M3: glass's light is a line on its bevel, and splay turns the bend towards the centre (#1888)

* M3: glass bends with the reference tool's thick-lens profile and splits its colours over 8 steps (#1885)

The backdrop bends most a 1-unit bevel in from the edge, falls off as the
3.5th power of the way out of the curve to nothing `depth` further in, and
moves a fifth as much on the bevel itself (it fell off as the square of the
way in from the very edge). Dispersion samples each channel at 8 steps along
the bend with a triangular window, red moving furthest and blue least (it was
three samples, blue furthest), as far apart as the reference tool's: at 100%
red and blue move 0.16 of the bend apart (`Glass::DISPERSION_SPREAD`).
`Glass::max_bend` bounds red's furthest step, so the backdrop read around a
pane covers every sample, and the field margin covers the bevel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* M3: glass's bend stays within `max_bend`, checked on a single bright pixel too (#1885)

Review follow-up: the bend reaches `depth` × `refraction` a bevel in from
the edge, so `Glass::max_bend` drops its 0.999 and bounds it, as a test now
checks for several refractions. A single white pixel on black pins the
dispersion's triangular window, which a ramp and a flat backdrop can't tell
from one centred sample. The parity assessment records the bend, the
dispersion and how they were measured against the reference tool.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* M3: glass's light is a line on its bevel, and splay turns the bend towards the centre (#1885)

The light is a line on the 1-unit bevel added to the backdrop, as the
reference tool's: fully where the edge faces the light, none below 0.35 of
facing it and 0.6 as much on the opposite side, with a faint gradient across
the curve, 5% darker facing the light and lighter opposite. It was a glow
mixed towards white, as deep as `splay` made it, so `light_reach` goes.

`splay` turns the bend from along the edge's normal towards the way away
from the layer's centre, and no longer widens the light: files with a splay
above 0 look different. The direction to the centre is worked out in the
shape's own frame (`own_frame`, shared with the field cache), so cut renders
match the whole one byte for byte.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* M3: cut renders of glass match with splay turning a dispersed bend (#1885)

Review follow-up: the cut-versus-whole test had splay only on looks without
refraction, so no turned bend reached the dispersion. A look with splay 1 and
the default bend and dispersion now renders the same however it is cut. The
parity assessment records the light on the bevel and splay's new meaning.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* M3: glass's flat inside skips the bend's curve, and full dispersion stays within `max_bend` (#1885)

Review follow-up: `bend_at` returns 0 straight away past `depth` + the
bevel, where the curve and the bevel's easing are both 0, instead of
working out a power of 0 for every pixel of a pane's flat inside. A test
moves one white pixel along a row under every bend of the profile at full
dispersion and checks that no channel ever reads it further out than
`Glass::max_bend`, which sizes the backdrop read around a pane.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* M3: glass's light shows over a transparent backdrop, and its flat inside skips the light (#1885)

Review follow-up: the light was added to the backdrop's colour and then
clamped to the backdrop's alpha, so with nothing under the glass (an
export of the pane alone) the line on the bevel vanished, where the old
glow raised the alpha and showed. Now the alpha rises as far as the lit
colour needs (`lit`), and stays the backdrop's wherever the light fits
under it, so renders over an opaque backdrop don't change.

Past `depth` + the bevel the rim light, the gradient and the bend are all
0, so those pixels take one sample of the backdrop and skip the light's
direction, facing and splay work, as the old glow's `reach` early-out did.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Oleksii Sukholita <oleksii.sukholita@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: FredApps <153173854+FredApps@users.noreply.github.com>

crates/doc/src/props.rs

@@ -662,7 +662,8 @@ pub struct Glass {
     pub dispersion: f64,
     /// Blur of the backdrop (a blur radius, document units, 0–[`Self::MAX_FROST`]).
     pub frost: f64,
-    /// How widely the light spreads over the glass (0–1).
+    /// Which way the curved edge bends the backdrop: along the edge's normal (0) to away from the
+    /// layer's centre (1).
     pub splay: f64,
 }
 

crates/render/src/backdrop_tests.rs

@@ -254,16 +254,65 @@ fn glass_edges_bend_split_and_catch_the_light() {
     let (rim, middle) = changed(&split, &bent);
     assert!(rim > 0 && middle == 0, "dispersion: rim {rim}, middle {middle}");
 
