Fog lit by rays: shadows cut through tiles, and walls stay visible

The fog asked one question per tile -- can I see its centre? -- and drew or
skipped the whole tile on the answer. Every shadow was a staircase, tiles
snapped in and out as you walked, and a wall beside you vanished whenever
its centre happened to sit behind its neighbour.

Visibility casts rays from the eye instead: a fixed sweep, plus two aimed
either side of every wall corner, which is what makes each shadow edge a
straight line rather than a smear between rays. The view now draws terrain
and actors first and then darkness over whatever the rays did not reach,
so a shadow runs straight through a tile and an enemy rounding a corner
appears a slice at a time. Bullets are a child node and are still drawn
over the dark; telegraphs are drawn after it, as they must be.

"A bit less strict" is three specific things. A wall face you can see is
lit 26 units in, measured perpendicular to the face; shadows end in a
short fade rather than a line; and sight fades over its last 64 units
rather than stopping at a circle. Still hard fog -- nothing remembered.

The first version measured that depth along the ray, and a screenshot
caught what no test had: a wall you look along got a sliver, so the wall
beside you in a tunnel looked missing -- the old fog's own complaint.
Perpendicular depth fixes it, and light stops the instant a ray steps
back into the open, so it never rounds a corner, never reaches the far
side of a one-tile wall, and never leaves a stepped wall through its
other face. Two tiles touching only at a corner are closed.

About half a millisecond a frame. Measured against the old fog in the
same scene: hub 68 fps against 62, dungeon 240 against 240. The hub's
own low rate predates this.

MapGrid gains a revision counter so the corner list is rebuilt only when
the map changes or the eye crosses a tile. screenshot.tscn now captures a
dungeon, a tunnel and the tunnel with F1's overlay, which draws where
each ray stopped -- and makes SHOT_DIR rather than failing every capture
when it does not exist.

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

CLAUDE.md

@@ -13,7 +13,7 @@ the obvious default.
 
 ```bash
 tools/check.sh      # parse-check every script (~5s) -- run after every edit
-tools/test.sh       # 468 GUT tests, headless (~4s)
+tools/test.sh       # 534 GUT tests, headless (~4s)
 tools/smoke.sh      # real server + 4 bot clients over ENet (~40s)
 tools/server.sh     # dedicated server
 tools/client.sh --listen            # host and play, no menu
@@ -29,7 +29,7 @@ godot --headless --path . res://tools/diag_loot.tscn          # drop -> pick up
 godot --headless --path . res://tools/diag_upgrades.tscn      # level -> choice -> taken at the NPC -> new stats
 godot --headless --path . --script tools/bench.gd             # sim cost per tick
 python3 tools/build_local_assets.py                           # rebuild the local-only bullet atlas
-SHOT_DIR=/tmp/shots godot --path . res://tools/screenshot.tscn # capture the menus and HUD (needs a display)
+SHOT_DIR=/tmp/shots godot --path . res://tools/screenshot.tscn # menus, HUD, roster, and the fog in a dungeon and a tunnel (needs a display)
 ```
 
 `diag_progression`, `diag_loot` and `diag_upgrades` exist because the bot smoke
@@ -243,8 +243,13 @@ ticks in milliseconds with no SceneTree.
   enemies and the boss all go through the same lerp. The boss did not for a
   long time, which is invisible while bosses stand still and looks broken the
   moment one moves.
+- **The fog works by draw order, not by deciding per object.** `WorldView._draw`
+  draws terrain and every actor within sight range, then the darkness from
+  `Visibility` over them, then whatever must stay visible. That is what lets a
+  tile or an enemy be half lit. Anything new that fog may hide goes *before*
+  `_draw_darkness`; anything it must not goes after.
 - **A telegraph must be visible through fog.** `WorldView._draw_telegraphs`
-  ignores line of sight on purpose; everything else in the view respects it. A
+  is drawn after the darkness on purpose; everything else in the view is under it. A
   warning you cannot see is an unavoidable hit with extra steps.
 - **No i-frames.** Every bullet that touches a player lands; `spawn_grace` is
   the only invulnerable state. Do not reintroduce post-hit immunity — it makes

docs/ARCHITECTURE.md

@@ -48,8 +48,10 @@ This project deliberately does not, for four reasons:
 The trade is that there is no physics engine, so collision is hand-written.
 Since Stage 1 that means a tile grid ([MapGrid](../src/sim/map_grid.gd)):
 circle-vs-tile for actors, a point test for bullets, and Bresenham line of sight
-shared by fog and aggro. Actor-vs-actor hits stay circle-vs-circle, which is the
-genre convention anyway.
+for aggro. Actor-vs-actor hits stay circle-vs-circle, which is the genre
+convention anyway. The client's fog no longer shares that line test: it casts
+exact rays through the same grid ([Visibility](../src/view/visibility.gd)), so
+shadows cut through tiles rather than snapping to them.
 
 That was the right call rather than a compromise: the same grid answers
 collision, sight, and per-peer interest management as array lookups. On arbitrary
@@ -186,7 +188,8 @@ nothing from `net`, `view` or `ui`, and never touches `Net`.**
 
 | File | Role |
 | --- | --- |
-| `src/view/world_view.gd` | Everything that is not a bullet. Fog, actors, loot, telegraphs. |
+| `src/view/world_view.gd` | Everything that is not a bullet. Fog, actors, loot, telegraphs. Draw order is the fog's mechanism: everything it may hide, then the darkness, then what it must not. |
+| `src/view/visibility.gd` | The fog's geometry: rays from the eye, wall corners, the shadow mesh. Pure, so `test_visibility.gd` asks it which points are lit. |
 | `src/view/bullet_renderer.gd` | The bullet field: one MultiMesh per bullet kind. |
 | `src/view/art.gd` | Every atlas rect and sound path, in one table. |
 | `src/view/ui_theme.gd` | The control theme, built in code from the UI pack. |

docs/DECISIONS.md

@@ -89,6 +89,28 @@ about as often as a good one.
 **Hard fog.** *(User, this session.)* No remembered terrain — anything outside
 current line of sight is not drawn, including ground already walked over.
 
+**The fog is geometry, not a verdict per tile — and a little less strict.**
+*(User asked: "no longer grid based ... able to light up cells partially", and
+"a bit less strict". How strict was left to me.)* `Visibility` casts rays from
+the eye — a fixed sweep plus two aimed either side of every wall corner, which
+is what makes each shadow edge a straight line — and the view draws terrain and
+actors first, then darkness over everything the rays did not reach. A shadow
+edge runs straight through a tile; an enemy rounding a corner appears a slice
+at a time instead of popping in whole.
+
+"Less strict" is three specific things. The face of a wall you can see is lit
+26 units deep, measured perpendicular to the face — along the ray it was a
+sliver for any wall seen at a grazing angle, and the wall beside you vanished,
+which is the complaint the old fog had. Shadows end in an 8-unit fade instead
+of a hard line. And the edge of sight fades over its last 64 units instead of
+stopping at a circle. Nothing is remembered: still hard fog.
+
+Two lines this deliberately does not cross: light stops the moment a ray steps
+back out of the wall it was in, so it never rounds a corner or reaches the far
+side of a one-tile wall; and two blocking tiles that touch only at a corner are
+closed, not a crack. Bullets are drawn over the darkness, as they always were —
+a shot out of the black is still one you can dodge.
+
 **Dungeon size scales with depth.** *(User, this session.)* `--depth N` is a dev
 flag; what raises depth in actual play is still open.
 

docs/ROADMAP.md

@@ -40,11 +40,14 @@ and more specific than the table above.
 
