Access dimensions in color histogram openCV

Question

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!

Hi, thanks for your answer, but there they're splitting the image before and computing the individual histograms as I mention. I'm asking about handling the 3 dimensions matrix resulted from computing the histogram directly form the colored image: cv::calcHist(&image,1,channels,cv::Mat(), hist,3,histSize,ranges); It's more a question about how to handle these dimensions (no channels) than about the histogram. — Pablo

Andrey Smorodov Andrey Smorodov · Accepted Answer · 2014-09-29T19:26:39

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.

Access dimensions in color histogram openCV

1 Answers