-    // The light (from the top left by default) brightens the top-left rim more than the
-    // bottom-right one, and only near the edge.
+    // The light (from the top left by default) is a line on the 1-unit bevel: the top-left corner
+    // catches it all, the bottom-right one (facing away) 0.6 as much, the top side (45° from it)
+    // 0.574 and the bottom side 0.6 × 0.574, and the top-right corner (side on) none. Further in
+    // only the faint gradient across the curve shows, and nothing past `depth`.
     let (d, f, _) = glass_scene(Glass { light_intensity: 1.0, ..plain });
     let lit = render_node(&d, f, 1.0).unwrap();
     let gain = |x: u32, y: u32| i32::from(px(&lit, x, y)[1]) - i32::from(px(&flat, x, y)[1]);
-    assert!(gain(82, 42) > gain(197, 117), "{} vs {}", gain(82, 42), gain(197, 117));
-    assert!(gain(82, 42) > 40, "{}", gain(82, 42));
+    let (top_left, bottom_right, top, bottom, top_right) = (gain(80, 40), gain(199, 119), gain(140, 40), gain(140, 119), gain(199, 40));
+    assert!(
+        top_left > bottom_right && bottom_right > top && top > bottom && bottom > top_right.abs(),
+        "{top_left} {bottom_right} {top} {bottom} {top_right}"
+    );
+    assert!(top_left > 120 && top_right.abs() <= 2, "{top_left} {top_right}");
+    assert!(gain(140, 44).abs() <= 13, "4 px in: the gradient only, {}", gain(140, 44));
     assert_eq!(gain(140, 80), 0);
 }
 
+#[test]
+fn splay_turns_the_bend_towards_the_centre() {
+    // Refraction only, so the bend alone moves the stripes' edges.
+    let bend = Glass { frost: 0.0, refraction: 1.0, dispersion: 0.0, light_intensity: 0.0, depth: 16.0, ..Glass::default() };
+    let (d, f, _) = glass_scene(bend.clone());
+    let edge = render_node(&d, f, 1.0).unwrap();
+    let (d, f, _) = glass_scene(Glass { splay: 1.0, ..bend });
+    let centre = render_node(&d, f, 1.0).unwrap();
+    let diff = |xs: std::ops::Range<u32>, ys: std::ops::Range<u32>| {
+        let mut most = 0;
+        for y in ys {
+            for x in xs.clone() {
+                most = most.max(px(&edge, x, y).iter().zip(px(&centre, x, y)).map(|(a, b)| a.abs_diff(b)).max().unwrap());
+            }
+        }
+        most
+    };
+    // Halfway along the left side the edge's normal and the way from the centre (140, 80) agree:
+    // nearly the same pixels. In the top-left corner they part, and so does what shows there.
+    assert!(diff(80..96, 78..82) <= 8, "mid-left: {}", diff(80..96, 78..82));
+    assert!(diff(80..96, 40..56) > 40, "corner: {}", diff(80..96, 40..56));
+}
+
+/// Glass with nothing under it (an export of the pane alone): the light on the bevel still shows,
+/// the alpha rising with it, and the flat inside and the outside stay clear.
+#[test]
+fn glass_over_nothing_still_catches_the_light() {
+    let mut d = Document::new("glass");
+    let p = d.first_page().unwrap();
+    let pane = d.create(p, NodeKind::Rectangle, "Glass", 40.0, 30.0, 120.0, 80.0).unwrap();
+    let n = d.get_mut(pane).unwrap();
+    n.fills.clear();
+    n.effects = vec![Effect { kind: EffectKind::Glass(Glass { light_intensity: 1.0, ..Glass::default() }), visible: true }];
+    let out = render_area(&d, &[pane], Rect::new(0.0, 0.0, 200.0, 140.0), 1.0).unwrap();
+    // The top-left corner faces the light (from the top left): white, as opaque as it is bright.
+    let corner = px(&out, 40, 30);
+    assert!(corner[3] > 100 && corner.iter().all(|v| v.abs_diff(corner[3]) <= 2), "{corner:?}");
+    assert!(out.data().chunks_exact(4).all(|q| q[0] <= q[3] && q[1] <= q[3] && q[2] <= q[3]), "premultiplied");
+    assert_eq!(px(&out, 100, 70), [0; 4]);
+    assert_eq!(px(&out, 10, 10), [0; 4]);
+}
+
 #[test]
 fn glass_and_backdrop_blur_survive_bands_and_partial_renders() {
     let (d, _, _) = glass_scene(Glass::default());

