Reading pixels data values from a bitmap image file

Question

here is the code i have written in c++

  int main()
   {
    Mat im = imread("C:/santhu/bitmap.bmp");

int rows = im.rows;
   int cols = im.cols;

cout<<"rows\n"<<rows;
cout<<"cols"<<cols;

if (im.empty()) 
 {
     cout << "Cannot load image!" << endl;
    return -1;
}


cout<<"the output for matrix of pixels";

for (int i = 0; i <cols ; i++)
{
    Vec3b *ptr = im.ptr<Vec3b>(i);
    for (int j = 0; j < rows; j++)
    {   
        Vec3b pixel = ptr[j];
        cout<<pixel<<"\t";
    }
    cout<<"\n";
}

getchar();
 imshow("Image", im);
waitKey(0);
}

the code works fine until it shows every pixels values in terms of Vec3b,but at last the exception like "Unhandled exception at 0x75afb9bc in san.exe: Microsoft C++ exception: cv::Exception at memory location 0x0043f9d0.." will come up in prompting windows asks to break or continue the flow

And in command console, where i m getting the pixels values to be displayed, in this it's showing me as opencv error:assertion failed(y==0 ||(data && dims)=1 &&(unsigned) y <(unsigned)size.p[0] in cv::Mat::ptr,file c:\opencv\build\include\opencv2\core\mat.hpp,line 428, after displaying pixels data.

i checked whole web and the mat.hpp also it's inline function given, so i'm frustrated, can anyone plz explain abt this error(exception) and help me out to get code run only until the data pixels is their in bitmap and execute nicely.plz

You are assuming, without checking or demanding, that the array contain Vec3b pixels. — David Schwartz, Feb 04 '14 at 11:23
David, the default flag for imread will force it into CV_8UC3, so that's ok. — berak, Feb 04 '14 at 11:32

score 2 · Accepted Answer · answered Feb 04 '14 at 11:35

2

you're confusing rows and cols here.

for (int i = 0; i <rows; i++)       // rows, not cols
{
    Vec3b *ptr = im.ptr<Vec3b>(i);
    for (int j = 0; j < cols; j++)  // cols, not rows
    {   
        Vec3b pixel = ptr[j];
        cout<<pixel<<"\t";
    }
    cout<<"\n";
}

answered Feb 04 '14 at 11:35

berak

39,159
9
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thanks beark,u made my day..!! and the output is like[int,int,int] in every pixel,the thing here is that how can i access and change the values permanently and store back in the same image and get it displayed..if u can ping some links that would be nice. and thanq for last answer again. – Santhosh Feb 04 '14 at 12:21
use a reference : ` Vec3b & pixel = ptr[j]; pixel[1] = 255;` green is full on now, or just `ptr[j][1] = 255;` – berak Feb 04 '14 at 14:32

RyanLiu · Answer 2 · 2014-02-14T09:22:49.287

1

Color format

for(int j = 0; j < img.rows; j++) {
   for(int i = 0; i < img.cols; i++) {

      uchar b = img.ptr<cv::Vec3b>(j)[i][0];
      ucahr g = img.ptr<cv::Vec3b>(j)[i][1]; 
      uchar r = img.ptr<cv::Vec3b>(j)[i][2]; 

      std::cout << "b = " << (int) b << std::endl
                << "g = " << (int) g << std::endl
                << "r = " << (int) r << std::endl;

    }
 }

Gray format

cv::Mat img;
cv::cvtColor(src,img,CV_BGR2RGB);

for(int j = 0; j < img.rows; j++) {
   for(int i = 0; i < img.cols; i++) {
      std::cout << "gray value = " << img.ptr<uchar>(j)[i] << std::endl;
   }
}

edited Feb 14 '14 at 09:22

answered Feb 04 '14 at 13:01

RyanLiu

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hi RyanLiu actually when wrote your lines w.r.t my code,i got error like:"a pointer to a bound function may only be used to call the function",can u plz look into it... – Santhosh Feb 05 '14 at 04:23
@user2978527 sorry for edition error, the origin one "img.ptr[j](i)(0)" to access image need to be changed to the new edition version "img.ptr(j)[i][0]" and I have already fixed the errors. More information http://stackoverflow.com/a/21346778/2469488 – RyanLiu Feb 10 '14 at 03:23

Reading pixels data values from a bitmap image file

2 Answers2