how to use cv::Mat imgbuf created by pointer to video data - c++

Question

I am reading the raw video data from the read buffer using
cv::Mat imgbuf(Size(640, 480), CV_8UC3, &mem[0], (640*3));
This variable imgbuf I am passing to face detection algorithm which detects the face & draws the rectangle around the face. after that I am getting output something like
I tried with below code where I am performing resize operation before pass to face detection algorithm. by using this method it is working fine. but without resizing function I am getting noticeable output with rectangle around the face.
while(1)
{
unsigned char *mem = (unsigned char*)mmap(NULL, page_offset + len,
PROT_READ |PROT_WRITE, MAP_PRIVATE, fd, page_base);
cv::Mat imgbuf(Size(640, 480), CV_8UC3, &mem[0], (640*3));
cv::resize(imgbuf,imgbuf,(640,480)); //Dummy function to get the right output.
auto result = v->facedetection(imgbuf);
for (const auto &r : result.rects) {
cv::rectangle(imgbuf,cv::Rect{ cv::Point(r.x * imgbuf.cols, r.y *
imgbuf.rows),cv::Size{(int)(r.width * imgbuf.cols), (int)(r.height *
imgbuf.rows) } },0xff);
}
imshow("face-detection", imgbuf);
waitKey(1);
can anybody help be to sort out this problem

Answer 1

Test this method:
unsigned char *mem = (unsigned char*)mmap(NULL, page_offset + len,
PROT_READ |PROT_WRITE, MAP_PRIVATE, fd, page_base);
cv::Mat imgbuf(480,640, CV_8UC3, &mem[0]);
cv::Mat img_2, img_3;
cv::resize(imgbuf,img_2,cv::Size(640,480));
img_2.copyTo(img_3);
auto result = v->facedetection(img_2);
for (const auto &r : result.rects)
{
cv::Rect myR = cv::Rect(r.x * img_2.cols, r.y * img_2.rows, (int)(r.width * img_2.cols),
(int)(r.height * img_2.rows));
cv::rectangle(img_3,myR,Scalar(0, 0, 255), 1);
}
imshow("Result", img_3);
waitKey(0);
After getting a valid result you can optimize this and use less of "Mat"s.