Accessing a matrix element in the Mat object not the CvMat object in OpenCV C

Question

How to access elements by row  col in OpenCV 2 0 s new  Mat  class  The documentation is linked below  but I have not been able to make any sense of it   http   opencv willowgarage com documentation cpp basic structures html mat

User · Answer

For cv  Mat  lt T gt  mat just use mat row  col  Accessing elements of a matrix with specified type cv  Mat  lt   Tp  gt  is more comfortable  as you can skip the template specification  This is pointed out in the documentation as well  code  cv  Mat1d mat0   cv  Mat1d  zeros 3  4   std  cout  lt  lt   quot mat0  n quot   lt  lt  mat0  lt  lt  std  endl  std  cout  lt  lt   quot element   quot   lt  lt  mat0 2  0   lt  lt  std  endl  std  cout  lt  lt  std  endl   cv  Mat1d mat1    cv  Mat1d 3  4   lt  lt      1  NAN  10 5  NAN      NAN  -99   5  NAN      -70  NAN  -2  NAN   std  cout  lt  lt   quot mat1  n quot   lt  lt  mat1  lt  lt  std  endl  std  cout  lt  lt   quot element   quot   lt  lt  mat1 0  2   lt  lt  std  endl  std  cout  lt  lt  std  endl   cv  Mat mat2   cv  Mat 3  4  CV 32F  0 0   std  cout  lt  lt   quot mat2  n quot   lt  lt  mat2  lt  lt  std  endl  std  cout  lt  lt   quot element   quot   lt  lt  mat2 at lt float gt  2  0   lt  lt  std  endl  std  cout  lt  lt  std  endl   output  mat0   0  0  0  0   0  0  0  0   0  0  0  0  element  0  mat1   1  nan  10 5  nan   nan  -99  0 5  nan   -70  nan  -2  nan  element  10 5  mat2   0  0  0  0   0  0  0  0   0  0  0  0  element  0

User · Answer

The ideas provided above are good  For fast access  in case you would like to make a real time application  you could try the following     suppose you read an image from a file that is gray scale Mat image   imread  Your path   CV 8UC1        do some processing uint8 t  myData   image data  int width   image cols  int height   image rows  int  stride   image step   in case cols    strides for int i   0  i  lt  height  i          for int j   0  j  lt  width  j                  uint8 t val   myData  i    stride   j             do whatever you want with your value           Pointer access is much faster than the Mat at lt   accessing  Hope it helps

User · Answer

Based on what  J  Calleja said  you have two choices  Method 1 - Random access  If you want to random access the element of Mat  just simply use   Mat at lt data Type gt  row num  col num    value    Method 2 - Continuous access  If you want to continuous access  OpenCV provides Mat iterator compatible with STL iterator and it s more C   style  MatIterator  lt double gt  it  end  for  it   I begin lt double gt     end   I end lt double gt     it    end    it          do something here     or  for int row   0  row  lt  mat rows    row        float  p   mat ptr row     pointer p points to the first place of each row     for int col   0  col  lt  mat cols    col              p        operation here           If you have any difficulty to understand how Method 2 works  I borrow the picture from a blog post in the article Dynamic Two-dimensioned Arrays in C  which is much more intuitive and comprehensible    See the picture below

User · Answer

On the documentation   http   docs opencv org 2 4 modules core doc basic structures html mat  It says            if you know the matrix element   type  e g  it is float  then you can   use at lt     method   That is  you can use   Mat M 100  100  CV 64F   cout  lt  lt  M at lt double gt  0 0     Maybe it is easier to use the Mat  class  It is a template wrapper for Mat   Mat  has the operator   overloaded in order to access the elements

User · Answer

OCV goes out of its way to make sure you can t do this without knowing the element type  but if you want an easily codable but not-very-efficient way to read it type-agnostically  you can use something like  double val mean someMat Rect x y 1 1    channel     To do it well  you do have to know the type though  The at lt   method is the safe way  but direct access to the data pointer is generally faster if you do it correctly

[c++] Accessing a matrix element in the "Mat" object (not the CvMat object) in OpenCV C++

For `cv::Mat_<T> mat` just use `mat(row, col)`

Examples related to c++

Examples related to matrix

Examples related to opencv

[c++] Accessing a matrix element in the "Mat" object (not the CvMat object) in OpenCV C++

For cv::Mat_<T> mat just use mat(row, col)

Examples related to c++

Examples related to matrix

Examples related to opencv

For `cv::Mat_<T> mat` just use `mat(row, col)`