av/filter/diff.go

//go:build withcv
// +build withcv

/*
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
  diff.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 (
	"fmt"
	"io"

	"gocv.io/x/gocv"

	"bitbucket.org/ausocean/av/revid/config"
)

const defaultDiffThreshold = 3

// NewDiff returns a pointer to a new difference motion filter.
func NewDiff(dst io.WriteCloser, c config.Config) *Motion {

	// Validate parameters.
	if c.MotionThreshold <= 0 {
		c.LogInvalidField("MotionThreshold", defaultDiffThreshold)
		c.MotionThreshold = defaultDiffThreshold
	}

	alg := &Diff{
		thresh:    c.MotionThreshold,
		prev:      gocv.NewMat(),
		debugging: newWindows("DIFF"),
	}

	return NewMotion(dst, alg, c)
}

// Diff is a motion detection algorithm. It calculates the absolute
// difference for each pixel between two frames, then finds the mean.
// If the mean is above a given threshold, it is considered motion.
type Diff struct {
	debugging debugWindows
	thresh    float64
	prev      gocv.Mat
}

// Close frees resources used by gocv. It has to be done manually,
// due to gocv using c-go.
func (d *Diff) Close() error {
	d.debugging.close()
	d.prev.Close()
	return nil
}

// Detect performs the motion detection on a frame. It returns true
// if motion is detected.
func (d *Diff) Detect(img *gocv.Mat) bool {
	if d.prev.Empty() {
		d.prev = img.Clone()
		return false
	}

	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.debugging.show(*img, imgDelta, mean > d.thresh, nil, fmt.Sprintf("Mean: %f", mean), fmt.Sprintf("Threshold: %f", d.thresh))

	// Return if there is motion.
	return mean > d.thresh
}