av/generator/mpegts_generator.go

/*
NAME
  mpegts_generator.go

DESCRIPTION
  See Readme.md

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

LICENSE
  mpegts_generator.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 http://www.gnu.org/licenses.
*/

package generator

import (
	"time"

	"bitbucket.org/ausocean/av/mpegts"
	"bitbucket.org/ausocean/av/pes"
)

const psiPacketSize = 184

// TODO: really need to finish the at and pmt stuff - this is too hacky
var (
	patTable = []byte{0, 0, 176, 13, 0, 1, 193, 0, 0, 0, 1, 240, 0, 42, 177, 4, 178}
	pmtTable = []byte{0, 2, 176, 18, 0, 1, 193, 0, 0, 0xE1, 0x00, 0xF0, 0, 0x1B, 0xE1, 0, 0xF0, 0, 0x15, 0xBD, 0x4D, 0x56}
)

func init() {
	for len(patTable) < psiPacketSize {
		patTable = append(patTable, 0xff)
	}
	for len(pmtTable) < psiPacketSize {
		pmtTable = append(pmtTable, 0xff)
	}
}

const (
	sdtPid   = 17
	patPid   = 0
	pmtPid   = 4096
	videoPid = 256
	streamID = 0xe0 // First video stream ID.
)

// Time related constants.
const (
	// ptsOffset is the offset added to the clock to determine
	// the current presentation timestamp,
	ptsOffset = 700 * time.Millisecond

	// pcrFreq is the base Program Clock Reference frequency.
	pcrFreq = 90000 // Hz
)

// tsGenerator encapsulates properties of an mpegts generator.
type tsGenerator struct {
	outputChan   chan []byte
	nalInputChan chan []byte

	clock         time.Duration
	frameInterval time.Duration
	ptsOffset     time.Duration

	continuity map[int]byte
}

// NewTsGenerator returns an instance of the tsGenerator struct
func NewTsGenerator(fps float64) (g *tsGenerator) {
	return &tsGenerator{
		outputChan:   make(chan []byte, 1),
		nalInputChan: make(chan []byte, 1),

		frameInterval: time.Duration(float64(time.Second) / fps),
		ptsOffset:     ptsOffset,

		continuity: map[int]byte{
			patPid:   0,
			pmtPid:   0,
			videoPid: 0,
		},
	}
}

// Start is called when we would like generation to begin, i.e. we would like
// the generator to start taking input data and creating mpegts packets
func (g *tsGenerator) Start() {
	go g.generate()
}

func (g *tsGenerator) Stop() {}

// InputChan returns a handle to the nalInputChan (inputChan) so that nal units
// can be passed to the generator and processed
func (g *tsGenerator) InputChan() chan []byte {
	return g.nalInputChan
}

// OutputChan returns a handle to the generator output chan where the mpegts
// packets will show up once ready to go
func (g *tsGenerator) OutputChan() <-chan []byte {
	return g.outputChan
}

const (
	hasPayload         = 0x1
	hasAdaptationField = 0x2
)

const (
	hasDTS = 0x1
	hasPTS = 0x2
)

// generate handles the incoming data and generates equivalent mpegts packets -
// sending them to the output channel
func (g *tsGenerator) generate() {
	for {
		nalu := <-g.nalInputChan

		// Write PAT
		patPkt := mpegts.Packet{
			PUSI:    true,
			PID:     patPid,
			CC:      g.ccFor(patPid),
			AFC:     hasPayload,
			Payload: patTable,
		}
		g.outputChan <- patPkt.Bytes()

		// Write PMT.
		pmtPkt := mpegts.Packet{
			PUSI:    true,
			PID:     pmtPid,
			CC:      g.ccFor(pmtPid),
			AFC:     hasPayload,
			Payload: pmtTable,
		}
		g.outputChan <- pmtPkt.Bytes()

		// Prepare PES data.
		pesPkt := pes.Packet{
			StreamID:     streamID,
			PDI:          hasPTS,
			PTS:          g.pts(),
			Data:         nalu,
			HeaderLength: 5,
		}
		buf := pesPkt.Bytes()

		pusi := true
		for len(buf) != 0 {
			pkt := mpegts.Packet{
				PUSI: pusi,
				PID:  videoPid,
				RAI:  pusi,
				CC:   g.ccFor(videoPid),
				AFC:  hasAdaptationField | hasPayload,
				PCRF: pusi,
			}
			n := pkt.FillPayload(buf)
			buf = buf[n:]

			if pusi {
				// If the packet has a Payload Unit Start Indicator
				// flag set then we need to write a PCR.
				pkt.PCR = g.pcr()
				pusi = false
			}

			g.outputChan <- pkt.Bytes()
		}

		g.tick()
	}
}

// tick advances the clock one frame interval.
func (g *tsGenerator) tick() {
	g.clock += g.frameInterval
}

// pts retuns the current presentation timestamp.
func (g *tsGenerator) pts() uint64 {
	return uint64((g.clock + g.ptsOffset).Seconds() * pcrFreq)
}

// pcr returns the current program clock reference.
func (g *tsGenerator) pcr() uint64 {
	return uint64(g.clock.Seconds() * pcrFreq)
}

// ccFor returns the next continuity counter for pid.
func (g *tsGenerator) ccFor(pid int) byte {
	cc := g.continuity[pid]
	const continuityCounterMask = 0xf
	g.continuity[pid] = (cc + 1) & continuityCounterMask
	return cc
}