open-standards/wai/extensions/format-probes.md

WAI Extension: Format Probes

Status: Draft. A header probe for each sink-supplied format the registry names whose payload WAI does not decode itself: it identifies a payload as the format its capability names and reads the parameters the format's headers state, or refuses it with one of the codes of §12. No probe decodes a picture or a sound. Probes are defined for JPEG XS (wai.image.jxs, wai.video.jxs), H.265 and MV-HEVC (wai.video.hevc, wai.video.mvhevc), H.266 (wai.video.vvc), LCEVC (wai.video.lcevc), MPEG-H 3D Audio (wai.audio.mpegh), AVS3 video and audio (wai.video.avs3, wai.audio.avs3), the picture header of IEEE 1857.11 (wai.image.ieee1857_11, a candidate) and the projection descriptor (wai.meta.projection, projection). §3.5 maps a JPEG XS payload onto the media-type parameters of RFC 9134 and the constant-rate carriage of SMPTE ST 2110-22. The reference implementation is the format_probe module of wai-rs (feature format_probe) and its tool wai_format_probe; the corpus is format-probe-conformance/, and its second implementation is format-probe-conformance/format_probe_verify.py. Keywords MUST, MUST NOT, SHOULD, MAY are RFC 2119/8174.

Texts read for this revision: ITU-T H.265 (V11, 01/2026) and ITU-T H.266 (V4, 01/2026), whole; ITU-T H.273 and ITU-T H.274 (V4, 01/2026) §8.15; RFC 9134; SMPTE ST 2110-22:2022; the public previews of ISO/IEC 21122-2:2022/Amd 1:2022 (Tables A.7 and A.16) and ISO/IEC 21122-4:2022 (§6.3, Annex B.2); ATSC A/342-3:2024-04 §5.2; ETSI TS 101 154 V2.10.1 §5.16.2 and §6.10.2. Not openly readable, and so not read: ISO/IEC 21122-1, ISO/IEC 23094-2, ISO/IEC 23008-3, T/AI 109.2, T/AI 109.3 and IEEE 1857.11-2024. For those formats §3 to §10 say where each layout was read instead.

1. Scope

§1.1 A probe reads a payload's headers. It identifies the format by the bytes the format begins with, where it has such bytes (§2.4), reads the parameters its headers state, and checks the structure the probe reads: the order of the format's required structures, the lengths they declare, and the values the format forbids or reserves among the fields read. It reads no coded sample data: a payload a probe accepts is not shown to decode, and a probe that accepts a payload says nothing of the pictures or sounds in it.

§1.2 What a probe gives is used to describe a payload without decoding it: in a catalogue, in a manifest a packager writes, in the codecs string or media-type parameters a delivery binding states (§3.5), and to refuse a payload that does not hold the format its capability names before any decoder sees it.

§1.3 An encoder MUST NOT emit, under a capability whose probe has basis standard or wai (§2.3), a payload that probe refuses; it SHOULD NOT emit one that a probe of basis implementation refuses. A sink MAY probe a payload before it dispatches it (SPEC §4); a sink that probes and gets a refusal does not dispatch the payload under that capability, and may still take dispatch step 5 with a fallback. A probe's refusal is not a statement that the payload does not conform to its format's standard: a probe of basis implementation reads the layout its sources give, and §12's outside_probe names payloads it cannot read at all.

§1.4 This extension adds no capability. It completes the registrations of wai.video.vvc, wai.video.lcevc, wai.audio.mpegh, wai.video.avs3 and wai.audio.avs3, which were registered with no probe, and gives the registrations of wai.image.jxs, wai.video.jxs, wai.video.mvhevc, wai.image.ieee1857_11 and wai.meta.projection (SPEC §5) theirs.

2. Common rules

§2.1 A probe's result is a JSON object whose members each section names; a member the payload does not state is null. Integers are JSON numbers, flags JSON booleans. The probe reports the first refusal its reading meets, in the order each section reads.

§2.2 A bit reader reads most significant bit first. u(n) is an n-bit unsigned integer; ue(v) is the Exp-Golomb code of ITU-T H.265 §9.2, and one with more than 31 leading zero bits is reserved_value. Reading past the payload's end is truncated.

§2.3 Every probe has a basis: standard, where its format's own text is openly readable and was read; implementation, where it is not and the layout was read from open implementations, each section naming which agree; wai, for a format this specification defines.