crates/render/src/glass.rs

@@ -3,10 +3,12 @@
 //! Within `depth` of the outline the surface curves like a thick lens: the backdrop there is bent
 //! towards the inside, most a [`BEVEL`] in from the edge (`refraction` × `depth` far), falling off
 //! as the 3.5th power of the way out of the curve and gone `depth` further in, with a fifth as
-//! much on the bevel itself ([`bend_at`]). Each colour channel bends by a different amount
-//! (`dispersion`, a rainbow fringe: red furthest, blue least, each smeared along the bend), and the
-//! rim catches the light: brightest where it faces `light_angle`, half as bright on the opposite
-//! side, spread wider by `splay`.
+//! much on the bevel itself ([`bend_at`]). `splay` turns which way it bends, from along the edge's
+//! normal (0) to away from the layer's centre (1). Each colour channel bends by a different amount
+//! (`dispersion`, a rainbow fringe: red furthest, blue least, each smeared along the bend). The
+//! bevel catches the light as a thin line added to the backdrop ([`rim_light_at`]): fully where the
+//! edge faces `light_angle`, none below 0.35 of facing it, 0.6 as much on the opposite side; and a
+//! faint gradient runs across the curve, 5% darker where it faces the light and lighter opposite.
 //! `frost` blurs the backdrop first. Distances to the outline come from an exact Euclidean
 //! distance transform of the shape's coverage.
 //!
@@ -181,11 +183,45 @@ const RIM_BEND: f32 = 0.2;
 /// by a triangle (the two end steps weigh nothing and are skipped).
 const DISPERSION_STEPS: usize = 8;
 
+/// The least an edge faces the light (the cosine between its outward normal and the light) and
+/// still catches some.
+const HL_LO: f32 = 0.35;
+
+/// How much of the light the side opposite it catches.
+const HL_SECONDARY: f32 = 0.6;
+
+/// The gradient across the curve at its strongest, a share of the light's intensity.
+const BROAD: f32 = 0.05;
+
 /// `x` from `a` to `b` as 0 to 1, clamped (a span under 1e-5 counts as 1e-5).
 fn lin(a: f32, b: f32, x: f32) -> f32 {
     ((x - a) / (b - a).max(1e-5)).clamp(0.0, 1.0)
 }
 
