README.md

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`c90277fa` 2026-10-09 Pavel Safronov	1	# Nordic Skating
	2
	3	Live — <https://pavelsafronov.gitlab.io/nordic-skating/> · [Source](https://gitlab.com/PavelSafronov/nordic-skating)
	4
	5	An endless skate across an endless lake of thin black ice, for a phone. The ice sings
	6	under the blades the way real black ice does, and its pitch tells you how thin it is.
	7	Leave it alone and it skates itself; steer, and you choose where the lake forks and
	8	how close to the thin ice you dare go.
	9
	10	Inspired by National Geographic's
	11	[Hear the Otherworldly Sounds of Skating on Thin Ice](https://www.youtube.com/watch?v=v3O9vNi-dkA),
	12	in which Mårten Ajne skates a small lake outside Stockholm.
	13
`e5499331` 2026-10-09 Pavel Safronov	14	<img src="docs/screenshots/landscape.png" alt="Black ice laced with white cracks, running between snowy banks of spruce to a foggy horizon" width="640" />
	15
`c90277fa` 2026-10-09 Pavel Safronov	16	## Status
	17
`df2b8215` 2026-10-10 Pavel Safronov	18	Step 5 of 6, and a good deal since. The lake runs on for ever between snowy, wooded
	19	shores, forking every few kilometres, pinching to narrow passes, opening out until no
	20	shore can be seen, and dotted in places with islets of reeds to weave between. The skater skates it, and the ice sings under every stride at
`2f7e86b7` 2026-10-10 Pavel Safronov	21	a pitch set by how thick it is there. Thin ice is black, sings high and cracks under
	22	you. Nobody falls through: how thick the ice is decides only how it sounds.
	23
	24	Every visit is to a different lake, one of a million, by day, at dusk, at night under
`0045befd` 2026-10-10 Pavel Safronov	25	the Milky Way, or under the northern lights; in clear air, in falling snow, or in a
	26	blizzard; with, some nights, a comet, and some days an aeroplane drawing its trail.
`2f7e86b7` 2026-10-10 Pavel Safronov	27
	28	<img src="docs/screenshots/aurora.png" alt="The lake at night under green curtains of aurora" width="640" />
`c90277fa` 2026-10-09 Pavel Safronov	29
	30	1. ✅ Scaffold: build, tests, deploy to GitLab Pages.
`e5499331` 2026-10-09 Pavel Safronov	31	2. ✅ An endless lake, skated on autopilot, with steering.
`9ff4c3fc` 2026-10-09 Pavel Safronov	32	3. ✅ Measure the reference video, then synthesize the ice sound from the ice's thickness.
`646446cd` 2026-10-09 Pavel Safronov	33	4. ✅ Ice zones that fall out of the physics, and falling through.
`1cb1fefc` 2026-10-09 Pavel Safronov	34	5. ✅ Forks in the lake, narrow passes, and stretches of open sea.
`e3bfc714` 2026-10-10 Pavel Safronov	35	6. Game controller, then multiplayer. Postponed: [PLAN.md](PLAN.md) has what comes first.
`c90277fa` 2026-10-09 Pavel Safronov	36
`e5499331` 2026-10-09 Pavel Safronov	37	## Controls
	38
`6e58e276` 2026-10-10 Pavel Safronov	39	The button at the top right turns the sound on and off, as does the M key, and shows
`3f3d6bce` 2026-10-11 Pavel Safronov	40	which it is: a speaker, or a speaker crossed out. Sound cannot start by itself: a
`6e58e276` 2026-10-10 Pavel Safronov	41	browser lets a page make sound only after it has been touched, so until then the
`3f3d6bce` 2026-10-11 Pavel Safronov	42	speaker is crossed out, and the first tap or key anywhere starts it. Turning it off is remembered for next time. On an iPhone the
`95f8bb94` 2026-10-09 Pavel Safronov	43	ring/silent switch also silences it.
`9ff4c3fc` 2026-10-09 Pavel Safronov	44
`1cb1fefc` 2026-10-09 Pavel Safronov	45	Do nothing and the skater follows the middle of the lake, round every bend and down
	46	one side of every fork, for as long as you leave it.
