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Count black pixel each column manual

asked 2017-05-30 21:56:10 -0500

Kenny Karnama gravatar image

Hi guys, i have already counted black pixel from binary image using countNonZero function. And i was curious to implement it manually. Here is my code to count black pixel on each column :

for(int aa = 0; aa < after_shear.cols;aa++){
    Mat aaTmp = after_shear.col(aa);
    int  jumlah = 0;
    for(int oalah = 0; oalah < aaTmp.rows;oalah++){
      for(int oalah2 = 0; oalah2<aaTmp.cols;oalah2++){
        //cout <<(int)<uchar>(oalah,oalah2)<<endl;


But, it gives me zero result.

Can anyone help me ? Thanks

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answered 2017-05-31 03:43:52 -0500

updated 2017-06-01 04:10:46 -0500

Why not start with naming your valuables into something senseworthy, like rows, cols, i and j. It would make your code much clearer. That being said, I am unsure why you need 3 for loops to go over a 2D matrix, when in principle 2 should be enough.

Some sample code that should work just fine

int counter = 0;
for (int row=0; row<image.rows; row++){
    for(int col=0; col<image.cols; col++){
        if(<uchar>(row,col) == 0){

This will only work IF

  • The image is 8UC1, because the uchar is for an 8 bit jump.
  • The black is actually black, so not very small values like 1 or 3 or so.
  • Your image/mask is single channel. For a 3 channel you will need to use Vec3b iterators.

ADDITION: adding sample code for looking at transitions in columns

int previous = 0, current = 0, int counter = 0;
for (int col=1; col<image.rows; col++){
    for(int row=1; row<image.cols; row++){
        previous =<uchar>(row-1,col-1);
        current =<uchar>(row,col);
        if(previous  != current){

Notice that I start calculating from the second element, because else we cannot compare. Also I am expecting white to be 255 or 1 and black to be 0, so only 2 possible values.

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if i want to iterate each column, it means i should change<uchar>(row,col) to<uchar>(col,row) ? Thanks.

Kenny Karnama gravatar imageKenny Karnama ( 2017-05-31 06:20:42 -0500 )edit

But why iterate over columns, that is so inefficient... it will have you jump through memory like crazy...

StevenPuttemans gravatar imageStevenPuttemans ( 2017-05-31 08:26:23 -0500 )edit

And no, the indexing has to stay row,col --> thats how OpenCV uses it coordinates. You can however, change the order of the loops but again, that will take longer than this code.

StevenPuttemans gravatar imageStevenPuttemans ( 2017-05-31 08:27:16 -0500 )edit

sorry, i still don't understand. could you give me an example ? Because actually i want to count number of transition from black to white pixel on each column. Thanks

Kenny Karnama gravatar imageKenny Karnama ( 2017-06-01 03:18:24 -0500 )edit

Well actually that is something completely different. You never talked about transitions before. So for transitions in columns I will add some extra sample code.

StevenPuttemans gravatar imageStevenPuttemans ( 2017-06-01 04:05:45 -0500 )edit

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Asked: 2017-05-30 21:56:10 -0500

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Last updated: Jun 01 '17