 ### Known flake
 
-`tools/smoke.sh` failed once in a pre-push hook and then passed on four
-consecutive runs with no stray processes. It was not reproduced and is not
-diagnosed. The suspicion is machine load — the smoke test has fixed tick budgets
-and was competing with a full suite run — but that is a guess. If it recurs,
-that is the first thing to check.
+`tools/smoke.sh` has failed twice, **both times inside the pre-push hook**, and
+then passed on every direct run after: four in a row the first time, eight the
+second (six in a background loop plus two more, one of them the hook itself).
+Neither failure's logs survived, because the hook runs smoke without `KEEP`.
+The first suspicion was machine load, but the second failure had nothing else
+running. What is different about the hook is its context — stdin is git's
+pipe of ref lines, which every bot inherits. Unconfirmed. Push with
+`KEEP=1 git push` so the logs of the next one survive.
 
 ---
 
@@ -90,7 +93,7 @@ against itself.
 | Walls, pillars, pits, barricades | done | `MapGrid.Kind` + the three `BLOCKS_*` tables |
 | Enemies placed per room at creation | done | `Instance._populate()` |
 | Scrolling camera | done | [src/view/game_scene.gd](../src/view/game_scene.gd) |
-| Hard fog of war | done | `WorldView._draw_terrain` / `_visible` |
+| Hard fog of war, lit by rays rather than per tile | done | [src/view/visibility.gd](../src/view/visibility.gd), `WorldView._draw_darkness` |
 | Per-peer map streaming (anti map-hack) | done | `ServerRuntime._stream_map` |
 | Aggro: range **and** line of sight | done | `SimWorld._aggro_target` |
 | Cursor-to-world aiming under a scrolling camera | done | `ClientRuntime.screen_to_world` |
@@ -427,7 +430,7 @@ feet is a coin flip, the same burst with a second of notice is a question.
 - **`EmitContext` gained an `events` list**, the only thing an emitter can do
   besides spawn bullets.
 - **Markers are drawn through fog and through walls.** Everything else in the
-  view respects line of sight; a warning you cannot see is not a warning. See
+  view is covered by the darkness; a warning you cannot see is not a warning. See
   `WorldView._draw_telegraphs`.
 
 ### The Cantor of the Vault

src/sim/map_grid.gd

@@ -54,6 +54,11 @@ var tiles := PackedByteArray()
 ## existing coordinate (spawn points, portal, boss placement) meaningful and
 ## avoids an all-positive coordinate space where "0" is a corner.
 var origin := Vector2.ZERO
+## Bumped by every write, so something that derives data from the tiles can tell
+## whether it is stale without comparing them. The client's fog extracts wall
+## corners from the map and would otherwise have to redo it every frame, or miss
+## a chunk arriving.
+var revision: int = 0
 
 
 func _init(w: int = 1, h: int = 1, fill: Kind = Kind.WALL) -> void:
@@ -65,6 +70,7 @@ func resize(w: int, h: int, fill: Kind = Kind.WALL) -> void:
 	height = maxi(h, 1)
 	tiles.resize(width * height)
 	tiles.fill(fill)
+	revision += 1
 
 
 func in_bounds(tx: int, ty: int) -> bool:
@@ -82,6 +88,7 @@ func at(tx: int, ty: int) -> Kind:
 func set_tile(tx: int, ty: int, kind: Kind) -> void:
 	if in_bounds(tx, ty):
 		tiles[ty * width + tx] = kind
+		revision += 1
 
 
 func fill_rect(rect: Rect2i, kind: Kind) -> void:
@@ -111,6 +118,7 @@ func to_tile(world: Vector2) -> Vector2i:
 ## Centre the map on the world origin.
 func centre_on_origin() -> void:
 	origin = -world_size() * 0.5
+	revision += 1
 
 
 ## World-space rectangle the map occupies.
@@ -175,8 +183,14 @@ func slide_circle(pos: Vector2, delta: Vector2, radius: float) -> Vector2:
 	return out
 
 
-## Bresenham-style sight test between two world points. Used for fog on the
-## client and for aggro on the server, so it has to agree on both.
+## Bresenham-style sight test between two world points, from tile to tile.
+## Used for aggro on the server.
+##
+## It used to drive the client's fog as well, which is why the two once had to
+## agree. The fog is exact geometry now (see Visibility), so this is the
+## server's question alone -- and it is coarser than what the player sees: its
+## diagonal steps pass between two tiles that touch only at a corner, which
+## the fog treats as closed.
 ##
 ## [param include_end] is false here: the endpoints themselves never block, so
 ## standing in a doorway, or shooting at something embedded in a wall, still
@@ -303,3 +317,4 @@ func apply_chunk(chunk_id: int, data: PackedByteArray) -> void:
 		for tx in range(r.position.x, r.end.x):
 			tiles[ty * width + tx] = data[i]
 			i += 1
+	revision += 1

src/view/debug_draw.gd

@@ -20,6 +20,7 @@ const COL_PIT := Color(0.4, 0.5, 1.0, 0.5)
 const COL_AGGRO := Color(1.0, 0.5, 0.2, 0.25)
 const COL_MUZZLE := Color(0.4, 1.0, 0.6, 0.9)
 const COL_LOS := Color(0.5, 1.0, 0.8, 0.18)
+const COL_SIGHT := Color(1.0, 0.95, 0.55, 0.55)  # where each fog ray stopped
 
 
 ## Terrain: one outline per tile, coloured by which of the three flags it sets.
@@ -82,3 +83,18 @@ static func draw_bullets(c: CanvasItem, pool: BulletPool) -> void:
 ## A ring at the fog radius, so "why can I not see that" has a visible answer.
 static func draw_ranges(c: CanvasItem, pos: Vector2) -> void:
 	c.draw_arc(pos, SimConfig.FOG_VIEW_RADIUS, 0.0, TAU, 64, COL_LOS, 1.0)
+
+
+## The outline the fog's rays traced: where each one stopped. Drawn as the hit
+## points rather than the soft edge, because what this answers is "why is THAT
+## dark", and the answer is always which ray hit what.
+static func draw_sight(c: CanvasItem, sight: Visibility) -> void:
+	var n := sight.angles.size()
+	if n < 2:
+		return
+	var outline := PackedVector2Array()
+	outline.resize(n + 1)
+	for i in n:
+		outline[i] = sight.eye + sight.dirs[i] * sight.hits[i]
+	outline[n] = outline[0]
+	c.draw_polyline(outline, COL_SIGHT, 1.0)