+/// `x` from `a` to `b` as a smooth 0-to-1 step (Hermite), clamped.
+fn smoothstep(a: f32, b: f32, x: f32) -> f32 {
+    let t = ((x - a) / (b - a).max(1e-5)).clamp(0.0, 1.0);
+    t * t * (3.0 - 2.0 * t)
+}
+
+/// How much of the rim light falls `x` pixels in from the edge, for a `bevel` pixels deep: all
+/// of it within a fifth of the bevel, none a bevel in.
+pub(crate) fn rim_light_at(x: f32, bevel: f32) -> f32 {
+    smoothstep(-bevel - 0.001, -0.2 * bevel, -x)
+}
+
+/// How much light an edge catches, `facing` being the cosine between its outward normal and the
+/// light: from [`HL_LO`] to fully facing it, and [`HL_SECONDARY`] as much facing away.
+fn catches(facing: f32) -> f32 {
+    smoothstep(HL_LO, 1.0, facing) + HL_SECONDARY * smoothstep(HL_LO, 1.0, -facing)
+}
+
+/// How strongly the gradient across the curve shows `x` pixels in from the edge (`depth` and
+/// `bevel` in pixels): fully on the bevel, falling as the square of the way in to none at `depth`.
+pub(crate) fn sheen_at(x: f32, depth: f32, bevel: f32) -> f32 {
+    lin(-depth, -bevel + 1e-4, -x).powi(2)
+}
+
 /// How far the backdrop moves at `x` pixels in from the edge, for a glass `depth` and [`BEVEL`]
 /// `bevel` deep (pixels) and `refraction` (0–1): a thick lens's profile, the most a bevel in from
 /// the edge, falling off as the 3.5th power of the way out of the curve to nothing `depth` further
@@ -229,6 +265,17 @@ fn dispersed(frosted: &Pixmap, x: f32, y: f32, dx: f32, dy: f32, k: f32) -> [f32
     [(acc[0] / sum).min(a), (acc[1] / sum).min(a), (acc[2] / sum).min(a), a]
 }
 
+/// `base`, a premultiplied backdrop pixel (channels 0–255), with `add` (levels, below 0 darkens)
+/// added to its colour, at coverage `c`. Where that is more than the backdrop's alpha holds (all of
+/// it over a transparent backdrop, an export with nothing under the glass), the alpha rises with
+/// it, so the light shows there too.
+fn lit(base: [f32; 4], add: f32, c: f32) -> [u8; 4] {
+    let [r, g, b, a] = base;
+    let (r, g, b) = (r + add, g + add, b + add);
+    let a = a.max(0.0).max(r).max(g).max(b).min(255.0);
+    [r, g, b, a].map(|v| (v.clamp(0.0, a) * c).round().clamp(0.0, 255.0) as u8)
+}
+
 /// Bilinear sample of premultiplied `p` at pixel coordinates `(x, y)` (centres at +0.5), each
 /// channel 0..=255; transparent outside.
 fn sample(p: &Pixmap, x: f32, y: f32) -> [f32; 4] {
@@ -256,17 +303,6 @@ fn sample(p: &Pixmap, x: f32, y: f32) -> [f32; 4] {
     })
 }
 
-/// The furthest in (pixels) the rim light is worth half a level or more: where
-/// `255 × intensity × e^(−d / band)` falls to 0.5, at most [`MAX_LIGHT_PX`] (0 for no light).
-fn light_reach(band: f64, intensity: f64) -> f64 {
-    let k = 255.0 * intensity / 0.5;
-    if k > 1.0 && band.is_finite() { (band * k.ln()).min(MAX_LIGHT_PX) } else { 0.0 }
-}
-
-/// The light reaches no further in than this many pixels (a wide light on a deep pane at a high
-/// zoom would otherwise need a huge margin of coverage around every cut).
-const MAX_LIGHT_PX: f64 = 512.0;
-
 /// A shape's coverage (alpha), distance to its edge and smoothed inward normals over a window of
 /// pixels whose top-left pixel is (`x0`, `y0`).
 struct Fields {
@@ -420,9 +456,19 @@ fn shape_fields(shape: &Outline, xf: Affine, w: u32, h: u32, m: u32, r: usize) -
     Some((Arc::new(Fields::new(&cov, -i64::from(m), -i64::from(m), r)), (0, 0)))
 }
 
