opencv2 resize image not working on Mat with user data

Question

I'm using opencv2. I created 2 cv::Mat with external user data ( matLoad and matResize . And resize matLoad to matResize . However, after the cv::resize function is called, the user data is still empty (all 0xCD ). But matResize.data is changed and no longer equal to the pointer to the user data. It seems that cv::resize resets the destination Mat's data property, with a new piece of memory space allocated inside opencv. See below:

cv::Mat matLoad(cutw, cuth, cvfmt, mDataLoad);
... // Read image data into `mDataLoad`
cv::Mat matResize(rcutw, rcuth, cvfmt, mDataProc);
printf("%s\n", (matResize.data == mDataProc) ? "equal" : "notequal"); // "equal" is called
cv::resize(matLoad, matResize, cv::Size(rcutw, rcuth));
printf("%s\n", (matResize.data == mDataProc) ? "equal" : "notequal"); // "notequal" is printed
for (int i = 0; i < matResize.elemSize() * matResize.cols * matResize.rows; i++) {
  if (matResize.data[i] != 205) {
    printf("hehe %d\n", matResize.data[i]); // many lines printed, with different values
  }
}
for (int i = 0; i < matResize.elemSize() * matResize.cols * matResize.rows; i++) {
  if (mDataProc[i] != 205) {
    printf("hehe %d\n", mDataProc[i]); // none is printed
  }
}

But if I create matResize without passing the user data (ie, let opencv allocate and maintain the memory space by itself), the resize works well.

cv::Mat matLoad(cutw, cuth, cvfmt, mDataLoad);
... // Read image data into `mDataLoad`
cv::Mat matResize(rcutw, rcuth, cvfmt);
cv::resize(matLoad, matResize, cv::Size(rcutw, rcuth));
for (int i = 0; i < matResize.elemSize() * matResize.cols * matResize.rows; i++) {
  if (matResize.data[i] != 205) {
    printf("hehe %d\n", matResize.data[i]); // many lines printed, with different values
  }
}

What's wrong with the cv::Mat created with external user data?

Answer 1

If the dimension of the resized matrix is different from the destination matrix, then the destination matrix will be re-allocated. So to avoid re-allocations you need to be sure that the dimensions agree.

In your code you're setting the destination size as:

cv::Mat matResize(rcutw, rcuth, cvfmt, mDataProc);

but you're asking for a resized image of size:

cv::Size(rcutw, rcuth)

You inverted rows (height) and columns (width).

So simply create the matrix with the correct size with:

 cv::Mat matResize(rcuth, rcutw, cvfmt, mDataProc);
                   ^^^^^^^^^^^^^