Convert Mat to Array Vector in OpenCV

Question

I am novice in OpenCV  Recently  I have troubles finding OpenCV functions to convert from Mat to Array  I researched with  ptr and  at methods available in OpenCV APIs  but I could not get proper data  I would like to have direct conversion from Mat to Array if available  if not to Vector   I need OpenCV functions because the code has to be undergo high level synthesis in Vivado HLS  Please help

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Instead of getting image row by row  you can put it directly to an array  For CV 8U type image  you can use byte array  for other types check here    Mat img     Should be CV 8U for using byte   int size    int img total     img channels    byte   data   new byte size   img get 0  0  data      Gets all pixels

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Here is another possible solution assuming matrix have one column  you can reshape original Mat to one column Mat via reshape    Mat matrix  Mat  zeros 20  1  CV 32FC1   vector lt float gt  vec  matrix col 0  copyTo vec

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Can be done in two lines     Mat to array  uchar   arr   image isContinuous    image data  image clone   data  uint length   image total   image channels      Mat to vector  cv  Mat flat   image reshape 1  image total   image channels     std  vector lt uchar gt  vec   image isContinuous    flat   flat clone      Both work for any general cv  Mat   Explanation with a working example      cv  Mat image      image   cv  imread argv 1   cv  IMREAD UNCHANGED        Read the file     cv  namedWindow  cvmat   cv  WINDOW AUTOSIZE      Create a window for display      cv  imshow  cvmat   image                         Show our image inside it          flatten the mat      uint totalElements   image total   image channels       Note  image total      rows cols      cv  Mat flat   image reshape 1  totalElements      1xN mat of 1 channel  O 1  operation     if  image isContinuous              flat   flat clone       O N                flat data is your array pointer     auto   ptr   flat data     usually  its uchar         You have your array  its length is flat total    rows 1  cols totalElements         Converting to vector     std  vector lt uchar gt  vec flat data  flat data   flat total            Testing by reconstruction of cvMat     cv  Mat restored   cv  Mat image rows  image cols  image type    ptr      OR vec data   instead of ptr     cv  namedWindow  reconstructed   cv  WINDOW AUTOSIZE       cv  imshow  reconstructed   restored        cv  waitKey 0          Extended explanation   Mat is stored as a contiguous block of memory  if created using one of its constructors or when copied to another Mat using clone   or similar methods  To convert to an array or vector we need the address of its first block and array vector length    Pointer to internal memory block  Mat  data is a public uchar pointer to its memory  But this memory may not be contiguous  As explained in other answers  we can check if mat data is pointing to contiguous memory or not using mat isContinous    Unless you need extreme efficiency  you can obtain a continuous version of the mat using mat clone   in O N  time   N   number of elements from all channels   However  when dealing images read by cv  imread   we will rarely ever encounter a non-continous mat    Length of array vector  Q  Should be row cols channels right  A  Not always  It can be rows cols x y channels  Q  Should be equal to mat total    A  True for single channel mat  But not for multi-channel mat Length of the array vector is slightly tricky because of poor documentation of OpenCV  We have Mat  size public member which stores only the dimensions of single Mat without channels  For RGB image  Mat size    rows  cols  and not  rows  cols  channels   Mat total   returns total elements in a single channel of the mat which is equal to product of values in mat size  For RGB image  total     rows cols  Thus  for any general Mat  length of continuous memory block would be mat total   mat channels     Reconstructing Mat from array vector  Apart from array vector we also need the original Mat s mat size  array like  and mat type    int   Then using one of the constructors that take data s pointer  we can obtain original Mat  The optional step argument is not required because our data pointer points to continuous memory  I used this method to pass Mat as Uint8Array between nodejs and C    This avoided writing C   bindings for cv  Mat with node-addon-api   References    Create memory continuous Mat     OpenCV Mat data layout     Mat from array

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byte   matToBytes Mat image       int size   image total     image elemSize       byte   bytes   new byte size      delete   later    std  memcpy bytes image data size   sizeof byte