`e5499331` 2026-10-09 Pavel Safronov	47
	48	\| To \| Touch or mouse \| Keyboard \|
	49	\|---\|---\|---\|
`fe6b506c` 2026-10-10 Pavel Safronov	50	\| Steer left or right \| Drag sideways from wherever you touch; or tilt the phone, with Tilt on \| ← → or A D \|
	51	\| Tilt to steer, on or off \| The Tilt button at the bottom left (phones and tablets only) \| \|
`df2b8215` 2026-10-10 Pavel Safronov	52	\| Turn right round, or circle an islet \| Hold the steer over \| the same \|
`b2c4db9f` 2026-10-10 Pavel Safronov	53	\| Choose a way at a fork, or a side of an island \| A touch towards it on the way up \| the same \|
`95f8bb94` 2026-10-09 Pavel Safronov	54	\| Sound on or off \| The button at the top right \| M \|
`3f3d6bce` 2026-10-11 Pavel Safronov	55	\| Go faster or slower \| The pace button at the bottom right: each press is the next pace up, and from the fastest back to the slowest \| ↑ ↓ or W S \|
`2f7e86b7` 2026-10-10 Pavel Safronov	56	\| Go to another lake \| Randomize, at the top left \| R \|
	57
	58	At the top left, About opens a box that explains the app and its controls, and
	59	Settings one that changes it. The lake waits behind either, skater and sound both,
	60	until it is closed: by its button, by Escape, or by a touch outside it.
	61
	62	Randomize goes to another lake, picked at random, at once and without reloading
	63	the page. The browser's Back button returns to the lake you left, and Forward goes on
	64	again.
	65
`8b498d51` 2026-10-10 Pavel Safronov	66	## Install
	67
	68	The app can be put on a phone's home screen, where it has its own icon, opens full
	69	screen without the browser's bars, and works with no network.
	70
	71	- Android (Chrome): the menu, then "Add to Home screen" or "Install app".
	72	- iPhone (Safari): the Share button, then "Add to Home Screen".
	73
	74	Nothing is fetched once it is installed: the whole app is kept on the phone and the
	75	lake is computed. A new version is taken up the next time it is opened with a network.
	76
`2f7e86b7` 2026-10-10 Pavel Safronov	77	## Settings
	78
	79	Each takes effect as it is touched, behind the box, and is remembered for next time.
	80
	81	\| Setting \| Choices \| To begin with \|
	82	\|---\|---\|---\|
	83	\| Time of day \| Surprise me, Day, Dusk, Night, Aurora \| Surprise me: whichever the lake comes with \|
`0045befd` 2026-10-10 Pavel Safronov	84	\| Weather \| Surprise me, Clear, Snow, Blizzard \| Surprise me: whichever the lake comes with \|
`3f3d6bce` 2026-10-11 Pavel Safronov	85	\| Pace \| Meander, Cruise, Brisk, Racing, Olympic \| Cruise \|
`2f7e86b7` 2026-10-10 Pavel Safronov	86	\| Impossible ice \| on or off \| off \|
	87	\| Lake \| Another lake (the same as Randomize); a link back to this one \| a different lake every visit \|
	88
	89	<img src="docs/screenshots/settings.png" alt="The settings box at the foot of the screen, under a night sky" width="320" />
	90
`9f2e2509` 2026-10-10 Pavel Safronov	91	### Tilt to steer
	92
`3f3d6bce` 2026-10-11 Pavel Safronov	93	On a phone or tablet there is a Tilt button at the bottom left, showing a phone held
	94	in a hand when tilt is on and a phone by itself when it is off; anywhere else it is not
	95	shown. Turned on, which is remembered, the phone is leaned
`9f2e2509` 2026-10-10 Pavel Safronov	96	left or right like a wheel, about the line the skater looks along: more lean, tighter
	97	turn, full steering at 28°. Within 4° of level counts as level, since nobody holds a
	98	phone dead level, and held level the skater skates itself. It works held upright or on
	99	its side, and dragging still works alongside it.
	100
`fe6b506c` 2026-10-10 Pavel Safronov	101	While tilt is on the screen is stopped from turning with the phone, since leaning a
	102	phone over is exactly what makes its screen flip. A browser does that only for a page
	103	that fills the screen: the installed app already does, and in a browser tab the page
	104	goes full screen while tilt is on and comes out of it when tilt is turned off. An
	105	iPhone will do neither for a web page, so there the phone's own rotation lock, in
	106	Control Centre, is the way.
	107
`9f2e2509` 2026-10-10 Pavel Safronov	108	An iPhone asks leave for this, each visit, at the first touch. If it has been refused
`df03cd8b` 2026-10-10 Pavel Safronov	109	it is not asked again, and the app says so where its hint is: Settings, Safari, Motion &
	110	Orientation Access turns it back on.
`9f2e2509` 2026-10-10 Pavel Safronov	111	Browsers give a page the phone's tilt only over HTTPS, which the live site is and
	112	`npm run dev` on a phone is not, so it cannot be tried on a phone from the dev server.
	113
`2f7e86b7` 2026-10-10 Pavel Safronov	114	### A different lake every visit
	115
	116	A lake is a number from 1 to 999,999, and everything about it follows from the number:
`0045befd` 2026-10-10 Pavel Safronov	117	its bends, widths and forks, its ice, its trees, the time of day and the weather it
	118	comes with, which way its wind blows, and what is in its sky.
