How to get all pixels in mask in C-CodePudding

In python, we can use such code to fetch all pixels under mask:

src_img = cv2.imread("xxx")
mask = src_img > 50
fetch = src_img[mask]

what we get is a ndarray including all pixels matching condition mask. How to implement the same function using C opencv ?

I've found that copyTo can select pixels under specified mask, but it can only copy those pixels to another Mat instead of what python did.

CodePudding user response：

This is not that straightforward in C (as expected). That operation breaks down in further, smaller operations. One way to achieve a std::vector with the same pixel values above your threshold is this, I'm using this test image:

// Read the input image:
std::string imageName = "D://opencvImages//grayDog.png";
cv::Mat inputImage =  cv::imread( imageName );

// Convert BGR to Gray:
cv::Mat grayImage;
cv::cvtColor( inputImage, grayImage, cv::COLOR_RGB2GRAY );

cv::Mat mask;
int thresholdValue = 50;
cv::threshold( grayImage, mask, thresholdValue, 255, cv::THRESH_BINARY );

The above bit just creates a cv::Mat where each pixel above the threshold is drawn with a value of 255, 0 otherwise. It is (one possible) equivalent of mask = src_img > 50. Now, let's mask the original grayscale image with this mask. Think about an element-wise multiplication between the two cv::Mats. One possible way is this:

// Create grayscale mask:
cv::Mat output;
grayImage.copyTo( output, mask );

Now we have the original pixel values and everything else is zero. Convenient, because we can find now the locations of the non-zero pixels:

// Locate the non-zero pixel values:
std::vector< cv::Point > pixelLocations;
cv::findNonZero( output, pixelLocations );

Alright, we have a std::vector of cv::Points that locate each non-zero pixel. We can use this info to index the original grayscale pixels in the original matrix:

// Extract each pixel value using its location:
std::vector< int > pixelValues;
int totalPoints = (int)pixelLocations.size();

for( int i = 0; i < totalPoints; i   ){
    // Get pixel location:
    cv::Point currentPoint = pixelLocations[i];

    // Get pixel value:
    int currentPixel = (int)grayImage.at<uchar>( currentPoint );
    pixelValues.push_back( currentPixel );

    // Print info:
    std::cout<<"i: "<<i<<" currentPoint: "<<currentPoint<<" pixelValue: "<<currentPixel<<std::endl;
}

You end up with pixelValues, which is a std::vector containing a list of all the pixels that are above your threshold.

CodePudding user response：

Why do you hate writing loop?

I think this is the easiest way:

cv::Mat Img = ... //Where, this Img is 8UC1

// * In this sample, extract the pixel positions
std::vector< cv::Point > ResultData;

const unsigned char Thresh = 50;
for( int y=0; y<Img.rows;   y )
{
    const unsigned char *p = Img.ptr<unsigned char>(y);
    for( int x=0; x<Img.cols;   x,   p )
    {
        if( *p > Thresh )
        {//Here, pick up this pixel's info you want.
            ResultData.emplace_back( x,y );
        }
    }
}