CODEWITHSUNDEEP
c++opencvmultidimensional-arrayhistogramchannel
Pablo
Pablo

Reputation: 11

Access dimensions in color histogram openCV

I'm a novice in openCV and here goes my first doubt about matrix dimension subjects.

I am computing the Histogram of a colored image by means of the function cv::calcHist(..).

The resulting matrix is, as I expected, a 3D matrix. I guess the third dimension means the color for each RGB color channel but I don't know how to access them. After computing it, I have a matrix of 3 dimensions and 1 channel, and I'd like to be able to access each single dimension.

I think split function can't help here since it only splits a matrix into its channels.

Debugging I obtain the following relevant info from the 3Dhistrogram matrix:

Dimensions: 3, Rows: -1, Columns: -1, Size: 256

I know I can obtain the individual color histogram by splitting first the image into the 3 channels and computing the 1D histogram of each one afterwards but I'm curious about knowing how work the dimensions in openCV.

Thanks in advance!

Upvotes: 1

Views: 389

Answers (1)

Andrey Smorodov
Andrey Smorodov

Reputation: 10852

Here is from Opencv's reference on MatND class:

   // return pointer to the element (versions for 1D, 2D, 3D and generic nD cases)
    uchar* ptr(int i0);
    const uchar* ptr(int i0) const;
    uchar* ptr(int i0, int i1);
    const uchar* ptr(int i0, int i1) const;
    uchar* ptr(int i0, int i1, int i2);
    const uchar* ptr(int i0, int i1, int i2) const;
    uchar* ptr(const int* idx);
    const uchar* ptr(const int* idx) const;

    // convenient template methods for element access.
    // note that _Tp must match the actual matrix type -
    // the functions do not do any on-fly type conversion
    template<typename _Tp> _Tp& at(int i0);
    template<typename _Tp> const _Tp& at(int i0) const;
    template<typename _Tp> _Tp& at(int i0, int i1);
    template<typename _Tp> const _Tp& at(int i0, int i1) const;
    template<typename _Tp> _Tp& at(int i0, int i1, int i2);
    template<typename _Tp> const _Tp& at(int i0, int i1, int i2) const;
    template<typename _Tp> _Tp& at(const int* idx);
    template<typename _Tp> const _Tp& at(const int* idx) const;

So, you can use array of 3 elements as an argument of .at method to set required element position.

Upvotes: 0

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