-/// [`shape_fields`] for a whole shape, from the cache; `None` when it isn't kept (first asked for
-/// in this render, too big, or its transform too far out).
-fn whole_shape(shape: &Outline, xf: Affine, m: u32, r: usize) -> Option<(Arc<Fields>, (i64, i64))> {
+/// A layer transform split so the same shape at the same zoom lands on the same pixels of its own
+/// frame wherever a render puts it or cuts it (see [`own_frame`]).
+struct OwnFrame {
+    /// The whole pixels of the translation: the shift from layer pixels to the shape's own frame.
+    shift: (i64, i64),
+    /// The translation's fraction of a pixel, in steps of 1/65536.
+    frac: (i64, i64),
+    /// Local coordinates → the shape's own frame: the linear part and that fraction.
+    xf: Affine,
+}
+
+/// `xf` (local → layer pixels) split into its [`OwnFrame`]; `None` for a translation too far out.
+fn own_frame(xf: Affine) -> Option<OwnFrame> {
     let [a, b, c, d, e, f] = xf.as_coeffs();
     // The translation in steps of 1/65536 px (far finer than the rasteriser's 1/64, coarse enough
     // that rounding errors in where a render starts land on the same step), its whole pixels (the
@@ -435,7 +481,14 @@ fn whole_shape(shape: &Outline, xf: Affine, m: u32, r: usize) -> Option<(Arc<Fie
     let (qe, qf) = (q(e)?, q(f)?);
     let shift = (qe.div_euclid(STEPS as i64), qf.div_euclid(STEPS as i64));
     let frac = (qe.rem_euclid(STEPS as i64), qf.rem_euclid(STEPS as i64));
-    let own = Affine::new([a, b, c, d, frac.0 as f64 / STEPS, frac.1 as f64 / STEPS]);
+    Some(OwnFrame { shift, frac, xf: Affine::new([a, b, c, d, frac.0 as f64 / STEPS, frac.1 as f64 / STEPS]) })
+}
+
+/// [`shape_fields`] for a whole shape, from the cache; `None` when it isn't kept (first asked for
+/// in this render, too big, or its transform too far out).
+fn whole_shape(shape: &Outline, xf: Affine, m: u32, r: usize) -> Option<(Arc<Fields>, (i64, i64))> {
+    let [a, b, c, d, _, _] = xf.as_coeffs();
+    let OwnFrame { shift, frac, xf: own } = own_frame(xf)?;
     let bb = (own * shape.0.clone()).bounding_box();
     let (x0, y0, x1, y1) = (bb.x0.floor(), bb.y0.floor(), bb.x1.ceil(), bb.y1.ceil());
     if ![x0, y0, x1, y1].iter().all(|v| v.is_finite() && v.abs() < 1e9) {
@@ -479,27 +532,25 @@ pub fn glass(g: &Glass, frame: EffectFrame<'_>) -> Option<Pixmap> {
     let g = g.sanitized();
     let depth = (g.depth * scale).clamp(1.0, 1024.0);
     let bevel = (BEVEL * scale).min(1024.0);
-    let band = (depth * (0.15 + 0.35 * g.splay)).max(1.0);
-    let reach = light_reach(band, g.light_intensity);
     let r = (depth * 0.1).clamp(1.0, 8.0) as usize;
     // Coverage with a margin, so wherever the layer is cut every distance the pixels read is
-    // exact: the bend's (within `depth` + `bevel`), the light's (within `reach`) and those the
-    // normals are smoothed from (2r + 1 further).
-    let m = (depth + bevel).max(reach).ceil() as u32 + 4 * r as u32 + 4;
+    // exact: the bend's and the gradient's (within `depth` + `bevel`; the light lies on the bevel)
+    // and those the normals are smoothed from (2r + 1 further).
+    let m = (depth + bevel).ceil() as u32 + 4 * r as u32 + 4;
     let (fields, shift) = shape_fields(shape, xf, w, h, m, r)?;
     let mut frosted = back.clone();
     fx::blur(&mut frosted, g.frost / 2.0 * scale);
     let ang = g.light_angle.to_radians();
     let light = (ang.sin() as f32, -ang.cos() as f32);
+    // The layer's centre in its own frame (see `own_frame`), which `splay` bends away from: the
+    // direction to it is the same wherever a render puts or cuts the layer.
+    let local_centre = shape.0.bounding_box().center();
+    let ((sx, sy), centre) = match own_frame(xf) {
+        Some(own) => (own.shift, own.xf * local_centre),
+        None => ((0, 0), xf * local_centre),
+    };
     let (refraction, dispersion, bevel) = (g.refraction as f32, (g.dispersion * Glass::DISPERSION_SPREAD) as f32, bevel as f32);
-    let power = (6.0 - 5.0 * g.splay) as f32;
-    let (band, reach) = (band as f32, reach as f32);
