av/generator/FLVGenerator.go

/*
NAME
  FLVGenerator.go

DESCRIPTION
  See Readme.md

AUTHOR
  Saxon Nelson-Milton <saxon@ausocean.org>

LICENSE
  FLVGenerator.go is Copyright (C) 2017 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
  along with revid in gpl.txt.  If not, see [GNU licenses](http://www.gnu.org/licenses).
*/
package generator

import (
	"../flv"
	_ "fmt"
	_ "time"
)

const (
	inputChanLength  = 1000
	outputChanLength = 1000
	audioSize        = 18
	videoHeaderSize  = 16
	interFrameCode   = 1
	keyFrameCode     = 5
	sequenceCode     = 6
)

// Data representing silent audio (required for youtube)
var dummyAudioTag1Data = []byte{0x00, 0x12, 0x08, 0x56, 0xe5, 0x00}
var dummyAudioTag2Data = []byte{0x01, 0xdc, 0x00, 0x4c, 0x61, 0x76, 0x63, 0x35,
	0x38, 0x2e, 0x36, 0x2e, 0x31, 0x30, 0x32, 0x00, 0x02, 0x30, 0x40, 0x0e}

// flvGenerator provides properties required for the generation of flv video
// from raw video data
type flvGenerator struct {
	fps              uint
	inputChan        chan []byte
	outputChan       chan []byte
	audioFlag        bool
	videoFlag        bool
	lastTagSize      int
	currentTimestamp uint32
	header           flv.Header
}

// GetInputChan returns the input channel to the generator. This is where the
// raw data frames are entered into the generator
func (g *flvGenerator) GetInputChan() chan []byte {
	return g.inputChan
}

// GetOutputChan retuns the output chan of the generator - this is where the
// flv packets (more specifically tags) are outputted.
func (g *flvGenerator) GetOutputChan() chan []byte {
	return g.outputChan
}

// NewFlvGenerator retuns an instance of the flvGenerator struct
func NewFlvGenerator(audio bool, video bool, fps uint) (g *flvGenerator) {
	g = new(flvGenerator)
	g.fps = fps
	g.audioFlag = audio
	g.videoFlag = video
	g.currentTimestamp = 0
	g.lastTagSize = 0
	g.inputChan = make(chan []byte, inputChanLength)
	g.outputChan = make(chan []byte, outputChanLength)
	return
}

// Start beings the generation routine - i.e. if raw data is given to the input
// channel flv tags will be produced and available from the output channel.
func (g *flvGenerator) Start() {
	go g.generate()
}

// GenHeader generates the flv header and sends it down the output chan for use
// This will generally be called once at the start of file writing/transmission.
func (g *flvGenerator) GenHeader() {
	header := flv.Header{
		AudioFlag: g.audioFlag,
		VideoFlag: g.videoFlag,
	}
	g.outputChan <- header.ToByteSlice()
}

// getNextTimestamp generates and returns the next timestamp based on the given
// fps rate
func (g *flvGenerator) getNextTimestamp() (timestamp uint32) {
	timestamp = g.currentTimestamp
	g.currentTimestamp += uint32(1000) / uint32(g.fps)
	return
}

// ResetTimestamp resets the current timestamp to 0 i.e. equivalent to start of
// transmission
func (g *flvGenerator) ResetTimestamp() {
	g.currentTimestamp = 0
}

// isKeyFrame checks the nature of the passed frame - returning true if the
// frame is keyframe and false otherwise
func isKeyFrame(frame []byte) bool {
	byteChannel := make(chan byte, len(frame))
	for i := range frame {
		byteChannel <- frame[i]
	}
	for len(byteChannel) >= 5 {
		aByte := <-byteChannel
		for i := 1; aByte == 0x00 && i != 4; i++ {
			aByte = <-byteChannel
			if (aByte == 0x01 && i == 2) || (aByte == 0x01 && i == 3) {
				aByte = <-byteChannel
				nalType := aByte & 0x1F
				switch nalType {
				case interFrameCode:
					return false
				case keyFrameCode:
					return true
				case 6:
					return true
				}
			}
		}
	}
	return false
}

// isSequenceHeader checks the nature of the passed frame and returns true
// if it is a sequnce header and false otherwise
func isSequenceHeader(frame []byte) bool {
	byteChannel := make(chan byte, len(frame))
	for i := range frame {
		byteChannel <- frame[i]
	}
	for len(byteChannel) >= 5 {
		aByte := <-byteChannel
		for i := 1; aByte == 0x00 && i != 4; i++ {
			aByte = <-byteChannel
			if (aByte == 0x01 && i == 2) || (aByte == 0x01 && i == 3) {
				aByte = <-byteChannel
				nalType := aByte & 0x1F
				switch nalType {
				case 1:
					return false
				case 5:
					return false
				case 6:
					return true
				case 7:
					return true
				case 8:
					return true
				}
			}
		}
	}
	return false
}

// generate takes in raw video data from the input chan and packetises it into
// flv tags, which are then passed to the output channel.
func (g *flvGenerator) generate() {
	g.GenHeader()
	for {
		select {
		case videoFrame := <-g.inputChan:
			var frameType byte
			if isKeyFrame(videoFrame) {
				frameType = flv.KeyFrameType
			} else {
				frameType = flv.InterFrameType
			}
			var packetType byte
			if isSequenceHeader(videoFrame) {
				packetType = flv.SequenceHeader
			} else {
				packetType = flv.AVCNALU
			}
			timeStamp := g.getNextTimestamp()
			// Do we have video to send off ?
			if g.videoFlag {
				g.outputChan <- flv.VideoTag{
					TagType:           uint8(flv.VideoTagType),
					DataSize:          uint32(len(videoFrame)) + flv.DataHeaderLength,
					Timestamp:         timeStamp,
					TimestampExtended: flv.NoTimestampExtension,
					FrameType:         frameType,
					Codec:             flv.H264,
					PacketType:        packetType,
					CompositionTime:   0,
					Data:              videoFrame,
					PrevTagSize:       uint32(videoHeaderSize + len(videoFrame)),
				}.ToByteSlice()
			}
			// Do we even have some audio to send off ?
			if g.audioFlag {
				// Not sure why but we need two audio tags for dummy silent audio
				g.outputChan <- flv.AudioTag{
					TagType:           uint8(flv.AudioTagType),
					DataSize:          7,
					Timestamp:         timeStamp,
					TimestampExtended: flv.NoTimestampExtension,
					SoundFormat:       flv.AACAudioFormat,
					SoundRate:         3,
					SoundSize:         true,
					SoundType:         true,
					Data:              dummyAudioTag1Data,
					PrevTagSize:       uint32(audioSize),
				}.ToByteSlice()

				g.outputChan <- flv.AudioTag{
					TagType:           uint8(flv.AudioTagType),
					DataSize:          21,
					Timestamp:         timeStamp,
					TimestampExtended: flv.NoTimestampExtension,
					SoundFormat:       flv.AACAudioFormat,
					SoundRate:         3,
					SoundSize:         true,
					SoundType:         true,
					Data:              dummyAudioTag2Data,
					PrevTagSize:       uint32(22),
				}.ToByteSlice()
			}
		}
	}
}