`nx_h264_nal.nx` source

1// nx_h264_nal.nx -- H.264 NAL unit splitter + header decode (rung 2b).
2// A coded video sample is a sequence of NAL (Network Abstraction Layer) units.
3// Two framings:
4//   AVCC   (MP4 sample): [length:lenSize BE][NAL bytes] repeated.  lenSize comes
5//          from the avcC config (lengthSizeMinusOne+1), usually 4.
6//   Annex B (raw .h264):  start code 00 00 01 (or 00 00 00 01) before each NAL.
7// First NAL byte = forbidden_zero(1) + nal_ref_idc(2) + nal_unit_type(5):
8//   type 7=SPS 8=PPS 6=SEI 5=IDR-slice 1=non-IDR-slice 9=AUD.
9// This locates each NAL + its type so the SPS/PPS/slice parsers (next rungs,
10// which use nx_h264_bits) know what to read.  Settled framing; KAT-gated.
11//
12// genealogy_id: itu_t_h264_nal_framing + iso_bmff_avcc_length_prefix
13// lineage_id:   nal_split_avcc + nal_split_annexb + nal_header_decode
14// license_tier: ORIGINAL
15import "nx_syscalls.nx"
16
17func nx_nal_type(first_byte: i64) -> i64 { return first_byte & 31 }
18func nx_nal_ref_idc(first_byte: i64) -> i64 { return (first_byte >> 5) & 3 }
19
20// read an n-byte big-endian integer at offset o
21func nal_be(d: *u8, o: i64, n: i64) -> i64 {
22    var v: i64 = 0
23    var i: i64 = 0
24    while i < n { v = (v << 8) | (d[o + i] as i64); i = i + 1 }
25    return v
26}
27
28// AVCC split over [start,end): each NAL = [len:len_size BE][bytes]. Records the
29// NAL payload offset (header byte) into offs[] and its type into types[].
30// Returns NAL count.
31func nx_nal_split_avcc(d: *u8, start: i64, end: i64, len_size: i64, offs: *i64, types: *i64, max: i64) -> i64 {
32    var pos: i64 = start
33    var cnt: i64 = 0
34    while pos + len_size < end {
35        let l: i64 = nal_be(d, pos, len_size)
36        if l <= 0 { pos = end }
37        if l > 0 {
38            let np: i64 = pos + len_size
39            if np >= end { pos = end }
40            if np < end {
41                if cnt < max { offs[cnt] = np; types[cnt] = (d[np] as i64) & 31; cnt = cnt + 1 }
42                pos = np + l
43            }
44        }
45    }
46    return cnt
47}
48
49// Annex B split over [start,end): find each 00 00 01 (handles 00 00 00 01 too,
50// matching the trailing 00 00 01).  Records NAL payload offset + type.
51func nx_nal_split_annexb(d: *u8, start: i64, end: i64, offs: *i64, types: *i64, max: i64) -> i64 {
52    var cnt: i64 = 0
53    var i: i64 = start
54    while i + 3 <= end {
55        var hit: i64 = 0
56        if (d[i] as i64) == 0 { if (d[i + 1] as i64) == 0 { if (d[i + 2] as i64) == 1 { hit = 1 } } }
57        if hit == 1 {
58            let np: i64 = i + 3
59            if np < end { if cnt < max { offs[cnt] = np; types[cnt] = (d[np] as i64) & 31; cnt = cnt + 1 } }
60            i = i + 3
61        }
62        if hit == 0 { i = i + 1 }
63    }
64    return cnt
65}
nx_h264_nal.nx source

`nx_h264_nal.nx` source
