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I have one function to compute convolution (to test whether we are using correct settings for filter2D), I think the function body is not important, so here's just header and the end:

template<typename T>
cv::Mat conv(const cv::Mat &input, const cv::Mat &kernel) {
    cv::Mat output(input); // or should I rather use output( input.rows, input.cols, input.depth())?

    ...
    return output;
}

cv::Mat result = conv( input, kernel);

At this point, I have completely useless results in result (those aren't even random data, they have some strange pattern which got repeated every time I run the function).

When I rewrite function to:

template<typename T>
void conv(const cv::Mat &input, cv::Mat &output, const cv::Mat &kernel) {
   ...
}

cv::Mat result(input);
conv( input, result, kernel);

Everything works just fine and result matrix contains exactly what it should.

So my question is: What's wrong on first approach? Am I doing something wrong? Why isn't assign operator/return from function working?

*Note: OpenCv version: extra/opencv 2.3.1_a-3 (archlinux package)*

Something similar happened to me when I was loading external data from opencv storage and data got lost until I used data( loaded.clone())

Vyktor
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1 Answers1

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Well, it seems that filter2d, or whatever you do, does not work 'in-place', that is, when input and output are the same. With your first line in the function,

cv::Mat output(input); // or should I rather use output( input.rows, input.cols, input.depth())?

you make output point to the same data as input! It is not a clone, it is another reference!

What you want to do is written in your comment. Another option may be (depending on your code) to let output completely uninitialized, as typically C++ OpenCV functions will initialize their output matrices for you, if they are empty.

Note that your conv(), even when giving a proper results, would always destroy your input matrix on the way. OpenCV does not respect const in its internal data reference meachanism. Yes, it is bad design.

ypnos
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  • If I understand correctly than the second you change the data (and I change them a lot, every cell) all data are copied to new "smart pointer item". If you take a look at the second approach, you will notice that I'm using `result(input)` and it works just fine than. – Vyktor Feb 02 '12 at 21:45
  • No, there is no copy-on-write in OpenCV, see http://stackoverflow.com/questions/6411476/opencv-matoperator-does-it-support-copy-on-write I don't know what your function in the second approach does, maybe it overwrites result in a way that it is re-initialized. Then input is left untouched. – ypnos Feb 02 '12 at 21:49
  • Yes I've read that question and answers and it make me believe that bot my approaches should work. Not just the second one. Body of function is in both codes same (it applies (Gabor) filter). – Vyktor Feb 02 '12 at 21:58
  • I'll be damned: http://pastebin.com/0NMQy7wU, http://pastebin.com/WgNAB6fm , thanks. – Vyktor Feb 03 '12 at 00:12