OpenCV pixel access pointr vs. at() - Different values

Question

I encountered weird behaviour when trying to access pixels as shown below:

void Dbscan::regionQuery(int i, int j, std::vector<Point>* res) const {
// check rect. grid around center point
const size_t row_min = std::max(0, i-eps_);
const size_t row_max = std::min(n_rows_, i+eps_+1);
const size_t col_min = std::max(0, j-eps_);
const size_t col_max = std::min(n_cols_, j+eps_+1);
assert(masked_img_.depth() == CV_8UC1);
for (int m = row_min; m<row_max; ++m) {
    const uchar* mask_ptr = masked_img_.ptr(m);
    for (int n = col_min; n<col_max; ++n) {
        assert(*mask_ptr == masked_img_.at<uchar>(m, n));
        if (masked_img_.at<uchar>(m, n) == 255) {
            res->emplace_back(Point(m,n));
        }
        mask_ptr++;
    }
}

Basically, the second assertion as shown fails and I'm rather clueless as to what is going on. Does anyone have an idea how to best approach debugging the problem above?

Bests regards

Felix

did you try `*(mask_ptr + n)` (without `mask_ptr++` at the end)? — slawekwin, Sep 11 '18 at 13:16
does it work if you remove `mask_ptr++` and choose `assert(mask_ptr[n] == masked_img_.at(m, n));` ? — Micka, Sep 11 '18 at 13:25
@Isma The assertion fails, e.g. the accessed pixel values are different — felix, Sep 11 '18 at 13:30
@Micka Thanks, that also works. Can you imagine why incrementing the pointer would fail? — felix, Sep 11 '18 at 13:31

score 1 · Accepted Answer · answered Sep 11 '18 at 13:31

1

cv::Mat::ptr returns a pointer to the beginning of the row from the argument, which is an address of an element in the first column of this row. cv::Mat::at returns a reference to the element in the row and column from the argument. In your code the row matches, but the column doesn't (unless your col_min evaluates to 0), thus you need to move the pointer from cv::Mat::ptr n elements to match your column as well:

for (int m = row_min; m<row_max; ++m) {
    const uchar* mask_ptr = masked_img_.ptr(m);
    for (int n = col_min; n<col_max; ++n) {
        assert(*(mask_ptr + n) == masked_img_.at<uchar>(m, n));
        if (masked_img_.at<uchar>(m, n) == 255) {
            res->emplace_back(Point(m,n));
        }
    }
}

answered Sep 11 '18 at 13:31

slawekwin

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Thanks a lot, that makes sense! – felix Sep 11 '18 at 13:35
but `mask_ptr++` should make sure that mask_ptr is always at the right col position, shouldn't it? – Micka Sep 11 '18 at 13:55
1

@Micka only as long as you start with the same element - `mask_ptr++` only guarantees that you move the pointer every iteration, but the initial offset is constant. You could keep the `mask_ptr++` (and skip the `+n` part) if you changed `mask_ptr` initialization to `masked_img_.ptr(m) + col_min` – slawekwin Sep 11 '18 at 13:57
1

yeah sorry, just saw the `col_min` myself. Thx. – Micka Sep 11 '18 at 13:58

OpenCV pixel access pointr vs. at() - Different values

1 Answers1