Mat::forEach operation is resulting in error

asked 2018-08-14 03:21:15 -0600

nobot
51 ●2 ●5

updated 2018-08-14 04:31:04 -0600

by going through this link I got some information , and tested it. I am working on opencv v3.4.2 in Ubuntu 18.04.

If I am using cv::Point_<uchar> Pixel, its giving me a wrong output, but if uchar Pixel is being used the error is given at the compilation time only.

In member function 'void Operator::operator()(Pixel&, const int*) const': for_each.cpp:23:78: error: request for member 'x' in 'pixel', which is of non-class type 'Pixel {aka unsigned char}' rmat("[%d,%d]= %d \n",position[0],position[1],(uchar)pixel.x);

code

    #include <opencv2/opencv.hpp>
#include <stdio.h>
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>

using namespace std;
using namespace cv;
using std::cout;




typedef cv::Point_<uchar> Pixel;//wrong
//typedef uchar Pixel;  //  correct
//typedef Point3_<uint8_t> Pixel;



struct Operator {
    void operator ()(Pixel &pixel, const int * position) const 
    {           
        cout << format("[%d,%d]= %d \n",position[0],position[1],(uchar)pixel.x);      
    }
};

int main( int argc, char* argv[])
{    
    Mat mTest(Size(3, 2), CV_8UC1,Scalar(0));   
    randn(mTest,Scalar(125),Scalar(125));
    cout<<"I am using ----> cv::Point_<uchar> Pixel  \n";
    //cout<<"I am using ----> uchar Pixel \n";
    cout<< format (" Size : %d , %d \n\n",mTest.rows,mTest.cols);

//for(;;)
 {
    for (int Rows = 0; Rows < mTest.rows; Rows++)
    {
        for (int Cols = 0; Cols < mTest.cols; Cols++)
        {
            cout << format("[%d,%d]= %d \t",Rows,Cols,mTest.at<uchar>(Rows,Cols));
        }
        cout << "\n";
    }

    cout << "\n\n";
  }

//for(;;)
    mTest.forEach<Pixel>(Operator());

    waitKey();
    return 0;
}

Is the problem with the new opencv version ?? And can it be resolved??

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answered 2018-08-14 03:43:29 -0600

berak
32993 ●7 ●81 ●312

updated 2018-08-14 06:29:45 -0600

again, there should not be any cv::Point's here at all, and your Operator should look like:

struct Operator {
    void operator ()(uchar &pixel, const int * position) const 
    {           
        cout << format("[%d,%d]= %d \n",position[0],position[1], pixel);      
    }
};

or, for a 3 channel bgr image:

struct Operator {
    void operator ()(Vec3b &pixel, const int * position) const 
    {           
        cout << format("[%d,%d]= (%d %d %d)\n",position[0],position[1], pixel[0], pixel[1], pixel[2]);      
    }
};

(also, those typedefs are more misleading than helpful, you're safe to skip them and use explicit types there)

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Comments

Thank You, the issue is resolved, leading me into another follow up question regarding forEach<pixel>(Operator()). Actually my objective is to compare the pixel value of 2 grayscale images ; will it be possible to do that by using forEach<pixel>(Operator()), as it is taking only 1 parameter as argument.

nobot ( 2018-08-14 08:11:02 -0600 )edit

no. and doing such a thing per-pixel is a terrible idea. please use functions, that work on whole images, like norm() absdiff() compare(), or such here instead.

berak ( 2018-08-14 08:49:40 -0600 )edit

likely I am doing pixel-wise comparison between two images , and depending on the pixel intensity I am manipulating the whole image, by doing so its getting slowed down a bit, so I am insisting on the forEach operator

for(int r1=0; r1 < data0.rows; r1++)
                {
                for(int c1 =0; c1 < data0.cols; c1++)
                    {
                        if (data0.at<float>(r1,c1) < data1.at<float>(r1,c1))
                            {
                            data0.at<float>(r1,c1) = data0.at<float>(r1,c1);
                            }
                        else 
                            { 
                            data0.at<float>(r1,c1) = data1.at<float>(r1,c1);
                            }

data 0&1 same type

nobot ( 2018-08-14 09:25:17 -0600 )edit

again, please don't do it this way. but:

Mat mask = data0 >= data1;
data1.copyTo(data0, mask);

for-loops are illegal in opencv ;)

berak ( 2018-08-14 09:29:07 -0600 )edit

Mat mask = data0 < data1;

will this compare the per pixel intensity of image data0 & data1

nobot ( 2018-08-14 09:31:25 -0600 )edit

is there any other way without copying the images, it is substantially increasing the overhead!!

nobot ( 2018-08-14 09:41:07 -0600 )edit

yes, it will compare it. and no, there is no overhead, in the opposite, you'll profit from the internal optimization

berak ( 2018-08-14 10:09:41 -0600 )edit

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answered 2018-08-16 04:21:43 -0600

nobot
51 ●2 ●5

I am working like this-->

        cv::merge(channels, added3);                   (1)
        added3.forEach<Pixel>(Operator());          (2)
        cv::extractChannel (added3, op1, 0 );         (3)

Then , both (1) & (2) has BGR format, but in (3) if I put index = 0 ( index of channel to extract ) then will I be extracting the Blue Channel only right??

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