-    // The falloff, lowered by its value at `reach` and scaled back to 1 at the edge, so the light
-    // fades out to 0 at `reach` instead of stopping there.
-    let floor = (-reach / band).exp();
-    let fall = 1.0 - floor;
-    let intensity = g.light_intensity as f32;
-    let depth = depth as f32;
+    let (splay, intensity, depth) = (g.splay as f32, g.light_intensity as f32, depth as f32);
     let mut out = Pixmap::new(w, h)?;
     let dst = out.data_mut();
     for y in 0..h as usize {
@@ -508,30 +559,37 @@ pub fn glass(g: &Glass, frame: EffectFrame<'_>) -> Option<Pixmap> {
             if c <= 0.0 {
                 continue;
             }
-            let bend = bend_at(d, depth, bevel, refraction);
             let (bx, by) = (x as f32 + pad as f32 + 0.5, y as f32 + pad as f32 + 0.5);
-            let base = if bend == 0.0 {
-                // Nothing bends (past `depth` + `bevel`, or no refraction): the channels' samples
-                // are at the same point, so one sample.
-                let s = sample(&frosted, bx, by);
-                [s[0].min(s[3]), s[1].min(s[3]), s[2].min(s[3]), s[3]]
+            let (base, add) = if d >= depth + bevel {
+                // Past the curve nothing bends and no light falls (the rim light and the gradient
+                // are 0 there): the channels' samples are at the same point, so one sample.
+                (dispersed(&frosted, bx, by, 0.0, 0.0, 0.0), 0.0)
             } else {
-                dispersed(&frosted, bx, by, inward.0 * bend, inward.1 * bend, dispersion)
-            };
-            // The rim catches the light where it faces it, and half as much opposite. It fades out
-            // by `reach` and is none past it, wherever the layer is cut.
-            let hl = if d < reach && fall > 0.0 {
+                // The curve's outward direction: the edge's normal, turned by `splay` towards the
+                // way away from the centre (not renormalised: where the two part, the bend is less).
+                let away = if splay > 0.0 {
+                    let px = ((x as i64 - sx) as f64 + 0.5 - centre.x) as f32;
+                    let py = ((y as i64 - sy) as f64 + 0.5 - centre.y) as f32;
+                    let len = (px * px + py * py).sqrt();
+                    if len > 1e-6 { (px / len, py / len) } else { (0.0, 0.0) }
+                } else {
+                    (0.0, 0.0)
+                };
+                let out_dir = (-inward.0 * (1.0 - splay) + away.0 * splay, -inward.1 * (1.0 - splay) + away.1 * splay);
+                // No refraction: nothing bends, so one sample (`dispersed` with no move).
+                let bend = bend_at(d, depth, bevel, refraction);
+                let k = if bend == 0.0 { 0.0 } else { dispersion };
+                let base = dispersed(&frosted, bx, by, -out_dir.0 * bend, -out_dir.1 * bend, k);
+                // The bevel catches the light as a line added to the backdrop, by how much the edge
+                // faces it; across the curve a faint gradient, darker on the side facing it.
                 let facing = -(inward.0 * light.0 + inward.1 * light.1);
-                let spec = facing.max(0.0).powf(power) + 0.5 * (-facing).max(0.0).powf(power);
-                (intensity * spec * ((-d / band).exp() - floor) / fall).clamp(0.0, 1.0)
-            } else {
-                0.0
+                let rim = rim_light_at(d, bevel) * catches(facing);
+                let sheen = -(out_dir.0 * light.0 + out_dir.1 * light.1) * sheen_at(d, depth, bevel) * BROAD;
+                (base, 255.0 * intensity * (rim + sheen))
             };
             let i = (y * w as usize + x) * 4;
             if let Some(p) = dst.get_mut(i..i + 4) {
-                for (o, v) in p.iter_mut().zip(base) {
-                    *o = ((v * (1.0 - hl) + 255.0 * hl) * c).round().clamp(0.0, 255.0) as u8;
-                }
+                p.copy_from_slice(&lit(base, add, c));
             }
         }
     }
@@ -673,14 +731,32 @@ mod tests {
         assert!(c.held <= 100 && !c.map.contains_key(&key(4)));
     }
 