§2.4 The probes, in order:

capability	basis	section
wai.image.jxs	implementation	§3
wai.video.jxs	implementation	§3.3
wai.video.hevc	standard	§4
wai.video.mvhevc	standard	§4
wai.video.vvc	standard	§5
wai.video.lcevc	implementation	§6
wai.audio.mpegh	implementation	§7
wai.video.avs3	implementation	§8
wai.audio.avs3	implementation	§9
wai.image.ieee1857_11	implementation	§10
wai.meta.projection	wai	§11

Any other capability has no probe: no_probe.

§2.5 Annex B byte streams. A payload of H.265, H.266 or LCEVC is a byte stream: after any zero bytes, it begins with the start code prefix 0x000001, and otherwise is no_signature (one zero byte before the prefix is not enough: at least two). A NAL unit is the bytes after a prefix up to the next prefix, less the zero bytes that end it. Its payload is read with every emulation-prevention byte removed: the 0x03 of every 0x000003 (ITU-T H.265 §7.4.2).

3. JPEG XS

§3.1 Sources. ISO/IEC 21122-1 is not openly readable. The marker codes, the segment layouts and the order of §3.2 are those two independent open implementations of the codestream agree on, a decoder and encoder under BSD-2-Clause-Patent and a decoder under MIT, at the revisions format-probe-conformance/tools/make_external.sh pins. The profile codes are ISO/IEC 21122-2:2022/Amd 1:2022 Table A.7, and the sublevel codes its Table A.16, read in the amendment's public preview. Level codes are named only where the two open readings agree (§3.4).

§3.2 The codestream. A wai.image.jxs payload is one codestream. A probe reads, in order:

1. SOC, 0xFF10, as the first two bytes; otherwise no_signature.
2. CAP, 0xFF50, immediately after, and otherwise out_of_order; its length Lcap, a big-endian u16 of at least 2 (else length_mismatch), counts itself and Lcap − 2 bytes of capability bits, read most significant bit first: bit i set is reported in cap_bits.
3. PIH, 0xFF12, immediately after CAP, else out_of_order, with Lpih 26, else length_mismatch. Its body, big-endian: Lcod 32 bits, Ppih 16, Plev 16, Wf 16, Hf 16, Cw 16, Hsl 16, Nc 8, Ng 8, Ss 8, Bw 8, Fq 4, Br 4, Fslc 1, Ppoc 3, Cpih 4, Nlx 4, Nly 4, Lh 1, Rl 1, Qpih 2, Fs 2, Rm 2. It is reserved_value unless Wf, Hf and Hsl are at least 1, Nc is 1 to 8, Ng is 4, Ss is 8, Br is 4, Cpih is 0, 1 or 3, and Qpih is 0 or 1.
4. Segments up to the first slice header SLH, 0xFF20, each a marker, a big-endian u16 length of at least 2 (else length_mismatch) and that many bytes less two: CDT 0xFF13, WGT 0xFF14, COM 0xFF15, NLT 0xFF16, CWD 0xFF17, CTS 0xFF18, CRG 0xFF19, in any order. CDT's length is 2 + 2·Nc, else length_mismatch; it holds, per component, its precision B (8 bits) and its sampling factors Sx and Sy (4 bits each), none of which is 0 (else reserved_value). SOC, EOC, PIH or CAP among these segments is out_of_order; any other marker is reserved_value.
5. CDT and WGT are required before SLH, and with Cpih 3 also CTS and CRG: missing_structure. The payload ends with EOC, 0xFF11: otherwise missing_structure.
6. Lcod is 0 (a variable-rate codestream) or the codestream's length in bytes, SOC to EOC; otherwise length_mismatch.

The probe does not read the slices.

§3.3 Video. A wai.video.jxs payload is five u32 little-endian values, n, h, w, fps_num and fps_den, then n codestreams, each preceded by its length as a u32 little-endian value: the framing of wai.video.av1 (SPEC §5), with a codestream for a temporal unit. Any of the five being 0 is framing_invalid; a length past the payload's end is truncated; each codestream is probed by §3.2, and one whose Wf exceeds w or whose Hf exceeds h is framing_invalid; bytes after the n-th codestream are trailing_bytes.