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None of the provided examples here work for the generic case  which are N dimensional matrices   Anything using  rows  assumes theres columns and rows only  a 4 dimensional matrix might have more   Here is some example code copying a non-continuous N-dimensional matrix into a continuous memory stream - then converts it back into a Cv  Mat   include  lt iostream gt   include  lt cstdint gt   include  lt cstring gt   include  lt opencv2 opencv hpp gt   int main int argc  char  argv        if   argc    2                 std  cerr  lt  lt   Usage     lt  lt  argv 0   lt  lt     lt Image Path gt  n           return -1            cv  Mat origSource   cv  imread argv 1  1        if   origSource data            std  cerr  lt  lt   Can t read image           return -1                this will select a subsection of the original source image - WITHOUT copying the data         the header will point to a region of interest  adjusting data pointers and row step sizes      cv  Mat sourceMat   origSource cv  Range origSource size 0  4  3 origSource size 0   4  cv  Range origSource size 1  4  3 origSource size 1   4            correctly copy the contents of an N dimensional cv  Mat        works just as fast as copying a 2D mat  but has much more difficult to read code           see http   stackoverflow com questions 18882242 how-do-i-get-the-size-of-a-multi-dimensional-cvmat-mat-or-matnd        copy this code in your own cvMat To Char Array   function which really OpenCV should provide somehow           keep in mind that even Mat  clone   aligns each row at a 4 byte boundary  so uneven sized images always have stepgaps     size t totalsize   sourceMat step sourceMat dims-1       const size t rowsize   sourceMat step sourceMat dims-1    sourceMat size sourceMat dims-1       size t coordinates sourceMat dims-1     0       std  cout  lt  lt   Image dimensions         for  int t 0 t lt sourceMat dims t                     calculate total size of multi dimensional matrix by multiplying dimensions         totalsize  sourceMat size t           std  cout  lt  lt   t gt 0   X        lt  lt  sourceMat size t                Allocate destination image buffer     uint8 t   imagebuffer   new uint8 t totalsize       size t srcptr 0 dptr 0      std  cout  lt  lt  std  endl      std  cout  lt  lt   One pixel in image has    lt  lt  sourceMat step sourceMat dims-1   lt  lt    bytes   lt  lt std  endl      std  cout  lt  lt   Copying data in blocks of    lt  lt  rowsize  lt  lt    bytes   lt  lt  std  endl       std  cout  lt  lt   Total size is    lt  lt  totalsize  lt  lt    bytes   lt  lt  std  endl      while  dptr lt totalsize               we copy entire rows at once  so lowest iterator is always  dims-2             this is legal since OpenCV does not use 1 dimensional matrices internally  a 1D matrix is a 2d matrix with only 1 row          std  memcpy  amp imagebuffer dptr   amp    uint8 t  sourceMat data  srcptr   rowsize              destination matrix has no gaps so rows follow each other directly         dptr    rowsize             src matrix can have gaps so we need to calculate the address of the start of the next row the hard way            see  brief  text in opencv2 core mat hpp for address calculation         coordinates sourceMat dims-2             srcptr   0          for  int t sourceMat dims-2 t gt  0 t--                if  coordinates t  gt  sourceMat size t                     if  t  0  break                  coordinates t  0                  coordinates t-1                               srcptr    sourceMat step t  coordinates t                         this constructor assumes that imagebuffer is gap-less  if not  a complete array of step sizes must be given  too     cv  Mat destination cv  Mat sourceMat dims  sourceMat size  sourceMat type     void  imagebuffer          and just to proof that sourceImage points to the same memory as origSource  we strike it through    cv  line sourceMat cv  Point 0 0  cv  Point sourceMat size 1  sourceMat size 0   CV RGB 255 0 0  3       cv  imshow  original image  origSource      cv  imshow  partial image  sourceMat      cv  imshow  copied image  destination      while  cv  waitKey 60    q

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cv  Mat m  m create 10  10  CV 32FC3    float  array    float   malloc  3 sizeof float  10 10    cv  MatConstIterator  lt cv  Vec3f gt  it   m begin lt cv  Vec3f gt     for  unsigned i   0  it    m end lt cv  Vec3f gt     it           for   unsigned j   0  j  lt  3  j                 array   i       it  j           i             Now you have a float array  In case of 8 bit  simply change float to uchar  Vec3f to Vec3b and CV 32FC3 to CV 8UC3

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You can use iterators  Mat matrix         std  vector lt float gt  vec matrix begin lt float gt     matrix end lt float gt

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If the memory of the Mat mat is continuous  all its data is continuous   you can directly get its data to a 1D array   std  vector lt uchar gt  array mat rows mat cols mat channels     if  mat isContinuous        array   mat data    Otherwise  you have to get its data row by row  e g  to a 2D array   uchar   array   new uchar  mat rows   for  int i 0  i lt mat rows    i      array i    new uchar mat cols mat channels      for  int i 0  i lt mat rows    i      array i    mat ptr lt uchar gt  i       UPDATE  It will be easier if you re using std  vector  where you can do like this   std  vector lt uchar gt  array  if  mat isContinuous           array assign mat datastart  mat dataend       lt - has problems for sub-matrix like mat   big mat row i    array assign mat data  mat data   mat total   mat channels       else     for  int i   0  i  lt  mat rows    i        array insert array end    mat ptr lt uchar gt  i   mat ptr lt uchar gt  i  mat cols mat channels             p s   For cv  Mats of other types  like CV 32F  you should do like this   std  vector lt float gt  array  if  mat isContinuous           array assign  float  mat datastart   float  mat dataend       lt - has problems for sub-matrix like mat   big mat row i    array assign  float  mat data   float  mat data   mat total   mat channels       else     for  int i   0  i  lt  mat rows    i        array insert array end    mat ptr lt float gt  i   mat ptr lt float gt  i  mat cols mat channels               UPDATE2  For OpenCV Mat data continuity  it can be summarized as follows    Matrices created by imread    clone    or a constructor will always be continuous  The only time a matrix will not be continuous is when it borrows data  except the data borrowed is continuous in the big matrix  e g  1  single row  2  multiple rows with full original width  from an existing matrix  i e  created out of an ROI of a big mat     Please check out this code snippet for demonstration

[c++] Convert Mat to Array/Vector in OpenCV

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