`2f7e86b7` 2026-10-10 Pavel Safronov	119	Opening the app picks a number, and so do Randomize and reloading the page: each is a
	120	new lake under a new sky. Settings shows the number and a link to it, and `?seed=1234`
	121	on the address asks for a lake by number, which is how to come back to one or send it
	122	to someone.
`95f8bb94` 2026-10-09 Pavel Safronov	123
`2f7e86b7` 2026-10-10 Pavel Safronov	124	Which lake a page is showing is kept with its place in the browser's history, not in
	125	its address. That is what lets Back return to the lake that was there while a reload,
	126	which is someone asking for something new, gets a new one. Randomize takes any
	127	`?seed=` off the address, since it no longer says which lake this is.
`e5499331` 2026-10-09 Pavel Safronov	128
`2f7e86b7` 2026-10-10 Pavel Safronov	129	### Times of day
	130
	131	\| Time \| The sky \| How often a lake comes with it \|
	132	\|---\|---\|---\|
	133	\| Day \| Flat, bright winter noon \| 35% \|
	134	\| Dusk \| An orange horizon under deepening blue, and the first stars \| 20% \|
	135	\| Night \| Moonless and clear: the Milky Way and thousands of stars \| 27% \|
	136	\| Aurora \| The same night under slow green curtains, violet where they reach highest \| 18% \|
	137
	138	The sky is painted by a shader (`src/render/sky.ts`), and each time has its own light:
	139	the snow is orange at dusk, blue at night and faintly green under the aurora, and the
	140	milkiness of thick ice shows as much as there is light to show it. `?time=night` on the
	141	address asks for a time whatever the settings say.
	142
	143	<img src="docs/screenshots/night.png" alt="The lake at night, the Milky Way overhead" width="640" />
	144
`0045befd` 2026-10-10 Pavel Safronov	145	### What else is in the sky
	146
	147	- Shooting stars, at night and under the aurora: one every quarter of a minute or
	148	so, somewhere in the sky, gone in under a second.
	149	- A comet, over one night lake in three: ahead and low, with a tail.
	150	- An aeroplane, over two day or dusk lakes in three: a bright speck crossing the
	151	sky ahead in four or five minutes, drawing a trail that widens and fades behind it,
	152	white by day and pink at dusk. When it has gone, a while later another comes.
	153
	154	<img src="docs/screenshots/comet.png" alt="A comet with a long tail over the lake at night" width="640" />
	155
	156	### Weather
	157
	158	\| Weather \| Snow \| How far you can see \| The sky \| Wind \| How often a lake comes with it \|
	159	\|---\|---\|---\|---\|---\|---\|
	160	\| Clear \| none \| 600 m \| as the time of day has it \| none \| 64% \|
	161	\| Snow \| falling quietly \| 420 m \| mostly cloud \| 0.5 to 3 m/s \| 26% \|
	162	\| Blizzard \| thick, and driven \| 200 m \| all cloud \| 7 to 16 m/s, in gusts \| 10% \|
	163
`893cda77` 2026-10-10 Pavel Safronov	164	Snow that falls lies, in drifts (`src/render/drifts.ts`): long pale streaks on the
	165	ice, all running the way that lake's wind blows, a few of them under falling snow and
	166	many in a blizzard. A drift is the lake's bad ice. In the middle of one a skater is
	167	dragged at four times as hard as on ice and slows to about half speed; the ice under
	168	it does not ring to a strike or sing to the glide, and the blade shushes instead. Bare
	169	ice between the drifts is as fast and as loud as ever, so in a blizzard there is a
	170	reason to pick a way.
	171
	172	Snow in the air is a box of flakes that stays round the skater (`src/render/snow.ts`): they fall,
`0045befd` 2026-10-10 Pavel Safronov	173	the wind carries them, and they stream past as the skater moves, which is most of what
	174	makes a blizzard feel fast. Each lake's wind blows from a direction of its own. The wind
	175	can be heard, louder and higher in its gusts. Cloud hides the stars, the aurora and
	176	whatever else is up there, and takes the colour out of a sunset. `?weather=snow` on the
	177	address asks for a weather whatever the settings say.
	178
	179	<img src="docs/screenshots/snow.png" alt="Snow falling over the lake by day" width="640" />
	180
`3f3d6bce` 2026-10-11 Pavel Safronov	181	A game controller works as well: either stick or the direction buttons steer, up
	182	and down on the direction buttons or the shoulder buttons change pace, Y or triangle
	183	goes to another lake, and X or square turns the sound on and off. It needs no
`893cda77` 2026-10-10 Pavel Safronov	184	setting up beyond being connected, though a browser shows a page a controller only
	185	once one of its buttons has been pressed.