src/view/visibility.gd

@@ -0,0 +1,348 @@
+class_name Visibility
+extends RefCounted
+## What the local player can see, as geometry rather than a verdict per tile.
+##
+## The fog this replaces asked one question per tile -- can I see its centre? --
+## and drew or skipped the whole tile on the answer. Every shadow was a
+## staircase, a wall beside you vanished whenever its centre happened to sit
+## behind its neighbour, and tiles snapped in and out as you walked. This casts
+## rays instead and lights the space they reach, so a shadow edge runs straight
+## through a tile and half of one can be lit.
+##
+## Client-side and rendering-only, like the fog it replaces. It decides what is
+## DRAWN; what the client is SENT is the server's business -- the actor interest
+## radius and map streaming -- and nothing in here can widen that.
+
+## Rays at fixed angles, on top of the ones aimed at corners. Corners alone give
+## an exact outline wherever there is geometry to hit; the sweep covers open
+## ground with nothing in it, and keeps the angle between neighbouring rays
+## small enough that the straight chord joining them stays inside the fade.
+const SWEEP_RAYS := 180
+## How far either side of a corner its two rays are aimed, in radians: one grazes
+## past and one hits. That pair is what makes the shadow leaving a corner a
+## straight line rather than a smear across the gap to the next ray.
+const CORNER_NUDGE := 0.0004
+## How deep into a wall its lit face goes, measured straight in from the face.
+##
+## Without any, the wall you are looking at is exactly where the dark begins and
+## is never drawn -- floor would end in a void. Measured perpendicular to the
+## face rather than along the ray, because along the ray a wall you are looking
+## ALONG gets a sliver: a fixed distance travelled at a grazing angle barely gets
+## under the surface, so the wall right beside you was the one you could not
+## see -- the very thing this replaced the old fog to fix.
+##
+## Under a tile, and light also stops the moment a ray leaves the wall it is
+## in, so neither the far side of a one-tile wall nor the space round a corner
+## is ever lit by it.
+const WALL_DEPTH := 26.0
+## The last part of that depth fades out instead of ending in a hard line.
+const FEATHER := 8.0
+## Soft edge at the limit of sight, in place of the old hard circle.
+const FADE := 64.0
+
+## One entry per ray, sorted by angle in [-PI, PI).
+var angles := PackedFloat32Array()
+var dirs := PackedVector2Array()
+## Where each ray stopped: at the first tile that blocks sight, or at the radius.
+var hits := PackedFloat32Array()
+## Along each ray: fully lit up to [member starts], dark from [member ends].
+var starts := PackedFloat32Array()
+var ends := PackedFloat32Array()
+var eye := Vector2.ZERO
+var radius: float = 0.0
+
+var _sweep := PackedFloat32Array()
+## Every corner of sight-blocking geometry near the eye, in world space. Derived
+## from the tiles alone, so it is only rebuilt when the map changes or the eye
+## moves into another tile.
+var _corners := PackedVector2Array()
+var _corner_map: int = 0
+var _corner_revision: int = -1
+var _corner_tile := Vector2i(-99999, -99999)
+var _corner_radius: float = -1.0
+
+
+func _init() -> void:
+	_sweep.resize(SWEEP_RAYS)
+	for i in SWEEP_RAYS:
+		_sweep[i] = -PI + TAU * float(i) / float(SWEEP_RAYS)
+
+
+## Recompute everything for an eye at [param at] seeing [param view_radius].
+func compute(map: MapGrid, at: Vector2, view_radius: float) -> void:
+	eye = at
+	radius = view_radius
+	_refresh_corners(map)
+
+	var want := PackedFloat32Array(_sweep)
+	var reach := view_radius + MapGrid.TILE * 1.5
+	var reach_sq := reach * reach
+	for c in _corners:
+		var d := c - at
+		var dsq := d.length_squared()
+		if dsq > reach_sq or dsq < 0.0001:
+			continue
+		var a := d.angle()
+		want.append(wrapf(a - CORNER_NUDGE, -PI, PI))
+		want.append(wrapf(a + CORNER_NUDGE, -PI, PI))
+	want.sort()
+
+	var n := want.size()
+	angles = want
+	dirs.resize(n)
+	hits.resize(n)
+	starts.resize(n)
+	ends.resize(n)
+	var fade_from := view_radius - FADE
+	for i in n:
+		var dir := Vector2.from_angle(want[i])
+		dirs[i] = dir
+		var traced := trace(map, at, dir, view_radius, WALL_DEPTH, FEATHER)
+		hits[i] = traced.x
+		var s := fade_from
+		var e := view_radius
+		if traced.x < view_radius:
+			s = minf(traced.y, fade_from)
+			e = minf(traced.z, view_radius)
+		starts[i] = s
+		ends[i] = maxf(e, s + 0.5)
+
+
+## How lit [param p] is, from 1 (fully) to 0 (dark). Matches what the shadow mesh
+## draws: the outline between two neighbouring rays is the straight chord
+## joining their ends, exactly as the triangles are laid out.
+func light_at(p: Vector2) -> float:
+	var n := angles.size()
+	if n == 0:
+		return 1.0
+	var d := p - eye
+	var dist := d.length()
+	if dist < 0.001:
+		return 1.0
+	var u := d / dist
+	var a := wrapf(d.angle(), -PI, PI)
+	var j := angles.bsearch(a) % n
+	var i := (j - 1 + n) % n
+	var s := _along(u, dirs[i] * starts[i], dirs[j] * starts[j])
+	var e := _along(u, dirs[i] * ends[i], dirs[j] * ends[j])
+	if dist <= s:
+		return 1.0
+	if dist >= e:
+		return 0.0
+	return 1.0 - (dist - s) / maxf(e - s, 0.001)
+
+
+## Darkness covering everything not lit, out to [param far], as one triangle
+## list: { "points", "colors", "indices" }. Three vertices per ray -- where the
+## fade starts (clear), where it is complete (dark) and the far edge (dark) --
+## and four triangles per pair of neighbouring rays.
+##
+## One batch rather than a polygon per segment: there are several hundred
+## segments a frame, and a draw call each was the expensive part.
+func shadow_mesh(far: float, colour: Color) -> Dictionary:
+	var n := angles.size()
+	var points := PackedVector2Array()
+	var colors := PackedColorArray()
+	var indices := PackedInt32Array()
+	points.resize(n * 3)
+	colors.resize(n * 3)
+	indices.resize(n * 12)
+	var clear := Color(colour, 0.0)
+	var dark := Color(colour, 1.0)
+	for i in n:
+		var dir := dirs[i]
+		points[i * 3] = eye + dir * starts[i]
+		points[i * 3 + 1] = eye + dir * ends[i]
+		points[i * 3 + 2] = eye + dir * far
+		colors[i * 3] = clear
+		colors[i * 3 + 1] = dark
+		colors[i * 3 + 2] = dark
+	for i in n:
+		var j := (i + 1) % n
+		var si := i * 3
+		var sj := j * 3
+		var k := i * 12
+		# The fade: clear at the inner edge, dark at the outer.
+		indices[k] = si
+		indices[k + 1] = sj
+		indices[k + 2] = sj + 1
+		indices[k + 3] = si
+		indices[k + 4] = sj + 1
+		indices[k + 5] = si + 1
+		# Solid dark from there out to the far edge.
+		indices[k + 6] = si + 1
+		indices[k + 7] = sj + 1
+		indices[k + 8] = sj + 2
+		indices[k + 9] = si + 1
+		indices[k + 10] = sj + 2
+		indices[k + 11] = si + 2
+	return {"points": points, "colors": colors, "indices": indices}
+
+
+## Distance along unit ray [param u] to the chord from [param a] to [param b],
+## both relative to the eye. Falls back to the nearer end when the ray runs
+## parallel to the chord, which only happens for a degenerate pair.
+static func _along(u: Vector2, a: Vector2, b: Vector2) -> float:
+	var ab := b - a
+	var denom := u.cross(ab)
+	if absf(denom) < 0.000001:
+		return minf(a.length(), b.length())
+	return a.cross(b) / denom
+
+
+## Distance from [param from] along unit [param dir] to the first tile that
+## blocks sight, or [param max_dist] if there is none that close.
+static func cast(map: MapGrid, from: Vector2, dir: Vector2, max_dist: float) -> float:
+	return trace(map, from, dir, max_dist, 0.0, 0.0).x
+
+
+## Follow one ray: where it hits, and how far its light carries into the wall.
+## Returns (hit, fade_start, fade_end), all distances along the ray.
+##
+## Grid traversal (Amanatides and Woo), so it visits exactly the tiles the ray
+## passes through and lands on the true edge of the one that stops it -- which
+## is the whole difference from the old fog. A ray passing exactly through a
+## corner shared by two blocking tiles is stopped there: two walls touching at
+## a point leave no gap to see through.
+##
+## Past the hit, the light runs on to [param depth] measured straight in from the
+## face it entered -- so along the ray that is depth / cos(angle of incidence),
+## long for a grazing ray and short for a head-on one -- and the last
+## [param feather] of it fades. It stops early, wherever it is, the moment the
+## ray steps back out into a tile that does not block sight.
+static func trace(map: MapGrid, from: Vector2, dir: Vector2, max_dist: float,
+		depth: float, feather: float) -> Vector3:
+	var tile := MapGrid.TILE
+	var px := (from.x - map.origin.x) / tile
+	var py := (from.y - map.origin.y) / tile
+	var tx := floori(px)
+	var ty := floori(py)
+	var limit := max_dist / tile
+	var w := map.width
+	var h := map.height
+	var tiles := map.tiles
+	var sight: Array = MapGrid.BLOCKS_SIGHT
+
+	var step_x := 0
+	var step_y := 0
+	var delta_x := INF
+	var delta_y := INF
+	var next_x := INF
+	var next_y := INF
+	if dir.x > 0.0:
+		step_x = 1
+		delta_x = 1.0 / dir.x
+		next_x = (float(tx + 1) - px) * delta_x
+	elif dir.x < 0.0:
+		step_x = -1
+		delta_x = -1.0 / dir.x
+		next_x = (px - float(tx)) * delta_x
+	if dir.y > 0.0:
+		step_y = 1
+		delta_y = 1.0 / dir.y
+		next_y = (float(ty + 1) - py) * delta_y
+	elif dir.y < 0.0:
+		step_y = -1
+		delta_y = -1.0 / dir.y
+		next_y = (py - float(ty)) * delta_y
+
+	# Phase 1 walks open ground to the first blocker; phase 2 walks on through
+	# the blocker while there is light left to carry.
+	var hit := -1.0
+	var stop := limit
+	var fade := limit
+	while true:
+		var t: float
+		var crossed_x := false
+		var diagonal := false
+		if next_x < next_y - 0.000000001:
+			t = next_x
+			tx += step_x
+			next_x += delta_x
+			crossed_x = true
+		elif next_y < next_x - 0.000000001:
+			t = next_y
+			ty += step_y
+			next_y += delta_y
+		else:
+			t = next_x
+			diagonal = true
+		if t > stop:
+			break
+		if diagonal:
+			var side_x := tx + step_x
+			var side_y := ty + step_y
+			var across_x := side_x < 0 or ty < 0 or side_x >= w or ty >= h \
+				or bool(sight[tiles[ty * w + side_x]])
+			var across_y := tx < 0 or side_y < 0 or tx >= w or side_y >= h \
+				or bool(sight[tiles[side_y * w + tx]])
+			tx = side_x
+			ty = side_y
+			next_x += delta_x
+			next_y += delta_y
+			crossed_x = true
+			if hit < 0.0:
+				if across_x and across_y:
+					hit = t
+					stop = minf(limit, t + depth / tile / maxf(absf(dir.x), absf(dir.y)))
+					fade = minf(stop, t + (depth - feather) / tile / maxf(absf(dir.x), absf(dir.y)))
+					continue
+			elif not (across_x and across_y):
+				stop = t
+				break
+		# Off the map reads as wall, the same rule MapGrid.at() keeps.
+		var blocked := tx < 0 or ty < 0 or tx >= w or ty >= h \
+			or bool(sight[tiles[ty * w + tx]])
+		if hit < 0.0:
+			if blocked:
+				hit = t
+				# Incidence against the face just crossed: an x step crosses a
+				# vertical face, whose normal is along x.
+				var c := maxf(absf(dir.x) if crossed_x else absf(dir.y), 0.000001)
+				stop = minf(limit, t + depth / tile / c)
+				fade = minf(stop, t + (depth - feather) / tile / c)
+		elif not blocked:
+			# Back out into the open: the light ends at this face, wherever the
+			# depth had reached. This is what keeps it from rounding a corner.
+			stop = t
+			break
+	if hit < 0.0:
+		return Vector3(max_dist, max_dist, max_dist)
+	stop = minf(stop, limit)
+	fade = minf(fade, stop)
+	return Vector3(hit * tile, fade * tile, stop * tile)
+
+
+## Rebuild the corner list if the map changed or the eye moved to another tile.
+##
+## A corner is any tile vertex where blocking and open ground meet other than
+## along a straight edge: one or three of the four surrounding tiles blocking, or
+## two blocking diagonally. Those are the only points a shadow edge can leave
+## from, which is why aiming rays at them is enough to make every edge exact.
+func _refresh_corners(map: MapGrid) -> void:
+	var t := map.to_tile(eye)
+	var id := map.get_instance_id()
+	if id == _corner_map and map.revision == _corner_revision \
+			and t == _corner_tile and radius == _corner_radius:
+		return
+	_corner_map = id
+	_corner_revision = map.revision
+	_corner_tile = t
+	_corner_radius = radius
+	_corners.clear()
+
+	var span := ceili(radius / MapGrid.TILE) + 2
+	var x0 := maxi(t.x - span, 0)
+	var x1 := mini(t.x + span + 1, map.width)
+	var y0 := maxi(t.y - span, 0)
+	var y1 := mini(t.y + span + 1, map.height)
+	for vy in range(y0, y1 + 1):
+		for vx in range(x0, x1 + 1):
+			var a := map.blocks_sight(vx - 1, vy - 1)
+			var b := map.blocks_sight(vx, vy - 1)
+			var c := map.blocks_sight(vx - 1, vy)
+			var d := map.blocks_sight(vx, vy)
+			var count := int(a) + int(b) + int(c) + int(d)
+			# Two blocking and a == d means the pair is diagonal (a+d or b+c).
+			if count == 1 or count == 3 or (count == 2 and a == d):
+				_corners.append(map.origin + Vector2(float(vx), float(vy)) * MapGrid.TILE)

