av/filter/difference.go

// +build !circleci

/*
DESCRIPTION
  A filter that detects motion and discards frames without motion. The
  algorithm calculates the absolute difference for each pixel between
  two frames, then finds the mean. If the mean is above a given threshold,
  then it is considered motion.

AUTHORS
  Scott Barnard <scott@ausocean.org>

LICENSE
  difference.go is Copyright (C) 2020 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 filter

import (
	"io"

	"gocv.io/x/gocv"
)

// Difference is a filter that provides basic motion detection. Difference calculates
// the absolute difference for each pixel between two frames, then finds the mean. If
// the mean is above a given threshold, then it is considered motion.
type Difference struct {
	debugWindows
	dst    io.WriteCloser
	thresh float64
	prev   gocv.Mat
}

// NewDifference returns a pointer to a new Difference struct.
func NewDifference(dst io.WriteCloser, threshold float64) *Difference {
	d := &Difference{
		dst:    dst,
		thresh: threshold,
		prev:   gocv.NewMat(),
	}
	d.newWindows("DIFF")
	return d
}

// Implements io.Closer.
// Close frees resources used by gocv, because it has to be done manually, due to
// it using c-go.
func (d *Difference) Close() error {
	d.prev.Close()
	d.closeWindows()
	return nil
}

// Implements io.Writer.
// Write applies the motion filter to the video stream. Only frames with motion
// are written to the destination encoder, frames without are discarded.
func (d *Difference) Write(f []byte) (int, error) {
	if d.prev.Empty() {
		var err error
		d.prev, err = gocv.IMDecode(f, gocv.IMReadColor)
		if err != nil {
			return 0, err
		}
		return len(f), nil
	}

	img, err := gocv.IMDecode(f, gocv.IMReadColor)
	defer img.Close()
	if err != nil {
		return 0, err
	}

	imgDelta := gocv.NewMat()
	defer imgDelta.Close()

	// Seperate foreground and background.
	gocv.AbsDiff(img, d.prev, &imgDelta)
	gocv.CvtColor(imgDelta, &imgDelta, gocv.ColorBGRToGray)

	mean := imgDelta.Mean().Val1

	// Update History.
	d.prev = img.Clone()

	// Draw debug information.
	d.showDebug(img, imgDelta, mean > d.thresh)

	// Don't write to destination if there is no motion.
	if mean < d.thresh {
		return len(f), nil
	}

	// Write to destination.
	return d.dst.Write(f)
}