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 (
	"fmt"
	"image"
	"image/color"
	"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 {
	dst     io.WriteCloser
	thresh  float64
	prev    gocv.Mat
	debug   bool
	windows []*gocv.Window
}

// NewDifference returns a pointer to a new Difference struct.
func NewDifference(dst io.WriteCloser, debug bool, threshold float64) *Difference {
	var windows []*gocv.Window
	if debug {
		windows = []*gocv.Window{gocv.NewWindow("Diff: Bounding boxes"), gocv.NewWindow("Diff: Motion")}
	}
	return &Difference{dst, threshold, gocv.NewMat(), debug, windows}
}

// 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()
	for _, window := range d.windows {
		window.Close()
	}
	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.
	if d.debug {
		if mean >= d.thresh {
			gocv.PutText(
				&img,
				fmt.Sprintf("motion - mean:%f", mean),
				image.Pt(32, 32),
				gocv.FontHersheyPlain,
				2.0,
				color.RGBA{255, 0, 0, 0},
				2,
			)
		}

		d.windows[0].IMShow(img)
		d.windows[1].IMShow(imgDelta)
		d.windows[0].WaitKey(1)
	}

	// 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)
}