§3.4 Parameters. For one codestream: profile and level (Ppih and Plev as four lowercase hexadecimal digits after 0x), width and height (Wf, Hf), components (per component [B, Sx, Sy]), structure, colour_transform (Cpih), decompositions ([Nlx, Nly]), precinct_width (Cw), slice_height (Hsl), wavelet_precision (Bw), fractional_bits (Fq), quantizer (Qpih), long_header (Lh), raw_mode (Rl), cbr_bytes (Lcod), cap_bits, and rtp (§3.5). structure is "4:4:4", "4:2:2" or "4:2:0" when there are at least three components, the first has Sx = Sy = 1 and the second and third have equal factors of (1, 1), (2, 1) or (2, 2) respectively; otherwise null.

For a video: frames (n), size ([w, h]), fps ([fps_num, fps_den]), constant_bytes (the common Lcod when every codestream states the same non-zero Lcod, otherwise null) and first, the first codestream's parameters.

The profile names (white space omitted, as RFC 9134 §7.1 writes them), by Ppih:

Ppih	profile	Ppih	profile
0x0000	Unrestricted	0x4240	High420.12
0x1500	Light422.10	0x4A40	High444.12
0x1A00	Light444.12	0x4E40	High4444.12
0x2500	Light-Subline422.10	0x6EC0	MLS.12
0x3240	Main420.12	0x9300	LightBayer
0x3540	Main422.10	0xB340	MainBayer
0x3A40	Main444.12	0xC340	HighBayer
0x3E40	Main4444.12

The level, by Plev's high byte: 0x00 Unrestricted, 0x10 2k-1, 0x20 4k-1, 0x24 4k-2, 0x28 4k-3, 0x30 8k-1, 0x34 8k-2, 0x38 8k-3, 0x40 10k-1. The high bytes 0x04 and 0x2C are named 1k-1 and 5k-1 by one open implementation and refused as reserved by the other; the probe names them too, and §3.6 records the disagreement. The sublevel, by Plev's low byte: 0x00 Unrestricted, 0x80 Full, 0x10 Sublev12bpp, 0x0C Sublev9bpp, 0x08 Sublev6bpp, 0x06 Sublev4bpp, 0x04 Sublev3bpp, 0x03 Sublev2bpp. A code in none of these lists is reported with a null name and is not refused: a later edition may define it.

§3.5 RTP and SMPTE ST 2110-22. RFC 9134 carries a JPEG XS codestream in RTP; SMPTE ST 2110-22:2022 carries constant-bit-rate compressed video over RTP and names no codec. A sender of a wai.video.jxs payload over RFC 9134:

• states the media type video/jxsv with the parameters of rtp (RFC 9134 §7.1): profile, level and sublevel by the names of §3.4 (omitted where null), depth (the largest component precision), width and height, and sampling where the codestream determines it: RGB where structure is 4:4:4 under a colour transform (Cpih 1 or 3), whose decoder outputs R, G and B; otherwise rtp.sampling is null and the sender states the sampling, since a codestream does not say whether its components are Y′C′BC′R, constant-luminance Y′C′BC′R or ICTCP (RFC 9134 §7.1 lists all three);
• sends each codestream as one picture segment of codestream mode (K = 0, RFC 9134 §4.1) at the frame rate fps, the RTP timestamp at 90 kHz (RFC 9134 §4.2), or each slice in slice mode (K = 1);
• for SMPTE ST 2110-22 (§4 there: a constant number of bytes, and of RTP packets, per frame), sends only a payload whose constant_bytes is not null; the codestream's Lcod is the frame's byte count.

A receiver rebuilds the payload by framing each received codestream as §3.3 does, fps from the session's exactframerate. WAI defines no transport (moq-streaming-format §6); this mapping says which of a payload's fields fill which of RFC 9134's.

§3.6 Exactness. A sink that advertises wai.image.jxs or wai.video.jxs decodes at the strict conformance point of ISO/IEC 21122-4 (Annex B.2: decoded images bitwise identical to the reference images), not only at its relaxed point (PSNR bounds), so every conforming sink outputs the same samples; the registry class is standard-defined on that basis. What was measured (format-probe-conformance/provenance.json): the open decoder's C path and SIMD path give byte-identical output on every JPEG XS vector of the corpus, on arm64; the second, independent decoder gives the same bytes on every vector whose profile it implements, five of six; it does not implement MLS.12 (it refuses Ppih 0x6EC0), and on a copy of that vector with Ppih and Plev set to 0 its output differs, while the first decoder's output equals the vector's source picture byte for byte, as a lossless codestream's must. No x86 decode was measured.