src/view/visibility.gd.uid

@@ -0,0 +1 @@
+uid://bikom644gux4e

src/view/world_view.gd

@@ -33,6 +33,16 @@ var client: ClientRuntime = null
 var _anim_time: float = 0.0
 ## F1 overlay: draws what the simulation collides against, over what is drawn.
 var debug_hitboxes: bool = false
+## What the local player can see this frame. See [Visibility].
+var _sight := Visibility.new()
+## The darkness is the window's clear colour, exactly, so where it is complete it
+## is indistinguishable from the undrawn void beyond it and there is no seam.
+var _dark: Color = ProjectSettings.get_setting(
+	"rendering/environment/defaults/default_clear_color", Color.BLACK)
+## How far past the edge of sight the darkness is drawn. It has to cover every
+## tile and sprite that is drawn at all -- a tile touching the circle of sight
+## reaches 45 units past it, a boss sprite further.
+const DARK_REACH := 128.0
 
 
 func _unhandled_input(event: InputEvent) -> void:
@@ -50,28 +60,56 @@ func _process(delta: float) -> void:
 	queue_redraw()
 
 
+## Draw order is the fog's whole mechanism, so it is worth spelling out.
+##
+## Everything that fog may hide is drawn first, unconditionally within sight
+## range; then the darkness goes over it. That is what lets a tile -- or an enemy
+## stepping round a corner -- be half lit: nothing is decided per object, the
+## shadow simply covers whatever is behind it. Telegraphs go on top of the
+## darkness because a warning you cannot see is no warning (see
+## _draw_telegraphs). The local player goes on top because it is the eye. And
+## bullets are a child node, so they are drawn after all of this, over the dark
+## -- as they always were: a shot coming out of the black is a shot you can
+## still dodge.
 func _draw() -> void:
 	if client == null:
 		return
+	var map: MapGrid = client.world.map
+	if map != null:
+		_sight.compute(map, client.predicted_pos, SimConfig.FOG_VIEW_RADIUS)
 	_draw_terrain()
 	_draw_portals()
 	_draw_upgrade_npc()
 	for l in client.ground_loot():
-		if _visible(l["pos"]):
+		if _in_view(l["pos"]):
 			_draw_loot(l)
 	for e in client.enemies():
-		if _visible(e["pos"]):
+		if _in_view(e["pos"]):
 			_draw_enemy(e)
 	_draw_boss()
-	_draw_telegraphs()
 	for p in client.remote_players():
-		if _visible(p["pos"]):
+		if _in_view(p["pos"]):
 			_draw_remote_player(p)
+	if map != null:
+		_draw_darkness()
+	_draw_telegraphs()
 	_draw_local_player()
 	if debug_hitboxes:
 		_draw_debug()
 
 
+## Everything the player cannot see, as one batch of triangles.
+##
+## Straight to the RenderingServer rather than through draw_polygon: a CanvasItem
+## draw call is a thin wrapper around exactly this, but it takes one polygon,
+## and the shadow is several hundred. Issued from inside _draw on this item's own
+## canvas, so it lands in order between what was drawn before and after it.
+func _draw_darkness() -> void:
+	var mesh := _sight.shadow_mesh(SimConfig.FOG_VIEW_RADIUS + DARK_REACH, _dark)
+	RenderingServer.canvas_item_add_triangle_array(get_canvas_item(),
+		mesh["indices"], mesh["points"], mesh["colors"])
+
+
 ## Overlay showing what actually collides, on top of what is drawn. Deliberately
 ## ignores fog: the point is to check that art and simulation line up, and
 ## hiding half the evidence would defeat that.
@@ -91,6 +129,7 @@ func _draw_debug() -> void:
 		DebugDraw.draw_player(self, p["pos"], p["aim"])
 	DebugDraw.draw_player(self, client.predicted_pos, client.aim)
 	DebugDraw.draw_ranges(self, client.predicted_pos)
+	DebugDraw.draw_sight(self, _sight)
 
 
 ## Aggro radius by visual index. Read from the content definitions rather than
@@ -103,15 +142,16 @@ func _aggro_for(visual: int) -> float:
 	return 0.0
 
 
-## Terrain, drawn only where the player can currently see it.
+## Terrain within sight range, all of it. Whether it can actually be SEEN is
+## decided afterwards, by the darkness drawn over it -- see _draw.
 ##
-## Hard fog: nothing outside line of sight is drawn, including ground already
-## walked over. Note what this is and is not -- it is a rendering rule, not a
-## secrecy mechanism. The client holds real tile data because it simulates
-## bullets, so a modified client could draw what is hidden here. What it cannot
-## do is see terrain the server never streamed (only a radius around the
-## player) or actors the server filtered out, which is where the line actually
-## sits.
+## Still hard fog: nothing is remembered, and ground you walked over is dark
+## again once it is behind you. Note what this is and is not -- a rendering
+## rule, not a secrecy mechanism. The client holds real tile data because it
+## simulates bullets, so a modified client could draw what is hidden here. What
+## it cannot do is see terrain the server never streamed (only a radius around
+## the player) or actors the server filtered out, which is where the line
+## actually sits.
 func _draw_terrain() -> void:
 	var map: MapGrid = client.world.map
 	if map == null:
@@ -121,15 +161,18 @@ func _draw_terrain() -> void:
 	var lo := map.to_tile(eye - Vector2(view, view))
 	var hi := map.to_tile(eye + Vector2(view, view))
 	var view_sq := view * view
+	var half := Vector2(MapGrid.TILE, MapGrid.TILE) * 0.5
 	for ty in range(lo.y, hi.y + 1):
 		for tx in range(lo.x, hi.x + 1):
 			var kind := map.at(tx, ty)
 			if kind == MapGrid.Kind.UNKNOWN:
 				continue
 			var centre := map.tile_centre(tx, ty)
-			if eye.distance_squared_to(centre) > view_sq:
-				continue
-			if not map.has_line_of_sight(eye, centre):
+			# The tile's nearest point, not its centre: a tile the circle of
+			# sight cuts through is partly inside it, and skipping it would put
+			# a notch in the edge.
+			var near := eye.clamp(centre - half, centre + half)
+			if eye.distance_squared_to(near) > view_sq:
 				continue
 			_draw_tile(kind, centre, tx, ty)
 
@@ -158,16 +201,14 @@ func _draw_tile(kind: MapGrid.Kind, centre: Vector2, tx: int, ty: int) -> void:
 			draw_texture_rect_region(Art.TILESET, dest, Art.BARRICADE)
 
 
-## Actors are only drawn where the player can actually see them. The server
-## already withholds distant ones; this hides the ones behind walls.
-func _visible(at: Vector2) -> bool:
-	var map: MapGrid = client.world.map
-	if map == null:
-		return true
-	var eye := client.predicted_pos
-	if eye.distance_squared_to(at) > SimConfig.FOG_VIEW_RADIUS * SimConfig.FOG_VIEW_RADIUS:
-		return false
-	return map.has_line_of_sight(eye, at)
+## Whether an actor is near enough to be worth drawing. Distance only: whether
+## it is behind a wall is the darkness's job, not this one's, and that is what
+## lets something stepping round a corner be half visible instead of popping in
+## whole the moment its centre clears the edge. The margin lets a large sprite
+## standing just past the circle of sight show the part of it that is inside.
+func _in_view(at: Vector2) -> bool:
+	var reach := SimConfig.FOG_VIEW_RADIUS + 48.0
+	return client.predicted_pos.distance_squared_to(at) <= reach * reach
 
