av/stream/rtp/encoder.go

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/*
NAME
encoder.go
DESCRIPTION
See Readme.md
AUTHOR
Saxon Nelson-Milton (saxon@ausocean.org)
LICENSE
encoder.go is Copyright (C) 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
along with revid in gpl.txt. If not, see http://www.gnu.org/licenses.
*/
package rtp
import (
"io"
"math/rand"
"time"
)
const (
yes = 1
no = 0
defaultPktType = 1
timestampFreq = 90000 // Hz
)
type Encoder struct {
dst io.Writer
ssrc uint32
seqNo uint16
clock time.Duration
frameInterval time.Duration
}
// NewEncoder returns a new Encoder type given an io.Writer - the destination
// after encoding and the desired fps
func NewEncoder(dst io.Writer, fps int) *Encoder {
return &Encoder{
dst: dst,
ssrc: rand.Uint32(),
frameInterval: time.Duration(float64(time.Second) / float64(fps)),
}
}
// Encode takes a nalu unit and encodes it into an rtp packet and
// writes to the io.Writer given in NewEncoder
func (e *Encoder) Encode(nalu []byte) error {
pkt := Pkt{
V: rtpVer, // version
P: no, // padding
X: no, // header extension
CC: no, // CSRC count
M: no, // NOTE: need to check if this works (decoders should ignore this)
PT: defaultPktType, // NOTE: 1-23 according to rtp-h264 specs (don't think we need this)
SN: e.nxtSeqNo(), // sequence number
TS: e.nxtTimestamp(), // timestamp
SSRC: e.ssrc, // source identifier
Payload: nalu,
Padding: no,
}
_, err := e.dst.Write(pkt.Bytes())
if err != nil {
return err
}
e.tick()
return nil
}
// tick advances the clock one frame interval.
func (e *Encoder) tick() {
e.clock += e.frameInterval
}
// nxtTimestamp gets the next timestamp
func (e *Encoder) nxtTimestamp() uint32 {
return uint32(e.clock.Seconds() * timestampFreq)
}
// nxtSeqNo gets the next rtp packet sequence number
func (e *Encoder) nxtSeqNo() uint16 {
e.seqNo += 1
return e.seqNo - 1
}