+    /// The rim light and the gradient across the curve at depth 20 and a 1-unit bevel, x units in
+    /// from the edge: the reference profile's values.
     #[test]
-    fn light_reach_is_where_the_light_falls_under_half_a_level() {
-        let r = light_reach(10.0, 1.0);
-        assert!((255.0 * (-r / 10.0).exp() - 0.5).abs() < 1e-9, "{r}");
-        // No light, too faint to show, or a band that isn't a number: it reaches nowhere.
-        for (band, intensity) in [(10.0, 0.0), (10.0, 0.5 / 255.0), (f64::NAN, 1.0), (f64::INFINITY, 1.0)] {
-            assert_eq!(light_reach(band, intensity), 0.0, "{band} {intensity}");
+    fn the_light_lies_on_the_bevel_and_the_gradient_across_the_curve() {
+        for (x, rim, sheen) in [
+            (0.10, 1.0, 1.0),
+            (0.25, 0.9888, 1.0),
+            (0.50, 0.6843, 1.0),
+            (0.75, 0.2330, 1.0),
+            (1.00, 0.0, 1.0),
+            (2.0, 0.0, 0.8975),
+            (4.0, 0.0, 0.7091),
+            (6.0, 0.0, 0.5429),
+            (8.0, 0.0, 0.3989),
+            (16.0, 0.0, 0.0443),
+            (20.0, 0.0, 0.0),
+            (30.0, 0.0, 0.0),
+        ] {
+            assert!((rim_light_at(x, 1.0) - rim).abs() < 1e-3, "rim at {x}: {}", rim_light_at(x, 1.0));
+            assert!((sheen_at(x, 20.0, 1.0) - sheen).abs() < 1e-3, "gradient at {x}: {}", sheen_at(x, 20.0, 1.0));
         }
-        assert_eq!(light_reach(1e6, 1.0), MAX_LIGHT_PX);
+        // Facing the light fully, at 45° (cos 0.707) and side on; the opposite side 0.6 as much.
+        assert!((catches(1.0) - 1.0).abs() < 1e-6);
+        assert!((catches(std::f32::consts::FRAC_1_SQRT_2) - 0.574).abs() < 1e-3, "{}", catches(std::f32::consts::FRAC_1_SQRT_2));
+        assert_eq!(catches(0.0), 0.0);
+        assert!((catches(-1.0) - 0.6).abs() < 1e-6);
+        assert!((catches(-std::f32::consts::FRAC_1_SQRT_2) - 0.6 * 0.574).abs() < 1e-3);
     }
 }