 
 ## One ring per entrance, coloured and labelled by the dungeon it opens. The
@@ -192,7 +233,7 @@ func _draw_upgrade_npc() -> void:
 	if client.instance_kind != Protocol.InstanceKind.LOBBY:
 		return
 	var at := client.upgrade_npc
-	if not _visible(at):
+	if not _in_view(at):
 		return
 	var src := Art.frame(Art.NPC_IDLE, Art.anim_frame(_anim_time, 3))
 	_draw_sprite(Art.TILESET, src, at, Color.WHITE, false, Art.PLAYER_ANCHOR)
@@ -256,8 +297,8 @@ func _draw_enemy(e: Dictionary) -> void:
 
 ## Attacks that have been announced but not yet landed.
 ##
-## Drawn through fog and through walls, deliberately. Everything else in this
-## view respects line of sight, but a warning you cannot see is not a warning --
+## Drawn over the darkness and through walls, deliberately. Everything else in
+## this view is covered by it, but a warning you cannot see is not a warning --
 ## it is an unavoidable hit with extra steps, and the entire reason telegraphs
 ## exist is to make the strike a decision. The marker is inside the boss arena
 ## you are already standing in, so this hides nothing a player could not walk
@@ -278,7 +319,7 @@ func _draw_telegraphs() -> void:
 
 func _draw_boss() -> void:
 	var b := client.boss_state()
-	if b.is_empty() or not _visible(b["pos"]):
+	if b.is_empty() or not _in_view(b["pos"]):
 		return
 	var pos: Vector2 = b["pos"]
 	var visual := client.boss_def.visual if client.boss_def != null else 0

