Best way not to have to copy a Mat in OpenCV - c++

Question

I have this code that basically does a "dumb" background subtraction on two frames.
void FrameDifferenceBGS::operator()(cv::InputArray _image, cv::OutputArray _fgmask, double learningRate)
{
cv::Mat img_input = _image.getMat();
if(img_input.empty())
return;
_fgmask.create(img_input.size(), CV_8U);
cv::Mat img_foreground = _fgmask.getMat();
if(img_input_prev.empty())
{
img_input.copyTo(img_input_prev);
return;
}
cv::absdiff(img_input_prev, img_input, img_foreground);
if(img_foreground.channels() == 3)
cv::cvtColor(img_foreground, img_foreground, CV_BGR2GRAY);
if(enableThreshold)
cv::threshold(img_foreground, img_foreground, threshold, 255, cv::THRESH_BINARY);
if(showOutput)
cv::imshow("Frame Difference", img_foreground);
img_input.copyTo(img_input_prev);
img_foreground.copyTo(_fgmask);
firstTime = false;
}
If I don't add img_foreground.copyTo(_fgmask) in the end, the output array isn't updated with the result of img_foreground, resulting on a black image when this is called.
What am I doing wrong / should be doing here?

Answer 1

I reviewed your code again. It looks like you are creating new object for _fgmask.
_fgmask.create(img_input.size(), CV_8U);
I think this is why you have the problem. Because of this reference in the argument is different from the one after this statement. Why don't you call the line before calling the function.

Answer 2

fix
change _fgmask.create(img_input.size(), CV_8U); to _fgmask.create(img_input.size(), CV_8UC3); or _fgmask.create(img_input.size(), img_input.type());
why
this is because cv::absdiff(img_input_prev, img_input, img_foreground); recreate a new array everytime internally. and it does update the img_foreground structure but after the allocation, the memory address data inside _fgmask fail to change since the headers are passed by value.
you can seemlingly fix this(but still incurs creation cost) by doing cv::Mat& img_foreground = _fgmask.getMatRef();
and that is because CV_8U is not the same as CV_8UC3 and therefore the check # Mat::create() in mat.hpp always end up allocating a new array due to type difference
opinion
i think...maybe use Mat instead?
#include "opencv2/opencv.hpp"
using namespace cv;
class FrameDifferenceBGS
{
public:
Mat prev;
Mat diff;
bool enableThreshold;
bool showOutput;
bool firstTime;
uchar threshold;
FrameDifferenceBGS():firstTime(false),enableThreshold(false),showOutput(false),threshold(0)
{
}
void FrameDifferenceBGS::operator()(cv::Mat& _in, cv::Mat &_fg, double _lr)
{
if(_in.empty())
return;
if(prev.empty())
{
prev=_in.clone();
_fg=cv::Mat::zeros(_in.size(),CV_8UC1);
return;
}
cv::absdiff(prev, _in, diff);
if(diff.channels() == 3)
cv::cvtColor(diff, _fg, CV_BGR2GRAY);
else
_fg=diff;
if(enableThreshold)
cv::threshold(_fg, _fg, threshold, 255, cv::THRESH_BINARY);
if(showOutput)
cv::imshow("Frame Difference", _fg);
prev=_in.clone();
firstTime = false;
}
};
int main()
{
VideoCapture cap(0);
FrameDifferenceBGS bgs;
Mat frame,fg;
for(;;)
{
cap >> frame;
bgs(frame,fg,0);
imshow("frame", frame);
imshow("fg", fg);
if(waitKey(1) ==27) exit(0);
}
return 0;
}
edit 2(modified original)
#include "opencv2/opencv.hpp"
class FrameDifferenceBGS
{
public:
cv::Mat img_input_prev;
cv::Mat diff;
cv::Mat img_foreground;//put this in class in stead of inside the function
bool enableThreshold;
bool showOutput;
bool firstTime;
uchar threshold;
FrameDifferenceBGS():firstTime(false),enableThreshold(false),showOutput(false),threshold(0)
{
}
void FrameDifferenceBGS::operator()(cv::InputArray _image, cv::OutputArray _fgmask, double learningRate)
{
cv::Mat img_input = _image.getMat();
if(img_input.empty())
return;
if(_fgmask.empty())
_fgmask.create(img_input.size(), CV_8UC1);
if(img_input_prev.empty())
{
img_input.copyTo(img_input_prev);
return;
}
cv::absdiff(img_input_prev, img_input, img_foreground);
if(img_foreground.channels() == 3)
cv::cvtColor(img_foreground, _fgmask, CV_BGR2GRAY);
if(enableThreshold)
cv::threshold(img_foreground, img_foreground, threshold, 255, cv::THRESH_BINARY);
if(showOutput)
cv::imshow("Frame Difference", img_foreground);
img_input.copyTo(img_input_prev);
//img_foreground.copyTo(_fgmask);
firstTime = false;
}
};
int main()
{
cv::VideoCapture cap(0);
FrameDifferenceBGS bgs;
cv::Mat frame,fg;
for(;;)
{
cap >> frame;
bgs(frame,fg,0);
cv::imshow("frame", frame);
cv::imshow("fg", fg);
if(cv::waitKey(1) ==27) exit(0);
}
return 0;
}