Reputation: 2678

opencv binary threshold with C API: set threshold very small will filter out most pixel

Here is the picture in grayscale mode:

if I apply a Thresholding and set threshold to 0. According to my understanding, the thesholded image will be mostly white. but the result is opposite.

Result is:

I also tried this:

build a image and set all pixel to 255. then apply the 0 threshold thresholding, the returned image is all 255.

The question is: in the picture is mostly zero (black) after apply thresholding.

Here are the code:

IplImage*   g_image = NULL;
IplImage*   g_gray = NULL;
int         g_thresh = 100;
CvMemStorage*   g_storage = NULL;


void on_tracker(int){
    if(g_storage == NULL){
        g_gray = cvCreateImage(cvGetSize(g_image), 8, 1);
        g_storage = cvCreateMemStorage(0);
    }else{
        cvClearMemStorage(g_storage);
    }

    CvSeq* contours = 0;
    cvCvtColor(g_image, g_gray, CV_BGR2GRAY);
    cvNamedWindow("Gray");
    cvShowImage("Gray", g_gray);
    cvThreshold(g_gray, g_gray, g_thresh, 255, CV_THRESH_BINARY);
    cvFindContours(g_gray, g_storage, &contours);
    cvShowImage("Contours", g_gray);
}
int main(int argc, char** argv){
    if( argc !=2 || !(g_image = cvLoadImage(argv[1]))){
        return -1;
    }

    cvNamedWindow("Contours", CV_WINDOW_AUTOSIZE);
    cvCreateTrackbar(
        "Threshold",
        "Contours",
        &g_thresh,
        255,
        on_tracker
    );
    on_tracker(0);
    cvWaitKey();
    return 0;
}

Upvotes: 0

Answers (3)

scott huang

Reputation: 2678

Accoring to @Miki's comments. this is caused by C API. I tried the same process with python API. the result is normal: if I do thresholding with 0 threshold, most of pixel will be set to 255.

Upvotes: 0

Itzik Chaimov

Reputation: 99

The basic Thresholding is to check the pixels value (say from 0 to 255) to be above the Threshold value and to assign to the pixel a value of maximum value (high intensity: black) this called Binary Thresholding. In your case, when setting a value of 0 to the threshold, you actually filtering all your pixels since all of them (the low intensities and the higher intensities) have values above zero (0).

Maybe you would like to make a brighter picture - in this case use Inverted Binary Thresholding: in this case, you will get white picture when value is 0.

Upvotes: 0