tests/unit/test_visibility.gd

@@ -0,0 +1,305 @@
+extends GutTest
+## The fog's geometry. Nothing here can look at the screen -- screenshot.tscn is
+## for that -- but every property that makes the fog fair or unfair is a
+## question about which points are lit, and those can be asked directly.
+
+const T := MapGrid.TILE
+const VIEW := 460.0
+
+var sight: Visibility
+
+
+func before_each() -> void:
+	sight = Visibility.new()
+
+
+## An open floor, walled at the border, with the origin at a tile corner so
+## world positions are easy to reason about.
+func _room(w: int = 40, h: int = 40) -> MapGrid:
+	var g := MapGrid.new(w, h, MapGrid.Kind.WALL)
+	g.fill_rect(Rect2i(1, 1, w - 2, h - 2), MapGrid.Kind.FLOOR)
+	return g
+
+
+func _centre(tx: int, ty: int) -> Vector2:
+	return Vector2(float(tx) + 0.5, float(ty) + 0.5) * T
+
+
+func test_open_ground_is_lit_and_beyond_sight_is_not() -> void:
+	var g := _room()
+	var eye := _centre(20, 20)
+	sight.compute(g, eye, VIEW)
+	assert_eq(sight.light_at(eye + Vector2(100.0, 0.0)), 1.0)
+	assert_eq(sight.light_at(eye + Vector2(0.0, -(VIEW - Visibility.FADE - 1.0))), 1.0,
+		"lit all the way to where the fade begins")
+	assert_eq(sight.light_at(eye + Vector2(VIEW + 5.0, 0.0)), 0.0,
+		"nothing past the radius")
+	var mid := sight.light_at(eye + Vector2(VIEW - Visibility.FADE * 0.5, 0.0))
+	assert_between(mid, 0.2, 0.8, "the edge of sight fades instead of cutting")
+
+
+func test_a_wall_hides_what_is_behind_it() -> void:
+	var g := _room()
+	for ty in range(1, 39):
+		g.set_tile(24, ty, MapGrid.Kind.WALL)
+	var eye := _centre(20, 20)
+	sight.compute(g, eye, VIEW)
+	assert_eq(sight.light_at(_centre(28, 20)), 0.0)
+	assert_eq(sight.light_at(_centre(22, 20)), 1.0, "and not what is in front")
+
+
+## The feature asked for: a shadow edge that runs THROUGH a tile, so part of it
+## is lit and part is not. The old fog could only light or hide a tile whole.
+func test_one_tile_can_be_part_lit() -> void:
+	var g := _room()
+	g.set_tile(22, 20, MapGrid.Kind.PILLAR)
+	var eye := _centre(20, 20) + Vector2(0.0, 7.0)
+	sight.compute(g, eye, VIEW)
+	# The pillar's shadow is the wedge between its top-left and bottom-left
+	# corners as seen from the eye: -25.6 to +10.6 degrees. Tile (25, 21) spans
+	# about +2.9 to +15.9, so the lower edge of the wedge cuts straight across
+	# it. Sample a grid inside it and require both answers.
+	var lit := 0
+	var dark := 0
+	for sy in 8:
+		for sx in 8:
+			var p := Vector2(25.0 * T + 2.0 + float(sx) * 4.0, 21.0 * T + 2.0 + float(sy) * 4.0)
+			var l := sight.light_at(p)
+			if l >= 0.99:
+				lit += 1
+			elif l <= 0.01:
+				dark += 1
+	assert_gt(lit, 0, "some of the tile is in the light")
+	assert_gt(dark, 0, "and some of the same tile is in shadow")
+
+
+func test_a_shadow_leaves_a_corner_as_a_straight_line() -> void:
+	var g := _room()
+	g.set_tile(22, 18, MapGrid.Kind.PILLAR)
+	var eye := _centre(20, 20)
+	sight.compute(g, eye, VIEW)
+	# The pillar's lower-right corner, and the line from the eye through it.
+	# orthogonal() turns towards the pillar's other silhouette corner, which is
+	# the shadowed side; its negation points away from the pillar, into light.
+	var corner := Vector2(23.0, 19.0) * T
+	var away := (corner - eye).normalized()
+	var side := -away.orthogonal()
+	for dist in [140.0, 220.0, 300.0]:
+		var on_line: Vector2 = eye + away * dist
+		assert_eq(sight.light_at(on_line + side * 3.0), 1.0,
+			"%d units out, just on the open side of the edge" % dist)
+		assert_eq(sight.light_at(on_line - side * 3.0), 0.0,
+			"%d units out, just on the shadowed side" % dist)
+
+
+## "A bit less strict": the face of the wall in front of you is lit, rather than
+## being exactly where the dark begins and so never drawn at all.
+func test_the_wall_you_are_facing_is_visible() -> void:
+	var g := _room()
+	for ty in range(1, 39):
+		g.set_tile(24, ty, MapGrid.Kind.WALL)
+	var eye := _centre(20, 20)
+	sight.compute(g, eye, VIEW)
+	var face := 24.0 * T
+	# A fixed depth, not one computed from the constant: a test that samples at
+	# WALL_DEPTH - 2 passes with the depth at zero, sampling the floor in front.
+	assert_eq(sight.light_at(Vector2(face + 12.0, eye.y)), 1.0,
+		"the first stretch into the wall is fully lit")
+	assert_lt(sight.light_at(Vector2(face + Visibility.WALL_DEPTH + 1.0, eye.y)),
+		0.01, "and it is dark again before the far side")
+
+
+## The regression the first version of this shipped with, caught by a
+## screenshot: light carried a fixed distance ALONG each ray, so a wall seen at a
+## grazing angle -- the one right beside you in a tunnel -- got a sliver and
+## looked missing. Depth is measured straight in from the face now.
+func test_a_wall_you_are_looking_along_is_lit_its_whole_length() -> void:
+	var g := _room()
+	for ty in range(1, 39):
+		g.set_tile(24, ty, MapGrid.Kind.WALL)
+	var eye := Vector2(24.0 * T - 40.0, 20.5 * T)   # just over a tile from the face
+	sight.compute(g, eye, VIEW)
+	var dim := 0
+	for along in [0.0, 60.0, 120.0, 200.0, 280.0, 350.0]:
+		for side in [-1.0, 1.0]:
+			var p := Vector2(24.0 * T + 10.0, eye.y + along * side)
+			if sight.light_at(p) < 0.99:
+				dim += 1
+	assert_eq(dim, 0, "ten units into the wall, all the way along it, is lit")
+
+
+func test_light_does_not_round_a_corner() -> void:
+	# A wall ending in a corner, seen at a grazing angle. A ray that enters the
+	# wall near its end would, if its light ran on regardless, come out through
+	# the end face into the open space round the corner. It must stop there.
+	var g := _room()
+	for ty in range(1, 21):
+		g.set_tile(24, ty, MapGrid.Kind.WALL)   # ends at row 20
+	var eye := Vector2(24.0 * T - 40.0, 6.0 * T)
+	sight.compute(g, eye, VIEW)
+	# Round the end of the wall: past its far face, below its last tile.
+	for p in [Vector2(25.0 * T + 6.0, 21.0 * T + 6.0),
+			Vector2(26.0 * T, 21.0 * T + 20.0),
+			Vector2(25.0 * T + 20.0, 22.0 * T)]:
+		assert_eq(sight.light_at(p), 0.0, "round the corner at %s is dark" % p)
+
+
+func test_light_never_reaches_the_far_side_of_a_one_tile_wall() -> void:
+	assert_lt(Visibility.WALL_DEPTH, T,
+		"the lit band inside a wall must stop inside a single tile of it")
+	var g := _room()
+	for ty in range(1, 39):
+		g.set_tile(24, ty, MapGrid.Kind.WALL)
+	var eye := _centre(20, 20)
+	sight.compute(g, eye, VIEW)
+	# Just past the wall, at every height the eye can see the wall at -- steep