	186
`2f7e86b7` 2026-10-10 Pavel Safronov	187	Steering turns the skater: the harder the steer, the tighter the curve, up to a circle
`df2b8215` 2026-10-10 Pavel Safronov	188	16 m in radius, with a moment to lean into it. Held over, the skater turns right round
	189	and can skate back up the lake, or round and round; that needs open water, about 24 m
	190	of it on the side being turned towards. A shore cannot be reached; near one the skater
`c2835cab` 2026-10-10 Pavel Safronov	191	is turned to run along it.
`2f7e86b7` 2026-10-10 Pavel Safronov	192
	193	The autopilot does not skate a straight line. It carves long, slow S-curves down the
`c2835cab` 2026-10-10 Pavel Safronov	194	lake, 6 m either side of wherever its line is centred.
	195
	196	Steering changes the autopilot's plan; it does not interrupt it. Let go, and the
	197	skater carries on the way you sent them, from where they had got to: the autopilot's
	198	line is re-centred there and its next curve begins in your direction, wide at first if
	199	you sent it off at an angle, and settling back to its usual 6 m over about twenty
	200	seconds. It does not return to where it was, or to the middle of the lake. Steer left
	201	out of a right-hand curve and you have a left-hand curve.
`2f7e86b7` 2026-10-10 Pavel Safronov	202
`3f3d6bce` 2026-10-11 Pavel Safronov	203	The speed breathes with the stride: each push adds about a seventh of it, and the glide
	204	after it gives that back. Strides are of different lengths, and no two the same, so
	205	neither are the speeds.
	206
	207	### Pace
	208
	209	\| Pace \| Speed \| Strides \| Turning circle \| \|
	210	\|---\|---\|---\|---\|---\|
	211	\| Meander \| 3.5 m/s, 13 km/h \| 2 to 4.5 s, long and lazy \| 8 m \| Standing tall. Tight enough to circle an islet closely \|
	212	\| Cruise \| 7 m/s, 25 km/h \| 1.4 to 3.2 s \| 16 m \| Where it starts \|
	213	\| Brisk \| 10 m/s, 36 km/h \| 1 to 2.4 s \| 22 m \| \|
	214	\| Racing \| 12.5 m/s, 45 km/h \| 0.8 to 1.9 s \| 31 m \| \|
	215	\| Olympic \| 15 m/s, 54 km/h \| 0.6 to 1.4 s, short and quick \| 45 m \| Down in a crouch. The pace of a long-track speed skater in a 1500 m, kept up for ever \|
	216
	217	A change of pace is worked up to, or eased down to, over a few strides. Everything
	218	else follows the speed. A faster skater is held to half a g sideways and so turns in a
	219	wider circle; looks further ahead, and steers round an islet from further off; keeps
	220	further from the shores, which in ordinary water leaves a racer little room to weave;
	221	swings wider on the autopilot's line; sees a wider view, which is most of what makes
	222	speed look like speed; and hears the air rush past. The pace is remembered, and
	223	`?pace=olympic` on the address asks for one for a visit.
`2f7e86b7` 2026-10-10 Pavel Safronov	224
	225	The screen is kept awake while the page is open, where the browser allows it.
`e5499331` 2026-10-09 Pavel Safronov	226
`df2b8215` 2026-10-10 Pavel Safronov	227	At a fork the lake parts round an island. A touch of steering one way, from 200 m
`c2835cab` 2026-10-10 Pavel Safronov	228	before the fork, is taken as asking for that side, and the autopilot does the rest. With
	229	no touch it takes the side it is already over towards, and down the middle, the side
	230	the fork prefers. Until the island begins, a little over
`df2b8215` 2026-10-10 Pavel Safronov	231	200 m after the fork, you can still change, by steering the other way. After that there
	232	is land between, though a skater who turns back can go round its tip.
`1cb1fefc` 2026-10-09 Pavel Safronov	233
`646446cd` 2026-10-09 Pavel Safronov	234	## The ice
	235
`2f7e86b7` 2026-10-10 Pavel Safronov	236	How thick the ice is decides how it looks and the note it sings. It does not decide
	237	whether it holds: for now nobody falls through, and the whole lake is safe to skate.
	238
	239	The zones are not chosen. They are the three thresholds of Gold's formula for what a
	240	sheet of ice will carry (`src/ice/strength.ts`), worked out for an 80 kg skater, and the
	241	note is the physics under [The sound](#the-sound).