crates/render/src/glass_cut_tests.rs

@@ -59,25 +59,31 @@ fn cut_diffs(d: &Document, p: NodeId) -> Vec<(u32, (u8, usize))> {
         .collect()
 }
 
-/// A pane deeper than the light may reach, over black and lit from the left: across its middle row
-/// each pixel is the light alone. It fades out smoothly where it stops, with no ring.
+/// A deep pane over black, lit from the left, at 2× (its 1-unit bevel 2 px wide): across its middle
+/// row the left rim catches the light, the right rim 0.6 as much, and in between only the faint
+/// gradient across the curve shows (black can't darken, so the lit half stays black), changing
+/// smoothly, with no ring.
 #[test]
-fn the_light_fades_out_where_it_stops() {
+fn the_light_is_a_line_on_the_bevel() {
     let mut d = Document::new("glass");
     let p = d.first_page().unwrap();
     let back = d.create(p, NodeKind::Rectangle, "Back", 0.0, 0.0, 700.0, 700.0).unwrap();
     d.get_mut(back).unwrap().fills = vec![Paint::solid(Color::BLACK)];
     let pane = d.create(p, NodeKind::Rectangle, "Glass", 50.0, 50.0, 600.0, 600.0).unwrap();
-    let g = Glass { depth: 300.0, splay: 1.0, light_angle: -90.0, light_intensity: 1.0, frost: 0.0, refraction: 0.0, ..Glass::default() };
+    let g = Glass { depth: 300.0, light_angle: -90.0, light_intensity: 1.0, frost: 0.0, refraction: 0.0, ..Glass::default() };
     let n = d.get_mut(pane).unwrap();
     n.fills.clear();
     n.effects = vec![Effect { kind: EffectKind::Glass(g), visible: true }];
     let full = render_view_single(&d, p, &View { origin: (0.0, 0.0), scale: 2.0, width: 1400, height: 1400 }, &Options::default()).unwrap();
     let row = &full.data()[700 * 1400 * 4..][..1400 * 4];
-    let red: Vec<u8> = row.chunks(4).map(|px| px[0]).skip(101).take(598).collect();
-    let step = red.windows(2).map(|w| w[0].abs_diff(w[1])).max().unwrap();
-    assert!(step <= 2, "the light jumps {step} levels between neighbouring pixels");
-    assert!(red[0] > 200 && red[597] < red[0], "lit at the edge, fading in: {} … {}", red[0], red[597]);
+    let red: Vec<u8> = row.chunks(4).map(|px| px[0]).collect();
+    // The pane covers pixels 100..1300 of the row.
+    let (left, right) = (red[100], red[1299]);
+    assert!(left > 230, "the left rim catches the light, less the gradient: {left}");
+    assert!((140..=175).contains(&right), "the right rim 0.6 as much, plus the gradient: {right}");
+    assert!(red[106] <= 14 && red[1293] <= 14, "3 units in, the gradient only: {} {}", red[106], red[1293]);
+    let step = red[104..1296].windows(2).map(|w| w[0].abs_diff(w[1])).max().unwrap();
+    assert!(step <= 2, "the gradient jumps {step} levels between neighbouring pixels");
 }
 
 /// Digests of glass renders over a spread of settings, scales and corners: compare two builds'
@@ -115,11 +121,13 @@ fn glass_digests() {
 
 #[test]
 fn a_cut_through_the_pane_matches_the_whole_render() {
-    // The last look is deep enough that the light is held to `MAX_LIGHT_PX`.
+    // The last look is deep (300 units), so the curve reaches far in from every cut.
     let looks = [
         wide_light(),
         Glass::default(),
         Glass { splay: 0.5, frost: 0.0, refraction: 0.0, ..Glass::default() },
+        // Splay with the bend and dispersion on: the dispersed samples follow the turned bend.
+        Glass { splay: 1.0, frost: 0.0, ..Glass::default() },
         Glass { depth: 300.0, ..wide_light() },
     ];
     // Rounded corners without the pane's fill: a fill's curved edge rasters 1–2 levels apart in a