4. H.265 and MV-HEVC

§4.1 Sources. ITU-T H.265 (V11, 01/2026): the NAL unit header (§7.3.1.2), the video parameter set and its extension (§F.7.3.2.1, §F.7.3.2.1.1, derivations (F-2) to (F-6)), the sequence parameter set (§7.3.2.2.1), profile_tier_level (§7.3.3, inference §F.7.4.4) and the HRD parameters (§E.2.2, §E.2.3).

§4.2 NAL units. Each NAL unit (§2.5) is at least 2 bytes (else truncated); its header is forbidden_zero_bit 1, nal_unit_type 6, nuh_layer_id 6, nuh_temporal_id_plus1 3 bits. forbidden_zero_bit 1 or nuh_temporal_id_plus1 0 is reserved_value. Types 0 to 31 are VCL. The probe reads the first VPS (type 32) and the first SPS (type 33) with nuh_layer_id 0. A VCL NAL unit before both is out_of_order if both occur later in the payload and missing_structure if either never does; a payload with no VCL NAL unit is missing_structure.

§4.3 The VPS. In order: vps_video_parameter_set_id u(4), vps_base_layer_internal_flag, vps_base_layer_available_flag, vps_max_layers_minus1 u(6), vps_max_sub_layers_minus1 u(3) (above 6 is reserved_value), vps_temporal_id_nesting_flag, vps_reserved_0xffff_16bits (any other value is reserved_value), profile_tier_level(1, vps_max_sub_layers_minus1), the sub-layer ordering information, vps_max_layer_id u(6), vps_num_layer_sets_minus1 ue(v) (above 1023 is reserved_value), the layer-id flags, the timing information with its HRD parameters (vps_num_hrd_parameters above vps_num_layer_sets_minus1 + 1 is reserved_value; cpb_cnt_minus1 above 31 is reserved_value), and vps_extension_flag. When the flag is 1 and vps_max_layers_minus1 is above 0, the probe reads the alignment bits (each 1, else reserved_value) and vps_extension( ) up to and including its profile_tier_level( ) structures: splitting_flag, the sixteen scalability_mask_flags, the dimension lengths, the layer ids (each above the one before, else reserved_value), the dimension ids, view_id_len and the view ids, the direct dependency flags, num_add_layer_sets and its highest_layer_idx_plus1 fields, the sub-layer and inter-layer temporal fields, default_ref_layers_active_flag, vps_num_profile_tier_level_minus1 (above 63 is reserved_value) and the structures it counts. A profile_tier_level( ) is read as §7.3.3 lays it out, its general profile part as 88 bits whatever its branches (the syntax's own note). A structure with no profile takes its profile, tier and compatibility flags from the structure before it (§F.7.4.4).

§4.4 The SPS of the base layer: sps_video_parameter_set_id u(4), sps_max_sub_layers_minus1 u(3) (above 6 is reserved_value), sps_temporal_id_nesting_flag, profile_tier_level(1, ·), sps_seq_parameter_set_id ue(v) (above 15 is reserved_value), chroma_format_idc ue(v) (above 3 is reserved_value), separate_colour_plane_flag when it is 3, pic_width_in_luma_samples and pic_height_in_luma_samples ue(v) (0 is reserved_value), the conformance window, and bit_depth_luma_minus8 and bit_depth_chroma_minus8 ue(v) (above 8 is reserved_value). The cropped size is the coded size less the window, whose offsets count SubWidthC and SubHeightC samples (2 and 2 at 4:2:0, 2 and 1 at 4:2:2, 1 and 1 otherwise); a window as large as the picture is reserved_value.

§4.5 MV-HEVC. A wai.video.mvhevc payload is also refused not_multiview unless the VPS extension's scalability mask has bit 1 (multiview, Table F.1), NumViews (F-3) is at least 2, and a VCL NAL unit with nuh_layer_id above 0 is present.

§4.6 Parameters. profile_idc, tier, level_idc (the base SPS's), chroma_format_idc, bit_depth ([luma, chroma]), coded and cropped ([width, height]), layers (Min(62, vps_max_layers_minus1) + 1), layers_present (the nuh_layer_ids of the VCL NAL units, ascending), and, when the VPS extension was read, multilayer: scalability_mask (bit i for scalability_mask_flag[ i ]), layer_ids, view_order and aux (ViewOrderIdx and AuxId per layer), views (NumViews when the multiview bit is set, else 1), view_ids (view_id_val, or zeros when view_id_len is 0), profiles (general_profile_idc of each extension structure, after inference) and multiview_profile (whether any of them indicates profile 6, Multiview Main or Multiview Main 10, by its idc or its compatibility flag).