`646446cd` 2026-10-09 Pavel Safronov	242
	243	\| Ice \| Thickness \| Looks \| Sings at \| What happens \|
	244	\|---\|---\|---\|---\|---\|
`6e58e276` 2026-10-10 Pavel Safronov	245	\| Impossible, if turned on \| 0.8 to 2.2 cm \| bright pink, a colour nothing on a lake is, strongest where it is thinnest \| 1390 to 3800 Hz \| Nothing: it sings, very high, and never cracks. Real ice this thin would not hold anyone \|
`2f7e86b7` 2026-10-10 Pavel Safronov	246	\| Cracks \| 2.2 to 3.4 cm \| nearly black \| 900 to 1390 Hz \| It cracks around you: more often and nearer the thinner it gets \|
`646446cd` 2026-10-09 Pavel Safronov	247	\| Sings \| 3.4 to 4.8 cm \| dark grey \| 630 to 900 Hz \| Sound ice near its limit. The ice of the film is here, at 4.1 cm \|
	248	\| Safe \| over 4.8 cm \| milky grey \| below 630 Hz \| Nothing. The thicker, the lower and plainer the note \|
	249
`2f7e86b7` 2026-10-10 Pavel Safronov	250	Of the ice a skater can reach, about 30% cracks, 35% sings and 35% is safe.
	251
	252	Impossible ice is a setting, and off to begin with. Off, the thinnest ice is 2.2 cm, the
	253	thinnest that is real, in patches that are on the point of giving way all the way
`5e8b9530` 2026-10-10 Pavel Safronov	254	across and crack every second or so. On, those patches go on thinning to 8 mm at
`6e58e276` 2026-10-10 Pavel Safronov	255	their middles and fall silent: about 4% of the ice a skater can reach. They are bright
	256	pink, as bright at midnight as at noon: lit from within, in a colour chosen because
	257	nothing on a lake is that colour.
`2f7e86b7` 2026-10-10 Pavel Safronov	258
	259	Falling through is written and switched off (`RULES` in `src/skater.ts`). Switched on,
	260	ice under 2.2 cm gives way and the skater is in the water for four seconds. It needs one
	261	thing first: a line the autopilot can be sure of, since nothing now keeps thin ice off
	262	its path.
`646446cd` 2026-10-09 Pavel Safronov	263
`2f7e86b7` 2026-10-10 Pavel Safronov	264	<img src="docs/screenshots/thin-ice.png" alt="Milky grey ice down the lake, with a black patch of the thinnest ice to the left of the middle" width="640" />
`646446cd` 2026-10-09 Pavel Safronov	265
`e5499331` 2026-10-09 Pavel Safronov	266	## How the lake works
	267
`1cb1fefc` 2026-10-09 Pavel Safronov	268	- The lake is a function, not a map (`src/world/lake.ts`). Where a channel's middle
	269	is, how wide it is and how the land rises from its shores are all computed from the
	270	distance skated and a seed. Any stretch can be made on demand, the same on every
	271	device, and nothing is stored.
	272	- It forks. A channel runs from the fork that made it to the fork that ends it: the
	273	first fork is 700 m from the start, and each one after comes 1.8 to 4.2 km after the
	274	last. A fork's two channels start as the one channel and draw 180 m apart over 600 m,
	275	which is always more than both are wide, so there is always an island. Each has a
	276	width and forks of its own from there on. Both are drawn for 1.2 km, and then only the
	277	one taken: the other is out of sight behind the island by then.
	278	- It changes character over kilometres. About 70% of the way it is ordinary water,
	279	12 to 44 m from the middle to a shore. About 10% of the way it pinches to a pass 5 m
	280	to a side. And now and then it opens until both shores are beyond the haze, 700 m off
	281	or more: a stretch of sea. Near a fork it is always ordinary, so that a fork reads as
	282	one.
	283
`df2b8215` 2026-10-10 Pavel Safronov	284	- It has islets. Tufts of reeds on humps of frozen ground, 5 to 15 m across,
	285	standing in the ice as they do in the film, where the skater weaves between them.
	286	They come in beds: about a quarter of the lake's half-kilometres have some, one every
	287	50 m or so, and the rest have none. Each has at least 9 m of ice between it and a
	288	shore, and there are none in a narrow pass or near a fork. The autopilot goes round
	289	them; a skater steering straight at one is turned aside at the last moment.
	290
`b2c4db9f` 2026-10-10 Pavel Safronov	291	- It has long islands. Low banks of reeds, 180 to 480 m long and up to 32 m across,
	292	standing in the middle of a channel with water both sides: the channel parts round
	293	one and is one again beyond it. The water either side is as wide as the channel
	294	would have been, so the lake swells where an island is. A touch of steering on the
	295	way up to one chooses the side; with none, the island has a side it prefers. About
	296	one every 5 km, and none near a fork or in a pass.
`94b3b29f` 2026-10-10 Pavel Safronov	297	- Its ice is cracked the way ice cracks (`src/ice/cracks.ts`). A few long cracks
	298	run nearly straight for tens of metres; later ones start from the side of an earlier
	299	one, leave it at a wide angle, and stop where they meet another, since a crack cannot
	300	cross a crack. So the cracks are a network of long runs joined at T's, with finer ones
	301	between, and each is painted as it looks in clear ice: a bright edge at the surface
	302	and its face going down into the ice beside it.
