CKey Format v0.28.0 — Profile 0.1: the print-robust layout

Status: draft. Added alongside Profile 0 (Format 0.27), which is unchanged.

Encoders emit frame format byte 0x1C (decimal 28), profile byte 0x00. The layout family is the print family (CKEY_LAYOUT_PRINT).

Why this layout exists

Profile 0's polar field is finer than its modules. Rays are 0.2 module wide, ring dots 0.4 module across, and ring slots are as little as 0.3 module apart on the inner rings. Printed small, those marks are the first to go:

Dark ink on paper: ink spread merges neighbouring ring dots.
Inked background: spread closes the light rays.

Profile 0.1 keeps every mark and every gap at about one module or more. The module can then shrink to whatever the printer and camera allow. In print simulations, the same payload prints at roughly half Profile 0's width (see "Measured" below).

Differences from Format 0.27

Everything not listed here is identical to Format 0.27: the skeleton, frame, finders, Grid Key, version table G(v), safe-area rule, Reed–Solomon framing and levels, header layout, and interleaving.

	Format 0.27 (P0)	Format 0.28 (P0.1)
Ring radial pitch	0.86p	1.0p
Rings R(v)	table in 0.27	2 3 3 3 4 4 4 5 5 6 7 7 8 8 9 10 11 12 13 14 15 16 17 19 20 22 24 25 27 29 32 (= ⌊(4.2 − 2.4p)/p⌋ + 1)
Slots per ring	one count S for all rings	equal arc: ring k at radius r gets 8·⌊2πr/(8p)⌋ slots, so dots are at least one module apart on every ring
Ring dot radius	0.2p	0.3p
Dial slots	S (≈ 72/p)	8·⌊2π·7.6/(8p)⌋ (≈ 48/p; rays at least one module apart)
Dial ray width	0.2p	0.35p
Long / short ray	2.2p / 1.0p	2.4p / 1.0p
Data ray outer ink (for the safe area)	7.6 + 2.3p	7.6 + 2.575p
Ray-Key bearings on rings	ring slots cleared on the three Ray Key slots	not cleared: rings have their own slot counts
Ring Key	16 slots: dots K, K+2, K+6; run K+7.5…K+15.5	8 slots: dots K, K+2; run K+3.5…K+7.5
Grid Key, v1	row 10	row 9, giving the longer rays a pitch of clearance
Placement key / whitening seed	`0x434B5030` / `0x434B5730` + v	`0x434B5031` / `0x434B5731` + v
Frame format byte	`1B`	`1C`

Normative and derived values:

The ring-count table is normative. For some versions it lands exactly on an integer boundary, so implementations must use the table.
Slot counts are derived and decided with at least 0.002 slack, so independent implementations agree.

Ink-to-ink gaps in every version: ring dots at least 0.39p along and across rings, and dial rays at least 0.6p apart. The test suite checks both.

Capacity (payload bytes)

v	Modules across	L	M	Q	H
1	29	11	7	3	0
2	31	21	17	9	3
3	33	31	23	15	7
4	36	35	27	19	9
5	38	46	36	26	16
6	41	62	50	36	22
7	44	75	61	45	29
8	47	98	80	60	40
9	50	116	96	72	48
10	54	152	124	94	64
11	58	191	157	119	81
12	62	221	185	141	97
13	67	274	230	174	122
14	71	295	245	189	131
15	77	367	303	235	163
16	82	433	361	277	193
17	88	518	428	332	232
18	94	608	506	392	278
19	101	694	574	446	314
20	108	814	678	526	374
21	116	958	798	618	438
22	124	1076	900	696	496
23	133	1272	1056	816	588
24	143	1504	1266	978	692
25	153	1705	1423	1103	783
26	164	2017	1677	1299	919
27	176	2318	1944	1504	1064
28	188	2655	2235	1719	1225
29	202	3056	2576	1990	1420
30	216	3564	2986	2306	1648
31	232	4123	3453	2679	1919

Reading

The decoder tests both layout families' key patterns (Grid, Ring and Ray Keys at each version's own pitch), so the family is identified the same way the version is. The frame's format byte confirms it. In the test suite, all 31 Profile 0.1 versions decode at 6 px per module and are identified as Profile 0.1.

Measured (simulation, informative)

The print model is binary device dots, ink spread (dot gain), paper blur, then a camera of a given resolution, then the decoder. The table gives the smallest width that decodes reliably at level M, P0 → P0.1:

Payload	Old inkjet, plain paper	Inkjet, photo paper	Weaker camera (12 px/mm), plain paper
8 B	0.38 → 0.19 in	0.21 → ≤ 0.15 in	0.47 → 0.24 in
30 B	0.42 → 0.24 in	0.29 → ≤ 0.15 in	0.56 → 0.29 in
120 B	0.68 → 0.32 in	0.38 → 0.21 in	0.68 → 0.42 in
400 B	0.91 → 0.56 in	0.62 → 0.38 in	1.22 → 0.75 in
1,200 B	1.47 → 0.91 in	1.11 → 0.56 in	1.78 → 1.22 in
2,400 B	2.37 → 1.34 in	1.34 → 0.83 in	2.61 → 1.62 in

The model's parameters:

Old inkjet, plain paper: 600 dpi, 0.045 mm spread.
Photo paper: 1,200 dpi, 0.015 mm spread.
Camera: 23 px/mm with 0.7 px blur.

These are estimates. ckey print-sheet produces the calibration ladder that replaces them with measurements.

Conformance vectors

FNV-1a-64 over every site in placement order of (identity little-endian, printed value), for payload "CKey renderer test" at level M:

Version	Hash
v3	`1908543413F369B8`
v31	`684C56624D3558A8`

CKey-Format-v0.28.0.md