OpenCV C# Frame Differencing-CodePudding

I'm new to computer vision and currently playing around with static frame differencing to try and determine whether there is motion in video.

My variables:

public Mat currentFrame = new Mat();
public Mat prevFrame = new Mat();
public Mat result = new Mat();
bool motion = false;

Simple differencing function (being called every frame):

public Mat getDifference(Mat videoFrame)
{
    currentFrame = videoFrame.Clone();
    Cv2.Absdiff(currentFrame, prevFrame, result);
    prevFrame = currentFrame.Clone();
    return result;
}

When motion exists the result matrix looks like this:

When motion doesn't exist the result matrix looks like this (empty):

My original idea was that if the result matrix is effectively empty (all black), then I could say motion = false. However, this is proving to be more difficult that anticipated since it is technically never empty, so I can't say:

if(!result.Empty())
{
    motion = true;
}

Without the need of for loops and pixel by pixel analysis, is there a simple/clean 'if' statement I can use that simply says (if the matrix contains anything that isn't black pixels, motion = true). Or... is this too simplistic? I'm open to hearing better ways of doing this, I had a look around on the web but there aren't many solid examples for C#. My video is playing within a WPF application in real-time so nested for loops are to be avoided.

Thanks for your time!

CodePudding user response：

You could for example convert the matrix to an image. That should give you access to all the image manipulation functions. For example ThresholdBinary to make pixels either zero or a given value, and CountNonZero. That should give you some tools balance how much things need to change, and how large area need to change.

CodePudding user response：

Found a simple way to do it, may not be the best but it does work.

public bool motion = false;
public Mat currentFrame = new Mat();
public Mat prevFrame = new Mat();
public Mat absDiffImage = new Mat();
public Mat grayImage = new Mat();

public Point[][] frameContours;
public HierarchyIndex[] external;
public Mat frameThresh = new Mat();

Cv2.CvtColor(currentFrame, currentFrame, ColorConversionCodes.BGRA2GRAY);
Cv2.Absdiff(currentFrame, prevFrame, absDiffImage);
Cv2.Threshold(absDiffImage, frameThresh, 80, 255, ThresholdTypes.Binary);
Cv2.FindContours(frameThresh, out frameContours, out external, RetrievalModes.List, ContourApproximationModes.ApproxSimple);

if (frameContours.Length > 20)
{
    motion = true;
}
else
{
    motion = false;
}