`b2c4db9f` 2026-10-10 Pavel Safronov	303	- It keeps the marks made on it (`src/render/marks.ts`). The skates leave a line,
	304	and each push a scratch to one side and then the other; on ice thin enough to crack,
`94b3b29f` 2026-10-10 Pavel Safronov	305	a crack opens where each near one is heard from, and a push on such ice starts more
	306	from under the skate. A crack is a star of a few arms that fork, and it runs out from
	307	its middle over a moment. Every mark is a hairline, a pixel wide however near. They
	308	stay where they were made, so a skater who turns back or circles an islet comes on
	309	their own tracks, as in the film; the lake remembers the last few thousand stretches
	310	of line and forgets the ones before.
`b2c4db9f` 2026-10-10 Pavel Safronov	311
`1cb1fefc` 2026-10-09 Pavel Safronov	312	<img src="docs/screenshots/fork.png" alt="The lake parting round a wooded island ahead" width="640" />
	313
`b2c4db9f` 2026-10-10 Pavel Safronov	314	<img src="docs/screenshots/island.png" alt="A long island of reeds ahead in the middle of the lake" width="640" />
	315
`df2b8215` 2026-10-10 Pavel Safronov	316	<img src="docs/screenshots/islets.png" alt="An islet of reeds standing in the ice, with more beyond it" width="640" />
	317
`1cb1fefc` 2026-10-09 Pavel Safronov	318	<img src="docs/screenshots/sea.png" alt="Ice to a hazy horizon with no shore in sight" width="640" />
	319
`2f7e86b7` 2026-10-10 Pavel Safronov	320	- The skater has a place, a direction and a speed (`src/skater.ts`), and steering is
	321	turning. The autopilot steers by the same means and within the same limits, aiming at
	322	a point 18 m ahead on its wandering line, so every path on the lake is an arc. The
`df2b8215` 2026-10-10 Pavel Safronov	323	skater may point any way at all, back up the lake included; left alone, the autopilot
	324	turns them down it again.
`2f7e86b7` 2026-10-10 Pavel Safronov	325	- The skater is kept on the ice. They have the width of the channel less 2 m at each
`df2b8215` 2026-10-10 Pavel Safronov	326	shore. Within 24 m of the edge of that they may point less and less towards it, down
	327	to not at all at the edge. Where the lake narrows faster than a skater could steer
	328	clear, their room starts closing 200 m early, whichever way they are going. Out at sea they may go no further than 150 m from the middle.
`646446cd` 2026-10-09 Pavel Safronov	329	- Thickness is a function too, and is painted on the ice from a small map of the 768 m
	330	around the skater that is redrawn every 32 m (`src/render/ice.ts`).
`1cb1fefc` 2026-10-09 Pavel Safronov	331	- The skater never leaves the origin of what is drawn. The shores and islands are
	332	built 100 m at a time ahead of the skater, thrown away behind, and placed relative to the skater
`e5499331` 2026-10-09 Pavel Safronov	333	every frame (`src/render/shores.ts`); the ice is one sheet whose texture slides the
	334	other way (`src/render/ice.ts`). So nothing drawn is ever a large number, and the
	335	picture is as sharp 10,000 km down the lake as at the start.
	336
`c90277fa` 2026-10-09 Pavel Safronov	337	## The sound
	338
`9ff4c3fc` 2026-10-09 Pavel Safronov	339	The sound is synthesized in the browser. None of it is a recording.
	340
	341	### The physics
	342
	343	A sheet of ice floating on water carries bending waves, and they are dispersive: the
	344	higher the frequency, the faster it travels. The two sounds of thin ice follow from that
	345	(`src/ice/acoustics.ts`).
	346
	347	- The singing note. A bending wave slower than sound in air cannot radiate into the
	348	air at all. The ice gives off sound most strongly at the one frequency whose bending
	349	wave travels exactly at the speed of sound in air, where the wave in the ice and the
	350	wave in the air keep step. That frequency is very nearly **30.3 Hz·m divided by the
	351	thickness**: 4 cm of ice sings at about 760 Hz, 3 cm at about 1010 Hz, 6 cm at about
	352	505 Hz. The pitch is a thickness gauge: the higher it sings, the thinner the ice.
	353	- The chirps. A crack is every frequency at once. From some way off the high ones
	354	arrive first and the low ones after, ending on the note: a falling "pew". The further
	355	the crack, the longer the fall — about a third of a second from 400 m.
	356
	357	The model is from Romeyn, Hanssen, Ruud and Johansen,
	358	[Sea ice thickness from air-coupled flexural waves](https://doi.org/10.5194/tc-15-2939-2021),
	359	The Cryosphere 15, 2939–2955 (2021), who took the sound of the very film this app is
	360	modelled on and read about 725 Hz and 4.3 cm of ice from it. With textbook values for
	361	freshwater ice and nothing tuned, the model here gives 4.2 cm for 725 Hz.
