@eluvio/elv-utils-js/lib/misc/mp4InitSeg.js

Utilities for the Eluvio Content Fabric

const R = require('@eluvio/ramda-fork')
const pipe = require('crocks/helpers/pipe')

const M = require('./mp4Atom')

// takes buffer containing MP4 init segment (or entire MP4 file) for a single x264/x265 video stream
// and returns codec descriptor string (e.g. "avc1.640028", "hev1.2.4.L150.90" etc.)
const codecDesc = buffer => {
  let resultContext = decoderConfigAtom(buffer)
  let {atomLength, atomType, headerLength} = M.readHeader(resultContext)

  // make new buffer that omits header (length and type fields)
  const decoderBodyBuf = resultContext.buffer.subarray(headerLength, atomLength)
  switch(atomType) {
    case 'avcC':
      return x264codecString(decoderBodyBuf)
    case 'hvcC':
      return x265codecString(decoderBodyBuf)
    default:
      throw Error(`Unknown decoder configuration atom type: '${atomType}'`)
  }
}

// buffer containing init segment -> MP4AtomReadContext with buffer containing only avcC or hvcC atom
const decoderConfigAtom = pipe(
  M.newContextFromBuffer,
  M.findAndEnter(['moov']),
  M.findAndEnter(['trak']),
  M.findAndEnter(['mdia']),
  M.findAndEnter(['minf']),
  M.findAndEnter(['stbl']),
  M.findAndEnter(['stsd']),
  M.moveWithin(8),
  M.findAndEnter(['avc1', 'encv', 'hev1']),
  M.moveWithin(78),
  M.find(['avcC', 'hvcC']),
  M.readAtom,
  M.newContextFromBuffer
)

// takes buffer containing decoder config atom (without the type/length fields at front) and returns x264 codec string
const x264codecString = atomBodyBuf => {
  // See https://dvb.org/wp-content/uploads/2019/10/a168_DVB_MPEG-DASH_Nov_2017.pdf

  if(atomBodyBuf.length < 4) throw Error('Not enough bytes in AVCDecoderConfiguration')

  const avcProfileIndication = atomBodyBuf.readUInt8(1).toString(16).padStart(2, '0')
  const profileCompatibility = atomBodyBuf.readUInt8(2).toString(16).padStart(2, '0')
  const avcLevelIndication = atomBodyBuf.readUInt8(3).toString(16).padStart(2, '0')
  return `avc1.${avcProfileIndication}${profileCompatibility}${avcLevelIndication}`
}

// takes buffer containing decoder config atom (without the type/length fields at front) and returns x265 codec string
const x265codecString = atomBodyBuf => {
  // See https://dvb.org/wp-content/uploads/2019/10/a168_DVB_MPEG-DASH_Nov_2017.pdf

  if(atomBodyBuf.length < 13) throw Error('Not enough bytes in HVCDecoderConfiguration')

  // parse bytes/bits into values
  const bitFlags = atomBodyBuf.readUInt8(1)
  const genProfileSpaceBits = (bitFlags & 0b11000000) >>> 6
  const genTierBit = (bitFlags & 0b00100000) >>> 5
  const genProfileIDC = bitFlags & 0b00011111
  const genProfileCompatibility = parseInt(R.reverse(atomBodyBuf.readUInt32BE(2).toString(2).padStart(32, '0')), 2).toString(16)
  let genConstraintFlags = [
    atomBodyBuf.readUInt8(6),
    atomBodyBuf.readUInt8(7),
    atomBodyBuf.readUInt8(8),
    atomBodyBuf.readUInt8(9),
    atomBodyBuf.readUInt8(10),
    atomBodyBuf.readUInt8(11)
  ]
  const genLevelIDC = atomBodyBuf.readUInt8(12)

  // Format values into appropriate strings

  // CODECSTRING = CODEC "." PROFILE "." LEVEL "." CONSTRAINTS

  // CODEC = ("hev1" | "hvc1" )
  let pieces = ['hcv1']

  // PROFILE = PROFILE_SPACE PROFILE_IDC "." PROFILE_COMPATIBILITY

  // PROFILE_SPACE = "" | "A" | "B" | "C" (for general_profile_space values 0,1,2,3)
  let profileSpace = ''
  switch(genProfileSpaceBits){
    case 1:
      profileSpace = 'A'
      break
    case 2:
      profileSpace = 'B'
      break
    case 3:
      profileSpace = 'C'
      break
  }

  // PROFILE_IDC is a decimal number, PROFILE_COMPATIBILITY is a hex string
  const profile = `${profileSpace}${genProfileIDC}.${genProfileCompatibility}`
  pieces.push(profile)

  // LEVEL = TIER LEVEL_IDC

  // TIER = "L" / "H" (general_tier_flag, 0=="L", 1=="H")
  const tier = genTierBit ? 'H' : 'L'
  const level = `${tier}${genLevelIDC}`
  pieces.push(level)

  // CONSTRAINTS = 2(HEXDIG) [ "." CONSTRAINTS ] (hexadecimal representation of the general_constraint_indicator_flags. Each byte is separated by a '.', and trailing zero bytes may be omitted.)

  while(genConstraintFlags.length > 0 && genConstraintFlags.slice(-1)[0] === 0) {
    genConstraintFlags.pop()
  }
  const constraints = genConstraintFlags.map(x => x.toString(16)).join('.')
  pieces.push(constraints)

  return pieces.join('.')
}

module.exports = {codecDesc, decoderConfigAtom}