Convert an image into an Occupancy Grid

Question

I've just started to learn OpenCV and I'm wondering how can I convert an image like this one:

Into an Occupancy Grid, like this one:

int grid[ROW][COL] = 
    { 
        { 1, 0, 1, 1, 1, 1, 0, 1, 1, 1 }, 
        { 1, 1, 1, 0, 1, 1, 1, 0, 1, 1 }, 
        { 1, 1, 1, 0, 1, 1, 0, 1, 0, 1 }, 
        { 0, 0, 1, 0, 1, 0, 0, 0, 0, 1 }, 
        { 1, 1, 1, 0, 1, 1, 1, 0, 1, 0 }, 
        { 1, 0, 1, 1, 1, 1, 0, 1, 0, 0 }, 
        { 1, 0, 0, 0, 0, 1, 0, 0, 0, 1 }, 
        { 1, 0, 1, 1, 1, 1, 0, 1, 1, 1 }, 
        { 1, 1, 1, 0, 0, 0, 1, 0, 0, 1 } 
    }; 

1: cell is not blocked (white pixel).
0: cell is blocked (black pixel).

I'm not going to use exactly that picture. I'm going to use a picture with only walls: no texts, no furniture, no windows and no door's symbols. There will be only walls with "holes" to show the doors.

I want to read the image and return 1 when the pixel is white, and 0 when the pixel is black. Only that.

How can I do that with OpenCV?

I will store that matrix into a text file, but I know how to do that.

Don't worry about what I'm going to do with that matrix. I'm not asking that.

Answer 1

Mat in OpenCV like the grid you mentioned. .pgm is the format used in the Robot Operating System for storing Occupancy Grid maps. Any image format is fine for representation.

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <iostream>

using namespace cv;
using namespace std;

int main( int argc, char** argv )
{   
   //read input image as gray
   Mat image_gray = imread("input.jpg", CV_LOAD_IMAGE_GRAYSCALE);   

   // convert gray image to binary image 
   // After threshold, all values are either (0 or 200)
   Mat imgage_bw;
   cv::threshold(image_gray, imgage_bw, 200, 255.0, THRESH_BINARY);

   // if you really want images with 0 for blocked cell and 1 for free cell
    Mat image_grid = imgage_bw/255;  

    // save to disk
    imwrite("output.pgm", image_grid);

//write result to text file
FileStorage file("ouput.yaml", cv::FileStorage::WRITE);    
file <<"grid " <<image_grid;
file.release(); 

    return 0;
}

---

ouput.yaml

%YAML:1.0
grid : !!opencv-matrix
   rows: 400
   cols: 800
   dt: u
   data: [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
       1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
       1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
       0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
       1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ....]