5. H.266

§5.1 Sources. ITU-T H.266 (V4, 01/2026) §7.3.1.2, §7.3.2.3, §7.3.2.4, §7.3.3.1 and §7.3.3.2.

§5.2 NAL units. The header is forbidden_zero_bit 1, nuh_reserved_zero_bit 1, nuh_layer_id 6, nal_unit_type 5, nuh_temporal_id_plus1 3 bits. Either of the first two set, nuh_temporal_id_plus1 0 or nuh_layer_id above 55 is reserved_value. Types 0 to 11 are VCL. The probe reads the first SPS (type 15) and the first VPS (type 14), of which it reads vps_max_layers_minus1 (the second field, after a 4-bit id). A VCL NAL unit before the SPS is out_of_order if an SPS occurs later and missing_structure if none does; no VCL NAL unit is missing_structure.

§5.3 The SPS to its conformance window: sps_seq_parameter_set_id u(4), sps_video_parameter_set_id u(4), sps_max_sublayers_minus1 u(3) (above 6 is reserved_value), sps_chroma_format_idc u(2), sps_log2_ctu_size_minus5 u(2) (above 2 is reserved_value), sps_ptl_dpb_hrd_params_present_flag and, when it is 1, profile_tier_level(1, sps_max_sublayers_minus1), sps_gdr_enabled_flag, sps_ref_pic_resampling_enabled_flag and the flag it conditions, sps_pic_width_max_in_luma_samples and sps_pic_height_max_in_luma_samples ue(v) (0 is reserved_value), and the conformance window. profile_tier_level( ) is read as §7.3.3.1 lays it out; general_constraints_info( ) is gci_present_flag, then, when it is 1, 71 bits of constraints, gci_num_additional_bits u(8) and that many bits, then zero bits to the byte boundary (a one among them is reserved_value). The cropped size is computed as in §4.4.

§5.4 Parameters. profile_idc, tier, level_idc, frame_only (ptl_frame_only_constraint_flag) and multilayer_enabled (ptl_multilayer_enabled_flag), all null when the SPS carries no profile_tier_level( ); sub_profiles (general_sub_profile_idcs); chroma_format_idc; ctu_size (1 << (sps_log2_ctu_size_minus5 + 5)); coded; cropped; layers (vps_max_layers_minus1 + 1, or 1 without a VPS); layers_present.

6. LCEVC

§6.1 Sources. ISO/IEC 23094-2 is not openly readable. The NAL unit header, the block structure and the two configurations are those an open decoder (BSD-3-Clause-Clear) and an open syntax reading under LGPL-2.1 agree on, each citing the standard's tables; the resolution table is the standard's Table 20 as the decoder reproduces it. The two readings disagree on the byte that extends a profile or level of 15; the probe does not read it (outside_probe).

§6.2 NAL units. A wai.video.lcevc payload is the byte stream of SPEC §5 (§2.5). Each NAL unit is at least 3 bytes (else truncated). Its first byte is forbidden_zero_bit (0), forbidden_one_bit (1), nal_unit_type (5 bits) and the first bit of reserved_flag (1); its second byte is the other eight bits of reserved_flag, all 1. Any other value of those bits is reserved_value. nal_unit_type is 29 (IDR) or 28 (non-IDR), else reserved_value; the first NAL unit is IDR, else out_of_order. The payload, emulation prevention removed, ends with the byte 0x80, else reserved_value.

§6.3 Blocks. The payload before that byte is a sequence of blocks: a header byte, its high three bits payload_size_type and its low five payload_type, then the body. A size type of 0 to 5 is the body's length; 6 is reserved_value; 7 is followed by the length as a multi-byte value: seven bits a byte, most significant first, each byte but the last with its top bit set, at most ten bytes (else reserved_value). A body past the payload's end is length_mismatch. Types: 0 sequence configuration, 1 global configuration, 2 picture configuration, 3 and 4 encoded data, 5 additional information, 6 filler; the probe reads the first 0 and the first 1, counts the 2s, and passes over the rest. A picture configuration before both configurations is out_of_order; a payload without both is missing_structure.

