av/revid/webcam.go

/*
DESCRIPTION
  webcam.go provides an implementation of AVDevice for webcams.

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

LICENSE
  Copyright (C) 2019 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 revid

import (
	"errors"
	"fmt"
	"io"
	"os/exec"
	"strings"

	"bitbucket.org/ausocean/utils/logger"
)

// Configuration defaults.
const (
	defaultWebcamInputPath = "/dev/video0"
	defaultWebcamFrameRate = 25
	defaultWebcamBitrate   = 400
	defaultWebcamWidth     = 1280
	defaultWebcamHeight    = 720
)

// Configuration field errors.
var (
	errWebcamBadFrameRate = errors.New("frame rate bad or unset, defaulting")
	errWebcamBadBitrate   = errors.New("bitrate bad or unset, defaulting")
	errWebcamWidth        = errors.New("width bad or unset, defaulting")
	errWebcamHeight       = errors.New("height bad or unset, defaulting")
)

// Webcam is an implementation of the AVDevice interface for a Webcam. Webcam
// uses an ffmpeg process to pipe the video data from the webcam.
type Webcam struct {
	out io.ReadCloser
	log Logger
	cfg Config
	cmd *exec.Cmd
}

// NewWebcam returns a new Webcam.
func NewWebcam(l Logger) *Webcam {
	return &Webcam{log: l}
}

// Set will validate the relevant fields of the given Config struct and assign
// the struct to the Webcam's Config. If fields are not valid, an error is
// added to the multiError and a default value is used.
func (w *Webcam) Set(c Config) error {
	var errs []error
	if c.Width == 0 {
		errs = append(errs, errBadWidth)
		c.Width = defaultWebcamWidth
	}

	if c.Height == 0 {
		errs = append(errs, errBadHeight)
		c.Height = defaultWebcamHeight
	}

	if c.FrameRate == 0 {
		errs = append(errs, errBadFrameRate)
		c.FrameRate = defaultWebcamFrameRate
	}

	if c.Bitrate <= 0 {
		errs = append(errs, errBadBitrate)
		c.Bitrate = defaultWebcamBitrate
	}
	w.cfg = c
	return multiError(errs)
}

// Start will build the required arguments for ffmpeg and then execute the
// command, piping video output where we can read using the Read method.
func (w *Webcam) Start() error {
	args := []string{
		"-i", w.cfg.InputPath,
		"-f", "h264",
		"-r", fmt.Sprint(w.cfg.FrameRate),
	}

	br := w.cfg.Bitrate * 1000
	args = append(args,
		"-b:v", fmt.Sprint(br),
		"-maxrate", fmt.Sprint(br),
		"-bufsize", fmt.Sprint(br/2),
		"-s", fmt.Sprintf("%dx%d", w.cfg.Width, w.cfg.Height),
		"-",
	)

	w.log.Log(logger.Info, pkg+"ffmpeg args", "args", strings.Join(args, " "))
	w.cmd = exec.Command("ffmpeg", args...)

	var err error
	w.out, err = w.cmd.StdoutPipe()
	if err != nil {
		return fmt.Errorf("failed to create pipe: %w", err)
	}

	err = w.cmd.Start()
	if err != nil {
		return fmt.Errorf("failed to start ffmpeg: %w", err)
	}

	return nil
}

// Stop will kill the ffmpeg process and close the output pipe.
func (w *Webcam) Stop() error {
	if w.cmd == nil || w.cmd.Process == nil {
		return errors.New("ffmpeg process was never started")
	}
	err := w.cmd.Process.Kill()
	if err != nil {
		return fmt.Errorf("could not kill ffmpeg process: %w", err)
	}
	return w.out.Close()
}

// Read implements io.Reader. If the pipe is nil a read error is returned.
func (w *Webcam) Read(p []byte) (int, error) {
	if w.out != nil {
		return w.out.Read(p)
	}
	return 0, errors.New("webcam not streaming")
}