OpenCV Error: Assertion failed in matrix.cpp line 522, /matrix.cpp:522: error: (-215)

Question

I am trying to create an ROI above the face detected to place a hat like shown in the image: Plz click here : ROI created above face to place a hat

I have made sure that the ROI created is withing the bounds of the image. It looks like this: // Create an ROI // where face is the detected face ROI

    if (0<=face.x && 0<=face.x-face.width*0.08<=image.cols && 0<=face.x+face.width+face.width*0.08<=image.cols 
         && 0<=face.y && 0<=face.y-face.height*0.28<=image.rows)
    {
      Mat ROI_hat = image(Rect(abs(face.x-face.width*0.08),abs(face.y-face.height*0.28),abs(face.x+face.width+face.width*0.08),abs(face.y)));
      rectangle(image,Point(abs(face.x-face.width*0.08),abs(face.y-face.height*0.28)),Point(abs(face.x+face.width+face.width*0.08),abs(face.y)),Scalar(255, 0, 0), 1, 4);

      cout<<"Within the bounds of Image"<<endl;
    }
    else{
     cout<<" Out of bounds of Image "<<endl;
        }

There are no negative values and for every frame it says ROI is withing the bounds. But I still get assertion error:

OpenCV Error: Assertion failed (0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= m.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= m.rows) in Mat, file /home/user/OpenCV_Installed/opencv-3.2.0/modules/core/src/ma‌​trix.cpp, line 522 terminate called after throwing an instance of 'cv::Exception' what(): /home/user/OpenCV_Installed/opencv-3.2.0/modules/core/src/ma‌​trix.cpp:522: error: (-215) 0 <= roi.x && 0 <= roi.width && roi.x + roi.width <= m.cols && 0 <= roi.y && 0 <= roi.height && roi.y + roi.height <= m.rows in function Mat Aborted (core dumped)

Can someone please tell me where I'm going wrong?

Answer 1

The error means that your ROI is outside of the image, so your conditions are wrong.

Since it's pretty easy to get confused, I usually apply this small trick that is based on the intersection of the roi with a dummy roi roiImg that contains all the image:

Rect roiImg(0, 0, image.cols, image.rows);
Rect roi = ... // Very complex way of setting up the ROI

if( (roi.area() > 0) && ((roiImg & roi).area() == roi.area()) ) {
    // roi is inside the image, and is non-empty
    // VALID roi
} else {
    // roi is at least partially outside of the image, or it's empty
    // INVALID roi
}