§6.4 The configurations. The sequence configuration: profile_idc 4, level_idc 4, sublevel_idc 2, conformance_window_flag 1, five reserved bits that are 0 (else reserved_value); a profile or level of 15 is outside_probe; with the flag, four multi-byte window offsets (left, right, top, bottom); bytes left in the body are length_mismatch. The global configuration: processed_planes_type_flag 1, resolution_type 6, transform_type 1, chroma_sampling_type 2, base_depth_type 2, enhancement_depth_type 2, temporal_step_width_modifier_signalled_flag 1, predicted_residual_mode_flag 1, temporal_tile_intra_signalling_enabled_flag 1, temporal_enabled_flag 1, upsample_type 3, level1_filtering_signalled_flag 1, scaling_mode_level1 2, scaling_mode_level2 2, tile_dimensions_type 2, user_data_enabled 2, level1_depth_flag 1, chroma_step_width_flag 1; then, each where its condition holds: a byte of planes type (the planes flag), a byte of step width modifier (its flag), four 16-bit kernel coefficients (upsample_type 4), a byte of two filter coefficients (the filtering flag), for tile_dimensions_type above 0 the custom tile width and height as 16-bit values when it is 3 and then a byte of tile compression, and, for resolution_type 63, the custom width and height as 16-bit values. resolution_type 0 or 51 to 62, an upsample_type above 4, or a custom width or height of 0 is reserved_value.

§6.5 Parameters. nal_units ([IDR, non-IDR] counts), pictures (picture configurations), profile_idc, level_idc, sublevel_idc, conformance_window (the four offsets as signalled, or null), resolution_type, size (Table 20's for types 1 to 50, the custom size for 63), transform_type, chroma_sampling_type, base_depth and enhancement_depth (8 + 2·type), temporal_enabled, upsample_type, tile_dimensions_type.

Table 20, types 1 to 50: 360×200, 400×240, 480×320, 640×360, 640×480, 768×480, 800×600, 852×480, 854×480, 856×480, 960×540, 960×640, 1024×576, 1024×600, 1024×768, 1152×864, 1280×720, 1280×800, 1280×1024, 1360×768, 1366×768, 1400×1050, 1440×900, 1600×1200, 1680×1050, 1920×1080, 1920×1200, 2048×1080, 2048×1152, 2048×1536, 2160×1440, 2560×1440, 2560×1600, 2560×2048, 3200×1800, 3200×2048, 3200×2400, 3440×1440, 3840×1600, 3840×2160, 3840×2400, 4096×2160, 4096×3072, 5120×2880, 5120×3200, 5120×4096, 6400×4096, 6400×4800, 7680×4320, 7680×4800.

7. MPEG-H 3D Audio

§7.1 Sources. ISO/IEC 23008-3 is not openly readable. The MHAS packet syntax, the packet types and the first fields of the configuration are those two independent open decoders agree on; the low-complexity profile-level codes 0x0B to 0x0D are confirmed by ATSC A/342-3:2024-04 §5.2.2.1. Neither open decoder is under a licence this corpus can use, so the corpus's MHAS vectors are written by its generator (§13.2).

§7.2 Packets. A wai.audio.mpegh payload is an MHAS stream: packets, each a header of MHASPacketType = escapedValue(3, 8, 8), MHASPacketLabel = escapedValue(2, 8, 32) and MHASPacketLength = escapedValue(11, 24, 24), then that many bytes. escapedValue(n1, n2, n3) reads v = u(n1); when v is 2^n1 − 1 it reads a = u(n2) and adds it; when a is 2^n2 − 1 it reads u(n3) and adds that too. Every header is a whole number of bytes. The first packet is a synchronisation packet (type 6) or a configuration packet (type 1), else no_signature; a packet past the end is truncated. A synchronisation packet has label 0 and the one byte 0xA5, else reserved_value. A frame packet (type 2) before the first configuration is out_of_order; a payload with no configuration is missing_structure. Other types are passed over.

§7.3 The configuration. The first configuration packet's payload: the profile-level indication u(8); usacSamplingFrequencyIndex u(5), its rate by the table below, or, for 31, a rate u(24) (0 is reserved_value); coreSbrFrameLengthIndex u(3); one reserved bit; receiverDelayCompensation u(1); speakerLayoutType u(2); then, for type 0, CICPspeakerLayoutIdx u(6), and for types 1 and 2 the speaker count, escapedValue(5, 8, 16) + 1. Rates by index 0 to 30: 96000, 88200, 64000, 48000, 44100, 32000, 24000, 22050, 16000, 12000, 11025, 8000, 7350, reserved, reserved, 57600, 51200, 40000, 38400, 34150, 28800, 25600, 20000, 19200, 17075, 14400, 12800, 9600, reserved, reserved, reserved; a reserved index is reserved_value.

§7.4 Parameters. profile_level; profile and level: low-complexity 1 to 5 for 0x0B to 0x0F, baseline 1 to 5 for 0x10 to 0x14, else null; sampling_rate; core_frame_length_index; speaker_layout_type; cicp_layout; speakers; and the counts packets, frames, syncs and configs.

8. AVS3 video

§8.1 Sources. T/AI 109.2 is not openly readable. The sequence header and the pseudo-start-code prevention are read as an open decoder (BSD-3-Clause) reads them, with the start codes it and an open syntax reading under LGPL-2.1 agree on. ETSI TS 101 154 V2.10.1 §5.16.2 requires the library flags to be 0 and confirms the profile codes 0x20, 0x22 and 0x32.

§8.2 Units. A wai.video.avs3 payload begins, after any zero bytes (at least two), with the prefix 0x000001, else no_signature. A unit is the byte after a prefix (its start code) and the bytes up to the next prefix. A unit's bytes are read with the prevention bits removed: in each byte 0x02 that follows two zero bytes of the unit, its two low bits. Start codes: 0xB0 sequence header, 0xB1 sequence end, 0xB3 intra picture, 0xB6 inter picture. A payload with no sequence header is missing_structure; one whose first unit is not one is out_of_order; one with no intra picture is missing_structure.

§8.3 The sequence header. profile_id u(8), level_id u(8), progressive_sequence u(1), field_coded_sequence u(1), library_stream_flag u(1) (1 is outside_probe), library_picture_enable_flag u(1) (1 is outside_probe: a flag follows whose place the open texts do not establish), marker_bit, horizontal_size u(14), marker_bit, vertical_size u(14), chroma_format u(2), sample_precision u(3), encoding_precision u(3) for profiles 0x22 and 0x32 only, marker_bit, aspect_ratio u(4), frame_rate_code u(4), marker_bit, bit_rate_lower u(18), marker_bit, bit_rate_upper u(12), low_delay u(1), temporal_id_enable_flag u(1), marker_bit, bbv_buffer_size u(18), marker_bit, max_dpb_size_minus1 u(4). A marker_bit of 0 is reserved_value, and so are a size, chroma format or sample precision of 0 and a frame rate code of 0 or above 13.

§8.4 Parameters. profile_id, level_id, progressive, field_coded, size, chroma_format, sample_precision, encoding_precision (or null), aspect_ratio, frame_rate_code, bit_rate (bit_rate_upper · 2^18 + bit_rate_lower), low_delay, temporal_id_enabled, bbv_buffer_size, max_dpb_size, pictures ([intra, inter] counts) and sequence_end (whether a sequence end unit is present).

9. AVS3 audio

§9.1 Sources. T/AI 109.3 is not openly readable. The frame header is read as an open demultiplexer under LGPL-2.1 reads it; ETSI TS 101 154 V2.10.1 §6.10.2 constrains the values a broadcast stream uses. The demultiplexer reads only the lossy codec and refuses ancillary data, and the probe does the same (outside_probe). It also skips two 8-bit fields it does not name, and so does the probe. The header states no frame length, so the probe reads the first frame's header only.

§9.2 The header. A sync word of 12 bits, 0xFFF (else no_signature); audio_codec_id u(4) (1, lossless, is outside_probe; anything but 2 is reserved_value); anc_data u(1) (1 is outside_probe); nn_type u(3) (above 1 is reserved_value); coding_profile u(3) (above 2 is reserved_value); sampling_frequency_index u(4), rates 192000, 96000, 48000, 44100, 32000, 24000, 22050, 16000, 8000 (above 8 is reserved_value); 8 bits; then for profile 0 channel_number_index u(7); for profile 1 soundbed_type u(2): for 0 the object count less one u(7) and the per-object bitrate index u(4), for 1 channel_number_index u(7), the bitrate index u(4), the object count less one u(7) and the per-object bitrate index u(4), and above 1 reserved_value; for profile 2 the ambisonic order less one u(4) (an order outside 1 to 3 is reserved_value); then resolution u(2) (above 2 is reserved_value), the bitrate index u(4) unless the profile is 1, and 8 bits.

§9.3 Parameters. codec_id, nn_type, coding_profile, sampling_rate, and as the profile gives them channel_number_index, soundbed_type, objects, bitrate_index, bitrate_index_per_object, ambisonic_order; and resolution_bits (8 · (resolution + 1)).

10. IEEE 1857.11

§10.1 wai.image.ieee1857_11 is a candidate (SPEC §5): IEEE 1857.11-2024 is not openly readable, and the open reference software (BSD-2-Clause) writes a picture header with no start code, from a working draft, marking the code that would write one as not yet written. So no payload format is pinned. This parse reads that header as the software's writer lays it out, at the revision the corpus README names. Its reader masks level_id differently from its writer; the parse follows the writer. With no signature, the parse cannot tell this format from another; it reads a payload declared to be one, and never refuses no_signature.

§10.2 The header, big-endian: 3 bytes holding profile_id (3 bits), level_id (6), model_id (3), parameter_set_id (7) and quality_id (5); the picture's height and width as 32-bit values; 2 bytes holding 2 unused bits (0), output_bit_depth (3), picture_format (3), log2_slice_size_h_minus6 (4) and log2_slice_size_w_minus6 (4). Fewer than 13 bytes is truncated. A profile_id above 2, an unused bit set, an output_bit_depth other than 1 (8 bits) or 2 (10 bits), or a height or width of 0 is reserved_value.

§10.3 Parameters. profile_id, level_id, model_id, parameter_set_id, quality_id, size ([width, height]), bit_depth, picture_format, slice_size ([2^(w+6), 2^(h+6)]).

11. The projection descriptor

§11.1 The probe of wai.meta.projection reads the descriptor as projection §3 lays it out and checks it as its §4 does; its parameters are those of projection.md §3. The check of a descriptor against the rendition it accompanies, which needs that rendition's probe, is projection.md §5's.

12. Refusals

code	meaning
no_signature	the payload does not begin with the format's signature
truncated	the bytes end inside a structure the probe reads
out_of_order	a structure is where the format does not allow it
reserved_value	a field holds a value the format forbids or reserves
length_mismatch	a length a structure declares disagrees with the bytes
missing_structure	a structure the format requires is absent
not_multiview	a wai.video.mvhevc payload with no second view
framing_invalid	the framing of a wai.video.jxs payload is wrong
trailing_bytes	bytes follow the end of the payload's last structure
descriptor_invalid	a projection descriptor's fields disagree with each other
layer_absent	a projection descriptor names a layer its rendition does not hold
outside_probe	the payload uses a feature whose syntax the probe's sources do not establish
no_probe	no probe is defined for the capability

13. Conformance

§13.1 format-probe-conformance/ holds vectors.json: every vector's capability, file, SHA-256, origin and outcome (its parameters, or its refusal), the projection descriptors' bindings with theirs (projection.md §5), and the facts of §13.2.

§13.2 External vectors (ext/) were written by permissively licensed encoders at pinned revisions from synthetic pictures, and each was decoded again; tools/make_external.sh writes them and provenance.json, which records every command and decode. Facts are what each encoder was asked for (a size, a profile, a view count): the probe must read them, which ties the probe to the encoder and not to itself. Generated vectors (gen/) are written by wai_format_probe gen from fixed values and from the external vectors: the formats no permissively licensed encoder writes (LCEVC, MHAS, AVS3 audio, the IEEE 1857.11 header, the projection descriptor) and the negative controls, each refused with a code of §12, every code given.

§13.3 Fetched vectors. fetch.sh fetches five LCEVC streams an open decoder's tests use, written by an LCEVC encoder, and checks each SHA-256; the corpus names them and does not hold them, because their licence notice names their owner. Each is a container of blocks (an 8-byte magic lcevcbin, a u32 version, then per block a u16 type, a u32 size and that many bytes, the first 16 of which are two indices) whose remaining bytes, concatenated, are the byte stream. vectors.json states, for each, what its encoder was asked for.

§13.4 wai_format_probe verify and wai-rs/tests/format_probe.rs require every outcome, binding and fact, and the generated files byte for byte; with --fetched the verifier requires every fetched stream. format_probe_verify.py, written from this text and projection.md and sharing no code with wai-rs, does the same, and requires every mutated copy of the corpus to fail.