+	# and grazing alike.
+	var leaks := 0
+	for ty in range(2, 38):
+		for fx in [0.5, 4.0, 12.0]:
+			if sight.light_at(Vector2(25.0 * T + fx, float(ty) * T + 16.0)) > 0.0:
+				leaks += 1
+	assert_eq(leaks, 0, "nothing beyond a one-tile wall may be lit")
+
+
+func test_a_staircase_wall_hides_its_far_side() -> void:
+	# A wall one tile thick that runs diagonally, as steps. A band of light
+	# measured straight in from the face it entered could leave a step through
+	# its other side -- which is the far side of the wall.
+	var g := _room()
+	for i in 14:
+		g.set_tile(18 + i, 6 + i, MapGrid.Kind.WALL)
+		g.set_tile(19 + i, 6 + i, MapGrid.Kind.WALL)
+	var eye := _centre(16, 16)   # below-left of the stairs
+	sight.compute(g, eye, VIEW)
+	var leaks := 0
+	for i in range(1, 12):
+		# The tile immediately above-right of each step: the far side.
+		var far := _centre(20 + i, 5 + i)
+		if g.at(20 + i, 5 + i) == MapGrid.Kind.FLOOR and sight.light_at(far) > 0.0:
+			leaks += 1
+	assert_eq(leaks, 0, "the far side of a stepped wall stays dark")
+
+
+func test_two_tiles_touching_at_a_corner_leave_no_gap_to_see_through() -> void:
+	var g := _room()
+	g.set_tile(22, 21, MapGrid.Kind.PILLAR)
+	g.set_tile(21, 22, MapGrid.Kind.PILLAR)
+	var eye := _centre(20, 20) + Vector2(16.0, 16.0)   # the shared corner's diagonal
+	sight.compute(g, eye, VIEW)
+	# Straight along the diagonal, through the point where the two pillars meet.
+	var through := eye + Vector2(1.0, 1.0).normalized() * 120.0
+	assert_eq(sight.light_at(through), 0.0,
+		"two walls meeting at a point are closed, not a crack")
+	# Vector2 is single precision and the cast is double, so "exactly" is to a
+	# thousandth of a unit.
+	assert_almost_eq(Visibility.cast(g, eye, Vector2(1.0, 1.0).normalized(), VIEW),
+		Vector2(22.0 * T, 22.0 * T).distance_to(eye), 0.001,
+		"the ray stops exactly at the shared corner")
+
+
+func test_a_ray_stops_exactly_on_the_face_it_hits() -> void:
+	var g := _room()
+	g.set_tile(25, 20, MapGrid.Kind.WALL)
+	var eye := _centre(20, 20)
+	assert_almost_eq(Visibility.cast(g, eye, Vector2.RIGHT, VIEW), 25.0 * T - eye.x, 0.0001)
+	assert_eq(Visibility.cast(g, eye, Vector2.UP, 100.0), 100.0,
+		"nothing within reach means the full reach")
+
+
+func test_only_what_blocks_sight_casts_a_shadow() -> void:
+	var g := _room()
+	var eye := _centre(20, 20)
+	for kind in [MapGrid.Kind.PIT, MapGrid.Kind.BARRICADE]:
+		g.set_tile(23, 20, kind)
+		sight.compute(g, eye, VIEW)
+		assert_eq(sight.light_at(_centre(26, 20)), 1.0,
+			"you can see across a %s" % MapGrid.Kind.keys()[kind])
+	g.set_tile(23, 20, MapGrid.Kind.PILLAR)
+	sight.compute(g, eye, VIEW)
+	assert_eq(sight.light_at(_centre(26, 20)), 0.0, "but not past a pillar")
+
+
+func test_a_streamed_wall_is_noticed() -> void:
+	# The corner list is cached between frames, so a chunk arriving with a new
+	# wall in it has to invalidate it. A stale list does not lose the shadow --
+	# rays read the tiles live -- it loses the EDGE: the new pillar's corners get
+	# no rays of their own, and its silhouette is drawn only as finely as the
+	# sweep, two degrees at a time. So the check is the edge, not the shadow.
+	var g := _room()
+	var eye := _centre(20, 20)
+	sight.compute(g, eye, VIEW)
+	g.set_tile(22, 18, MapGrid.Kind.PILLAR)
+	sight.compute(g, eye, VIEW)
+	var corner := Vector2(23.0, 19.0) * T
+	var away := (corner - eye).normalized()
+	var side := -away.orthogonal()
+	var on_line := eye + away * 300.0
+	assert_eq(sight.light_at(on_line + side * 2.0), 1.0,
+		"the new pillar's edge is as sharp as one that was always there")
+	assert_eq(sight.light_at(on_line - side * 2.0), 0.0)
+
+
+func test_light_falls_off_monotonically_along_every_ray() -> void:
+	var g := _room()
+	g.set_tile(23, 18, MapGrid.Kind.PILLAR)
+	g.set_tile(17, 23, MapGrid.Kind.WALL)
+	var eye := _centre(20, 20)
+	sight.compute(g, eye, VIEW)
+	for i in 64:
+		var dir := Vector2.from_angle(TAU * float(i) / 64.0)
+		var last := 1.0
+		var r := 4.0
+		while r < VIEW + 20.0:
+			var l := sight.light_at(eye + dir * r)
+			assert_lte(l, last + 0.0001, "light never comes back along a ray")
+			last = l
+			r += 6.0
+
+
+func test_the_shadow_mesh_is_well_formed() -> void:
+	var g := _room()
+	g.set_tile(23, 18, MapGrid.Kind.PILLAR)
+	sight.compute(g, _centre(20, 20), VIEW)
+	var mesh := sight.shadow_mesh(VIEW + 64.0, Color(0.05, 0.05, 0.08))
+	var points: PackedVector2Array = mesh["points"]
+	var colors: PackedColorArray = mesh["colors"]
+	var indices: PackedInt32Array = mesh["indices"]
+	assert_eq(points.size(), sight.angles.size() * 3)
+	assert_eq(colors.size(), points.size(), "a colour per vertex")
+	assert_eq(indices.size() % 3, 0, "whole triangles")
+	for k in indices:
+		assert_between(k, 0, points.size() - 1)
+	for c in colors:
+		assert_true(c.a == 0.0 or c.a == 1.0, "clear at the fade's start, dark beyond")
+
+
+func test_neighbouring_rays_are_never_far_apart() -> void:
+	# The outline between two rays is a straight chord. If the gap were wide, the
+	# chord would cut inside the circle of sight on open ground, and a wedge of
+	# darkness would appear where nothing is in the way.
+	var g := _room()
+	sight.compute(g, _centre(20, 20), VIEW)
+	var widest := 0.0
+	var n := sight.angles.size()
+	for i in n:
+		var gap := wrapf(sight.angles[(i + 1) % n] - sight.angles[i], 0.0, TAU)
+		widest = maxf(widest, gap)
+	var dip := (VIEW - Visibility.FADE) * (1.0 - cos(widest * 0.5))
+	assert_lt(dip, 1.0,
+		"the chord dips %.2f units into open ground; under a pixel is invisible" % dip)
+	# And the behaviour that follows from it: halfway between two rays, right at
+	# the start of the fade, open ground is still fully lit.
+	var mid := (sight.angles[0] + sight.angles[1]) * 0.5
+	assert_eq(sight.light_at(_centre(20, 20)
+		+ Vector2.from_angle(mid) * (VIEW - Visibility.FADE - 1.0)), 1.0)
+
+
+func test_a_real_dungeon_is_cheap_enough_to_do_every_frame() -> void:
+	# Not a benchmark -- the machine running the suite varies -- but a guard
+	# against the ray count exploding: corners are the variable part, and a
+	# map with thousands of them in view would be a regression worth knowing.
+	var inst := Instance.make_dungeon(2, 4242, 1, Dungeons.STANDARD)
+	sight.compute(inst.world.map, inst.world.spawn_point, VIEW)
+	assert_lt(sight.angles.size(), 1200,
+		"%d rays for one frame of a depth-1 dungeon" % sight.angles.size())

