av/codec/h264/parse.go

/*
DESCRIPTION
  parse.go provides H.264 NAL unit parsing utilities for the extraction of
	syntax elements.

AUTHORS
  Saxon A. Nelson-Milton <saxon@ausocean.org>
  Dan Kortschak <dan@ausocean.org>

LICENSE
  Copyright (C) 2017-2018 the Australian Ocean Lab (AusOcean).

  It is free software: you can redistribute it and/or modify them
  under the terms of the GNU General Public License as published by the
  Free Software Foundation, either version 3 of the License, or (at your
  option) any later version.

  It is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
  for more details.

  You should have received a copy of the GNU General Public License
  in gpl.txt.  If not, see http://www.gnu.org/licenses.
*/

package h264

import (
	"errors"

	"bitbucket.org/ausocean/av/codec/h264/h264dec"
)

var errNotEnoughBytes = errors.New("not enough bytes to read")

// NALType returns the NAL type of the given NAL unit bytes. The given NAL unit
// may be in byte stream or packet format.
// NB: access unit delimiters are skipped.
func NALType(n []byte) (int, error) {
	sc := frameScanner{buf: n}
	for {
		b, ok := sc.readByte()
		if !ok {
			return 0, errNotEnoughBytes
		}
		for i := 1; b == 0x00 && i != 4; i++ {
			b, ok = sc.readByte()
			if !ok {
				return 0, errNotEnoughBytes
			}
			if b != 0x01 || (i != 2 && i != 3) {
				continue
			}

			b, ok = sc.readByte()
			if !ok {
				return 0, errNotEnoughBytes
			}
			nalType := int(b & 0x1f)
			if nalType != h264dec.NALTypeAccessUnitDelimiter {
				return nalType, nil
			}
		}
	}
}

type frameScanner struct {
	off int
	buf []byte
}

func (s *frameScanner) readByte() (b byte, ok bool) {
	if s.off >= len(s.buf) {
		return 0, false
	}
	b = s.buf[s.off]
	s.off++
	return b, true
}

// Trim will trim down a given byte stream of video data so that a key frame appears first.
func Trim(n []byte) ([]byte, error) {
	sc := frameScanner{buf: n}
	for {
		b, ok := sc.readByte()
		if !ok {
			return nil, errNotEnoughBytes
		}
		for i := 1; b == 0x00 && i != 4; i++ {
			b, ok = sc.readByte()
			if !ok {
				return nil, errNotEnoughBytes
			}
			if b != 0x01 || (i != 2 && i != 3) {
				continue
			}

			b, ok = sc.readByte()
			if !ok {
				return nil, errNotEnoughBytes
			}
			nalType := int(b & 0x1f)
			if nalType == 7 {
				sc.off = sc.off - 4
				return sc.buf[sc.off:], nil
			}
		}
	}
}