	362
	363	### What the film measures
	364
	365	`npm run analyse-video` measures a local copy of the film (it is not in this repository):
	366
	367	\| \| Measured \| Used for \|
	368	\|---\|---\|---\|
	369	\| The note \| median 740 Hz; 664–826 Hz over the middle 90% \| Ice 4.1 cm thick, 3.7–4.6 cm; the middle of the lake's range of thickness \|
	370	\| Time between skate strikes \| median 0.66 s \| Not used: at that rate the view rocks from leg to leg without rest. Strides here are long glides of 1.4 to 3.2 s, each a different length \|
	371	\| How fast a ring dies \| 36 dB a second \| The ring's decay: to 1/e in 0.24 s \|
	372
`8b498d51` 2026-10-10 Pavel Safronov	373	The chirps cannot be checked against the film. Three ways of doing it were tried,
	374	and the last is still in the tool: split each burst of sound into frequency bands, time
	375	when each band arrives, and see whether higher bands arrive earlier by as much as the
	376	model says. Of 122 bursts, 119 show no spread to measure: every band arrives within
	377	30 ms of every other. The other three do not agree with each other or with the model.
	378	The likely reason is that the film's microphones were close to the skater, and a sound
	379	from close by has had no distance to spread out over. So the film gives the note and
	380	not the chirp. The chirps here are the model's, which is the paper's, and the paper
	381	measured the spreading directly, with instruments on the ice.
`9ff4c3fc` 2026-10-09 Pavel Safronov	382
	383	### What is synthesized
	384
	385	`src/audio/iceSound.ts`, from noise and sine waves only:
	386
	387	- The ring: at each stride, every 1.4 to 3.2 s, a skate strikes the ice, which answers with a narrow band
	388	of noise at its note, starting a little high and falling onto it, then dying away.
	389	- The hum and the scrape: the gliding blade keeps the note faintly alive and hisses.
`0045befd` 2026-10-10 Pavel Safronov	390	- The wind, when there is one: low, loose noise, louder and higher in its gusts.
`893cda77` 2026-10-10 Pavel Safronov	391	- Snow under the blade, in a drift: a soft, low shush in place of the ring, the hum
	392	and the scrape.
`9ff4c3fc` 2026-10-09 Pavel Safronov	393	- The cracks: every few seconds the ice cracks somewhere between 40 and 600 m off,
`5e8b9530` 2026-10-10 Pavel Safronov	394	and the chirp for that distance and that thickness arrives. The falling chirp is a
	395	sine wave and stops the moment it lands on the note; what rings on after it is the
	396	ice, in the same band of noise as after a strike. (Left to ring by itself the sine was
	397	a clean, struck, high note, like a sonar ping.) On ice thin enough to crack under the
	398	skater the cracks come from a few metres away, and faster the thinner it is, up to
	399	one every second or so.
`2f7e86b7` 2026-10-10 Pavel Safronov	400	Impossible ice does not crack at all.
`c90277fa` 2026-10-09 Pavel Safronov	401
`9ff4c3fc` 2026-10-09 Pavel Safronov	402	`npm run sound-check` renders twelve seconds of it without playing it and draws its
	403	spectrogram: six seconds on the film's 4.1 cm, then onto ice thinning to 2.8 cm, with
	404	three cracks.
`c90277fa` 2026-10-09 Pavel Safronov	405
`9ff4c3fc` 2026-10-09 Pavel Safronov	406	<img src="docs/sound/synth.png" alt="Spectrogram: a bright narrow band near 740 Hz struck every two seconds or so and fading between, three thin lines falling steeply onto it, and the band climbing past 1 kHz in the last six seconds" width="800" />
`c90277fa` 2026-10-09 Pavel Safronov	407
	408	## Develop
	409
	410	Needs Node 22.
	411
	412	```sh
	413	npm install
	414	npm run dev # dev server on every interface, with a QR code for a phone on the same Wi-Fi
	415	npm test # vitest
	416	npm run build # typecheck, then build into dist/
	417	npm run preview # serve dist/
`e5499331` 2026-10-09 Pavel Safronov	418	npm run screenshots # build, then photograph the app at phone size into docs/screenshots/
`9ff4c3fc` 2026-10-09 Pavel Safronov	419	npm run sound-check # render the synthesized sound and draw its spectrogram into docs/sound/
	420	npm run analyse-video # measure a local copy of the reference film
`8b498d51` 2026-10-10 Pavel Safronov	421	npm run icons # remake the home-screen and tab icons from assets/skate-shoes.png
`c90277fa` 2026-10-09 Pavel Safronov	422	```
	423
`2f7e86b7` 2026-10-10 Pavel Safronov	424	On the address: `?seed=1` asks for lake number 1, `?time=night` for a time of day (`day`,
`3f3d6bce` 2026-10-11 Pavel Safronov	425	`dusk`, `night`, `aurora`), `?weather=snow` for a weather (`clear`, `snow`, `blizzard`), `?pace=olympic` for a pace
	426	(`meander`, `cruise`, `brisk`, `racing`, `olympic`),
`0045befd` 2026-10-10 Pavel Safronov	427	and `?s=2500` starts that many metres down the lake. The
`2f7e86b7` 2026-10-10 Pavel Safronov	428	screenshots use all three, so that they are of the same thing every run.