tests/unit/test_visibility.gd.uid

@@ -0,0 +1 @@
+uid://beivpa4pxiu3f

tools/screenshot.gd

@@ -29,7 +29,12 @@ const OPEN_UPGRADES := 330
 const AT_UPGRADES := 360
 const OPEN_ROSTER := 370
 const AT_ROSTER := 400
-const DONE := 410
+const ENTER_DUNGEON := 410
+const AT_DUNGEON := 480
+const TO_TUNNEL := 490
+const AT_TUNNEL := 550
+const AT_TUNNEL_DEBUG := 560
+const DONE := 570
 
 var _frame: int = 0
 var _game: Node = null
@@ -44,6 +49,10 @@ func _ready() -> void:
 	var from_env := OS.get_environment("SHOT_DIR")
 	if not from_env.is_empty():
 		_dir = from_env
+		# Made rather than assumed. The example in CLAUDE.md points at a fresh
+		# /tmp/shots, and a missing directory fails every capture one at a time
+		# with an error that names the file rather than the folder.
+		DirAccess.make_dir_recursive_absolute(_dir)
 	get_window().size = Vector2i(1280, 720)
 	_menu = preload("res://src/ui/main_menu.gd").new()
 	add_child(_menu)
@@ -118,6 +127,23 @@ func _process(_delta: float) -> void:
 			_show_the_roster()
 		AT_ROSTER:
 			_grab("roster")
+		ENTER_DUNGEON:
+			# The fog is what these two are for, and the hub is the worst place to
+			# judge it: one big room with little in it to cast a shadow.
+			_game.set("_roster_open", false)
+			Net.server._send_to_dungeon(Net.LOCAL_PEER)
+		AT_DUNGEON:
+			_grab("dungeon")
+		TO_TUNNEL:
+			_stand_in_a_tunnel()
+		AT_TUNNEL:
+			_grab("tunnel")
+			# The same view with F1's overlay: the outline is where each fog ray
+			# stopped, which is the quickest way to tell a rendering mistake from
+			# a geometry one.
+			_game.get_node("WorldView").debug_hitboxes = true
+		AT_TUNNEL_DEBUG:
+			_grab("tunnel_debug")
 		DONE:
 			Net.shutdown()
 			get_tree().quit(0)
@@ -152,6 +178,46 @@ func _show_the_roster() -> void:
 	_game.get_node("CharacterSelect").refresh(roster, "c4")
 
 
+## Move the player, on both sides, into the tunnel closest to where it spawned:
+## somewhere with walls close on two sides and a corner in view, which is where
+## the old fog looked worst.
+func _stand_in_a_tunnel() -> void:
+	var inst := Net.server.instance_of(Net.LOCAL_PEER)
+	if inst == null or inst.rooms.is_empty():
+		return
+	var map := inst.world.map
+	var start: Rect2i = inst.rooms[0]
+	var spawn := map.to_tile(inst.world.spawn_point)
+	var best := Vector2i(-1, -1)
+	var best_d := INF
+	for ty in map.height:
+		for tx in map.width:
+			if map.at(tx, ty) != MapGrid.Kind.FLOOR:
+				continue
+			var here := Vector2i(tx, ty)
+			var in_room := inst.boss_room.has_point(here)
+			for r: Rect2i in inst.rooms:
+				if r.grow(1).has_point(here):
+					in_room = true
+			if in_room:
+				continue
+			# A couple of tiles clear of the first room, so the tunnel's walls
+			# are on screen and not just the doorway.
+			if start.grow(3).has_point(here):
+				continue
+			var d := float((here - spawn).length_squared())
+			if d < best_d:
+				best_d = d
+				best = here
+	if best.x < 0:
+		return
+	var at := map.tile_centre(best.x, best.y)
+	Net.client.predicted_pos = at
+	var p: SimPlayer = inst.world.players.get(Net.LOCAL_PEER)
+	if p != null:
+		p.pos = at
+
+
 ## One roster row in the shape decode_characters produces.
 func _roster_entry(who: String, level: int, ceiling: int, current: int,
 		tint: Color) -> Dictionary: