbers and obvious the moment a
// handle is dragged on a photograph, which is what this exists for.
fn frame_delta(uv: vec2<f32>) -> vec2<f32> {
    let aspect = vec2<f32>(f32(p.width) / f32(max(p.height, 1u)), 1.0);
    return (uv - p.centre) * aspect;
}

fn linear_mask(uv: vec2<f32>) -> f32 {
    // Signed distance along the ramp direction, from the centre.
    let d = dot(frame_delta(uv), p.axis);
    if (p.softness <= 0.0) {
        return select(0.0, 1.0, d >= 0.0);
    }
    return smoothstep(-p.softness * 0.5, p.softness * 0.5, d);
}

fn radial_mask(uv: vec2<f32>) -> f32 {
    let ca = cos(-p.angle);
    let sa = sin(-p.angle);
    let d = frame_delta(uv);
    // Into the ellipse's own frame, then normalised by its semi-axes so the
    // problem becomes a unit circle.
    let local = vec2<f32>(d.x * ca - d.y * sa, d.x * sa + d.y * ca);
    let r = length(local / max(p.axis, vec2<f32>(1e-6)));

    let edge = clamp(p.softness, 0.0, 1.0);
    if (edge <= 0.0) {
        return select(0.0, 1.0, r <= 1.0);
    }
    return 1.0 - smoothstep(1.0 - edge, 1.0, r);
}

// Coverage for one object, from its distance field.
//
// Bilinear on the *distance*, which is the reason this is a distance field at
// all: distance varies smoothly across the boundary where coverage does not,
// so interpolating it gives a clean sub-pixel edge even though the model's
// own mask was quarter-resolution.
fn subject_mask(uv: vec2<f32>) -> f32 {
    let dims = vec2<f32>(textureDimensions(subject));
    let last = vec2<i32>(dims) - vec2<i32>(1);

    let t = uv * dims - vec2<f32>(0.5);
    let base = vec2<i32>(floor(t));
    let f = fract(t);

    let p0 = clamp(base, vec2<i32>(0), last);
    let p1 = clamp(base + vec2<i32>(1), vec2<i32>(0), last);

    let a = textureLoad(subject, vec2<i32>(p0.x, p0.y), 0).r;
    let b = textureLoad(subject, vec2<i32>(p1.x, p0.y), 0).r;
    let c = textureLoad(subject, vec2<i32>(p0.x, p1.y), 0).r;
    let d = textureLoad(subject, vec2<i32>(p1.x, p1.y), 0).r;

    // `angle` carries the morphology offset in pixels: positive grows the
    // mask, negative shrinks it. Adding it before the falloff is what makes
    // dilation move the boundary rather than merely brighten the edge.
    let dist = mix(mix(a, b, f.x), mix(c, d, f.x), f.y) + p.angle;

    // `softness` is the feather half-width, also in pixels.
    if (p.softness <= 0.0) {
        return select(0.0, 1.0, dist >= 0.0);
    }
    let t_norm = dist / p.softness;

    // Every curve is 0.5 at the boundary, so changing the falloff changes how
    // the transition looks and never where it sits.
    switch p.falloff {
        case 0u: { return select(0.0, 1.0, dist >= 0.0); }
        case 1u: { return clamp(t_norm * 0.5 + 0.5, 0.0, 1.0); }
        case 3u: { return 1.0 / (1.0 + exp(-3.0 * t_norm)); }
        case 4u: {
            if (t_norm >= 0.0) {
                return 1.0 - 0.5 * exp(-3.0 * t_norm);
            }
            return 0.5 * exp(3.0 * t_norm);
        }
        default: {
            let x = clamp(t_norm * 0.5 + 0.5, 0.0, 1.0);
            return x * x * (3.0 - 2.0 * x);
        }
    }
}

// ---------------------------------------------------------------------------
// Range masks (FR-DEV-10)
// ---------------------------------------------------------------------------
//
// The masks that select by what a pixel *is* rather than by where it sits.
// Nothing below reads `frame_delta`, and that absence is the point: a range is
// not a function of position, so it cannot be stretched by an aspect ratio,
// cannot drift under a crop, and comes out the same at a proxy size and at an
// export because the only thing it depends on is the photograph's own values.
//
// The band arrives entirely in the fields the gradients use — `axis` is the
// pair of bounds, `centre` is a colour range's arc, `softness` is the fade —
// so a range costs nothing in the uniform beyond the image transform above.

// Display-encoded sRGB back to linear.
//
// A JPEG is uploaded with its bytes untouched, so its values are gamma-encoded
// where the demosaicer's are linear. The same undoing the generated adjust
// shader does, at the same point and for the same reason: a band over
// brightness is meaningless if two sources disagree about what a value means.
fn decode_srgb(c: vec3<f32>) -> vec3<f32> {
    let lo = c / 12.92;
    let hi = pow((max(c, vec3<f32>(0.04045)) + 0.055) / 1.055, vec3<f32>(2.4));
    return select(hi, lo, c <= vec3<f32>(0.04045));
}

// One source texel, as linear sRGB.
//
// This is the prologue of the generated adjust shader, repeated: decode,
// balance, pull a clipped pixel back to neutral, then the camera matrix. It is
// repeated rather than shared because the composer emits WGSL for the *edit*
// and this pass is not one — but it must agree with it, since a range mask
// exists to select the values the layer's own adjustments will then see.
//
// The highlight desaturation is the part that looks skippable and is not. A
// fully clipped photosite arrives as (1,1,1), carrying no colour at all; the
// as-shot multipliers are far from neutral, so balancing it and passing it
// through the matrix produces a strong magenta. A colour range would then
// select every blown sky as if the photographer had asked for magenta.
fn source_texel(px: vec2<i32>) -> vec3<f32> {
    var c = textureLoad(image, px, 0).rgb;
    if (p.as_shot_wb.w > 0.5) {
        c = decode_srgb(c);
    }

    let clipped = smoothstep(0.985, 1.0, max(c.r, max(c.g, c.b)));
    c = c * p.as_shot_wb.rgb;
    if (clipped > 0.0) {
        c = mix(c, vec3<f32>(max(c.r, max(c.g, c.b))), clipped);
    }

    return vec3<f32>(
        dot(p.cam_to_srgb_0.rgb, c),
        dot(p.cam_to_srgb_1.rgb, c),
        dot(p.cam_to_srgb_2.rgb, c),
    );
}

// The most taps one mask texel averages, per axis.
//
// A cap rather than the true footprint. At a 1600 px proxy over a 24 MP frame
// the ratio is under four, so this is the whole footprint for every ordinary
// photograph; past it the taps stride across the footprint instead of
// covering it, which is a sample of the area rather than its mean. That is the
// right way to run out of budget here — the estimate gets noisier, it does not
// start measuring somewhere else.
const MAX_SOURCE_TAPS: i32 = 4;

// The photograph's value under one mask texel, in linear sRGB.
fn image_value(px: vec2<i32>) -> vec3<f32> {
    let dims = vec2<i32>(textureDimensions(image));
    let last = dims - vec2<i32>(1);

    // The footprint's top-left corner in source texels. Not a centre plus a
    // radius: the mask texel is a *box* over the source, and sampling
    // symmetrically about its centre would weight the middle of every
    // footprint twice at odd tap counts.
    let origin = vec2<f32>(px) * p.source_step;
    let taps = clamp(vec2<i32>(ceil(p.source_step)), vec2<i32>(1), vec2<i32>(MAX_SOURCE_TAPS));
    // `stride`, not `step`: WGSL has a builtin of that name, and a local that
    // shadows one is legal and unreadable in the same breath.
    let stride = p.source_step / vec2<f32>(taps);

    var total = vec3<f32>(0.0);
    for (var y = 0; y < taps.y; y = y + 1) {
        for (var x = 0; x < taps.x; x = x + 1) {
            let at = origin + (vec2<f32>(f32(x), f32(y)) + vec2<f32>(0.5)) * stride;
            total = total + source_texel(clamp(vec2<i32>(at), vec2<i32>(0), last));
        }
    }
    return total / f32(taps.x * taps.y);
}

// A soft band: one inside, nothing outside, a smooth ramp across each edge.
//
// The `min` rather than a product of the two ramps. A band narrower than twice
// its softness has no plateau, and multiplying the rising and falling ramps
// would then peak well below one — so "select the highlights" would come out
// at sixty per cent and the photographer would compensate with opacity,
// against a mask that was quietly weaker than it said. `min` keeps the
// plateau where there is one and degrades to a single peak where there is not.
fn band(v: f32, lo: f32, hi: f32, soft: f32) -> f32 {
    if (soft <= 0.0) {
        return select(0.0, 1.0, v >= lo && v <= hi);
    }
    return min(smoothstep(lo - soft, lo, v), 1.0 - smoothstep(hi, hi + soft, v));
}

fn luminance_mask(px: vec2<i32>) -> f32 {
    let y = dot(image_value(px), vec3<f32>(0.2126, 0.7152, 0.0722));
    // Onto the perceptual position `tone_position` in `ops/_helpers.yaml`
    // establishes, which is where the stored bounds are measured. Linear light
    // puts middle grey at 0.18, so a band stated in it would spend four fifths
    // of its travel inside the shadows.
    let t = clamp(pow(max(y, 0.0), 1.0 / 3.0), 0.0, 1.0);
    return band(t, p.axis.x, p.axis.y, p.softness);
}

// Hue in turns, 0 at red and increasing through yellow.
//
// The plain six-sector definition. Zero for a neutral, which is a value the
// caller must not act on — the chroma bound below is what keeps a colour range
// away from the greys where this number is rounding noise.
fn hue_of(c: vec3<f32>) -> f32 {
    let hi = max(c.r, max(c.g, c.b));
    let lo = min(c.r, min(c.g, c.b));
    let d = hi - lo;
    if (d <= 0.0) {
        return 0.0;
    }
    var h = 0.0;
    if (hi == c.r) {
        h = (c.g - c.b) / d;
    } else if (hi == c.g) {
        h = (c.b - c.r) / d + 2.0;
    } else {
        h = (c.r - c.g) / d + 4.0;
    }
    return fract(h / 6.0);
}

fn colour_mask(px: vec2<i32>) -> f32 {
    let c = max(image_value(px), vec3<f32>(0.0));
    let hi = max(c.r, max(c.g, c.b));
    let lo = min(c.r, min(c.g, c.b));
    // The max-minus-min chroma `colour_saturation` uses, so the number the
    // band is stated in is the one the rest of the pipeline means by
    // 'colourfulness'.
    var chroma = 0.0;
    if (hi > 0.0) {
        chroma = (hi - lo) / hi;
    }

    // Distance round the circle, so an arc centred near red reaches both ways
    // past zero. Written as a wrap rather than as two comparisons because red
    // is exactly where skin sits, and an arc that stopped at the seam would
    // select half of it.
    let d = abs(fract(hue_of(c) - p.centre.x + 0.5) - 0.5);
    var arc = 0.0;
    if (p.softness <= 0.0) {
        arc = select(0.0, 1.0, d <= p.centre.y);
    } else {
        arc = 1.0 - smoothstep(p.centre.y, p.centre.y + p.softness, d);
    }

    // Both, not either: an arc alone selects a haze of noise everywhere the
    // picture is nearly grey, because a hue rounded out of three almost-equal
    // channels is still a hue.
    return min(arc, band(chroma, p.axis.x, p.axis.y, p.softness));
}

@fragment
fn fs(@builtin(position) pos: vec4<f32>) -> @location(0) vec4<f32> {
    let px = vec2<i32>(i32(pos.x), i32(pos.y));

    // Normalised, so a gradient's geometry survives a crop or an export at
    // another size — the mask is defined on the frame, not on a pixel count.
    let uv = vec2<f32>(pos.x / f32(p.width), pos.y / f32(p.height));

    var m = 0.0;
    switch p.mode {
        case 0u: { m = region_mask(px); }
        case 1u: { m = linear_mask(uv); }
        case 2u: { m = radial_mask(uv); }
        case 3u: { m = subject_mask(uv); }
        case 5u: { m = luminance_mask(px); }
        case 6u: { m = colour_mask(px); }
        default: { m = 0.0; }
    }

    return vec4<f32>(clamp(m, 0.0, 1.0), 0.0, 0.0, 1.0);
}

// ---------------------------------------------------------------------------
// Brush strokes (ARCH §5.4)
// ---------------------------------------------------------------------------
//
// The mask the architecture was written for. What arrives is a list of
// positions, a radius, a hardness and a flow; what leaves is pixels. Nothing
// between the two ever exists in CPU memory, which is the whole difference from
// darktable, where the same strokes are rasterised on the CPU and the lag makes
// painting unusable.
//
// # Why the strokes are not drawn by the full-screen triangle above
//
// Cost. A swept disc is the minimum distance to any segment of its polyline, so
// evaluating one stroke costs a distance per segment *per pixel*. Over the
// whole frame that is `pixels × segments`, and a stroke that wandered across
// the photograph has both terms large at once.
//
// So each stroke is drawn over its own bounding box instead, expanded by the
// radius. The rasteriser then never invokes the fragment shader for a pixel the
// stroke cannot reach, and the cost becomes `area(box) × segments` — for the
// ordinary case, a dab or a swipe, a small fraction of the frame. The model
// splits a long gesture into strokes of bounded length for the same reason:
// both terms of that product grow with how far one stroke travelled.
//
// # Why the strokes composite with fixed-function blending
//
// Add is `dst + a(1 - dst)` and erase is `dst(1 - a)`, which are exactly a
// source-over and a one-minus-source blend. Expressing them as blend state
// rather than as arithmetic in the shader is what allows one draw per stroke:
// the accumulating mask is the attachment, and no pass ever has to read the
// slice it is writing.

struct StrokeHeader {
    // Bounding box in normalised coordinates, already grown by the radius and
    // a texel — the vertex shader trusts it and draws nothing outside it.
    lo: vec2<f32>,
    hi: vec2<f32>,
    // Radius in units of the frame's shorter edge, so a dab is round on a frame
    // that is not square.
    radius: f32,
    // Fraction of the radius that is fully covered.
    hardness: f32,
    // Coverage deposited where the stroke is solid.
    flow: f32,
    // Window into `stroke_points`.
    first: u32,
    count: u32,
    _pad: u32,
}

@group(0) @binding(4) var<storage, read> strokes: array<StrokeHeader>;
@group(0) @binding(5) var<storage, read> stroke_points: array<vec2<f32>>;

struct BrushVertex {
    @builtin(position) pos: vec4<f32>,
    // Flat: a stroke index interpolated across its own quad would name a
    // different stroke in the middle of it.
    @location(0) @interpolate(flat) stroke: u32,
}

// Six vertices per stroke, non-instanced.
//
// Deliberately not one instance per stroke: `@builtin(instance_index)` with a
// non-zero first instance needs base-instance support, which the GL backend
// this has to run on under Android cannot promise. Dividing the vertex index
// costs one integer operation and works everywhere.
@vertex
fn vs_brush(@builtin(vertex_index) v: u32) -> BrushVertex {
    var quad = array<vec2<f32>, 6>(
        vec2<f32>(0.0, 0.0), vec2<f32>(1.0, 0.0), vec2<f32>(0.0, 1.0),
        vec2<f32>(0.0, 1.0), vec2<f32>(1.0, 0.0), vec2<f32>(1.0, 1.0),
    );

    let i = v / 6u;
    let s = strokes[i];
    let uv = mix(s.lo, s.hi, quad[v % 6u]);

    var out: BrushVertex;
    // y is flipped because normalised mask coordinates run downwards, the way
    // the fragment shader above reads them, and clip space runs upwards. A
    // stroke drawn without this lands mirrored about the horizon, which is
    // plausible enough on a symmetric test image to survive a careless check.
    out.pos = vec4<f32>(uv.x * 2.0 - 1.0, 1.0 - uv.y * 2.0, 0.0, 1.0);
    out.stroke = i;
    return out;
}

// Into units of the frame's shorter edge.
//
// Without this the brush would be a circle in normalised coordinates, which on
// a 3:2 frame is an ellipse half again as wide as it is tall. A brush whose dab
// is not round is not a brush.
fn to_square(uv: vec2<f32>) -> vec2<f32> {
    let dims = vec2<f32>(f32(p.width), f32(p.height));
    return uv * dims / min(dims.x, dims.y);
}

fn segment_distance(q: vec2<f32>, a: vec2<f32>, b: vec2<f32>) -> f32 {
    let ab = b - a;
    let len2 = dot(ab, ab);
    // A finger that stopped and went back leaves a zero-length segment, and
    // dividing by its length is a NaN — which propagates through the min()
    // below and takes the whole stroke with it.
    if (len2 <= 1e-12) {
        return length(q - a);
    }
    let t = clamp(dot(q - a, ab) / len2, 0.0, 1.0);
    return length(q - (a + ab * t));
}

@fragment
fn fs_brush(in: BrushVertex) -> @location(0) vec4<f32> {
    let s = strokes[in.stroke];
    let q = to_square(vec2<f32>(in.pos.x / f32(p.width), in.pos.y / f32(p.height)));

    // The *minimum* over the segments, which is the maximum of their coverage.
    // Accumulating the segments instead would make a stroke that crosses itself
    // — every circle, every scribble — build up a bright patch where it did,
    // and a soft brush would go blotchy along any curve tight enough for
    // consecutive dabs to overlap, which is all of them.
    var d = 1e30;
    if (s.count == 1u) {
        // A tap. One point is a legitimate stroke, and it paints one dab.
        d = length(q - to_square(stroke_points[s.first]));
    } else {
        for (var k = 0u; k + 1u < s.count; k = k + 1u) {
            d = min(
                d,
                segment_distance(
                    q,
                    to_square(stroke_points[s.first + k]),
                    to_square(stroke_points[s.first + k + 1u]),
                ),
            );
        }
    }

    // Even at full hardness the edge keeps a one-pixel ramp. A true step would
    // alias into a staircase, and the mask is sampled bilinearly at whatever
    // zoom the user is inspecting it at — which is where an edge is judged.
    let texel = 1.0 / f32(min(p.width, p.height));
    let inner = min(s.radius * clamp(s.hardness, 0.0, 1.0), max(s.radius - texel, 0.0));
    let coverage = 1.0 - smoothstep(inner, s.radius, d);

    return vec4<f32>(clamp(coverage * s.flow, 0.0, 1.0), 0.0, 0.0, 1.0);
}

// ---------------------------------------------------------------------------
// Joining one part to the mask so far
// ---------------------------------------------------------------------------
//
// A layer's mask is a fold over its parts, and the set operation is the *blend
// state* rather than arithmetic here: union is `max(dst, src)`, subtraction is
// `dst * (1 - src)`. Both are fixed-function, so joining a part costs one
// full-screen draw and no second texture beyond the one being read.
//
// # Why a part is drawn aside first, rather than straight onto the mask
//
// Because an erase stroke inside a part means "a hole in *this* part", not "a
// hole in the mask". Painted straight onto the accumulator it would take away
// whatever the parts before it had put there — so a correction that tidied its
// own edge would punch through the subject underneath, and the failure would
// look like the model's mask had holes in it.

@group(0) @binding(7) var part_mask: texture_2d<f32>;

@fragment
fn fs_combine(@builtin(position) pos: vec4<f32>) -> @location(0) vec4<f32> {
    let v = textureLoad(part_mask, vec2<i32>(i32(pos.x), i32(pos.y)), 0).r;
    let m = select(v, 1.0 - v, p.invert != 0u);
    return vec4<f32>(clamp(m, 0.0, 1.0), 0.0, 0.0, 1.0);
}