`e5499331` 2026-10-09 Pavel Safronov	429
`9ff4c3fc` 2026-10-09 Pavel Safronov	430	The screenshots and the sound check need Playwright's Chromium once:
	431	`npx playwright install chromium`. The sound check and the analysis need ffmpeg.
`e5499331` 2026-10-09 Pavel Safronov	432
`c90277fa` 2026-10-09 Pavel Safronov	433	## Deploy
	434
	435	`.gitlab-ci.yml` tests and builds every push, and publishes the default branch to
	436	GitLab Pages. Asset paths are relative (`vite.config.ts`), so the build works at
	437	`/nordic-skating/` or at the root of a unique Pages domain without configuration.
	438
	439	Static files go in `static/`, not `public/`: GitLab Pages needs its artifact to be
	440	named `public`, and a `public/` already in the repository would collide with it.
	441
`e3bfc714` 2026-10-10 Pavel Safronov	442	## Licence
	443
`3f3d6bce` 2026-10-11 Pavel Safronov	444	[MIT](LICENSE), for everything here but the icons. What it builds on is credited below.
`e3bfc714` 2026-10-10 Pavel Safronov	445
`c90277fa` 2026-10-09 Pavel Safronov	446	## Attribution
	447
`3f3d6bce` 2026-10-11 Pavel Safronov	448	- The app's icon and the pictures on its buttons are by
	449	[Icongeek26](https://www.flaticon.com/authors/icongeek26), from Flaticon, used under
	450	the Flaticon free licence, which requires attribution. The credit is in the app's
	451	About box as well as here. None of them is covered by this project's MIT licence.
	452	- Of those, the app's icon is a skate. The artwork is `assets/skate-shoes.png`; the icons in `static/` are made from it by `npm run icons`,
`8b498d51` 2026-10-10 Pavel Safronov	453	which uses only its shape: pale on the colour of black ice for a home screen, and
	454	as it was drawn, black on a see-through background, for a browser's tab (white where
`3f3d6bce` 2026-10-11 Pavel Safronov	455	the browser is dark). The same skate is on the pace button.
	456	- And the pictures on the sound button, a speaker and a speaker crossed out, are
	457	`assets/sound-on.png` and `assets/sound-off.png`. On a button only a picture's shape
	458	is used, in the colour of the button's words.
	459	- The pictures on the Tilt button, a phone held in a hand for on and a phone by itself
	460	for off, are `assets/tilt-on.png` and `assets/tilt-off.png`. **Whether these two are
	461	by the same author has not been confirmed**; if they are not, the credit here and in
	462	the About box needs their author added.
`c90277fa` 2026-10-09 Pavel Safronov	463	- [three.js](https://threejs.org/) — MIT licence.
`e5499331` 2026-10-09 Pavel Safronov	464	- [simplex-noise](https://github.com/jwagner/simplex-noise.js) — MIT licence.
`8b498d51` 2026-10-10 Pavel Safronov	465	- [Workbox](https://developer.chrome.com/docs/workbox), by way of
	466	[vite-plugin-pwa](https://vite-pwa-org.netlify.app/) — MIT licence. It is the service
	467	worker that keeps the app on the phone.
`9ff4c3fc` 2026-10-09 Pavel Safronov	468	- The physics of the sound: Romeyn, R., Hanssen, A., Ruud, B. O., and Johansen, T. A.,
	469	[Sea ice thickness from air-coupled flexural waves](https://doi.org/10.5194/tc-15-2939-2021),
	470	The Cryosphere 15, 2939–2955, 2021 (CC BY 4.0).
`646446cd` 2026-10-09 Pavel Safronov	471	- What ice will carry: Gold, L. W., "Use of ice covers for transportation", *Canadian
	472	Geotechnical Journal* 8, 170–181, 1971.
`c90277fa` 2026-10-09 Pavel Safronov	473	- The National Geographic video above is the reference this is modelled on. It is
	474	copyrighted, and neither it nor any audio taken from it is part of this repository
	475	or the app: it is measured locally, and the app's sound is synthesized from those
	476	measurements.